Moessbauer spectroscopy - American Chemical Society

Anal. Chem. 1986, 58,250 R-264 R (432) Shalaby, A.; Budvarl-Brany, 2.; Szasz, G.; Bauer, H. J . Li9. Chromatogr. 1984, 7 , 1151-68. (442) Shalaby. A.; Budvarl-Barany, 2.; Hanko-Novak, K.; Szasz, G. J . Li9. Chromatogr. 1984, 7, 2493-506. (452) Sun, S. F.; Kuo, S. W.; Nash, R. A. J . Chromafogr. 1984, 288, 377-88. (462) Sybilska, D.; Debowskl, J.; Jurczak, J.; Zukowskl, J. J . Chromatogr. 1984,286, 163-70. (472) Thus, J. L. G.; Kraak, J. C. J . Chromatogr. 1985, 302,271-9. (482) Urushigawa, Y.; Yonezawa, Y.; Masunaga, S.; Matsul, Y.;Masunaga, H.; Hlral, M. Kogai, Shigen Kenkyusho Iho 1984, 14, 27-33 (Japan);

Chem. Abstr. 102, 3 1 3 9 7 ~ . (492) Valko, K. J . Li9. Chromatogr. 1984, 7 , 1405-24. (502) Valko, K.; Friedmann. T.: Batl, J.; Nagykaldi, A. J . Li9. Chromatogr. w a 4 , 7, 2073-92. (512) Wilson, T. D.;Forde, M. D.; Craln, A. V. R. J . Pharm. Sci. 1985, 7 4 , 3 12-1 5. (522) Xie, T. M.; Hulthe, B.; Folestad, S. Chemosphere 1984, 13,445-59. (532) Yamagami, C.; Takaml, H.; Yamamoto, K.; Mlyoshi, K.; Takao, N. Chem. Pharm. Bull. 1984, 32,4994-5002. (542) Yoza, N.; Shuto, T.; Baba, Y.; Tanaka, A,; Ohashl, S. J . Chromatogr. 1984, 298,419-26.

Mossbauer Spectroscopy John G. Stevens*

Department of Chemistry, University of NQrth Carolina-Asheuille,

Asheuille, North Carolina 28804-3299

Lawrence H. Bowen

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204

Katherine M. Whatley

Department of Physics, University of North Carolina-Asheuille, Asheuille, North Carolina 28804-3299

This review on Mossbauer spectroscopy is based on the papers that have been received and surveyed by the Mossbauer Effect Data Center since our 1984 review (259, and therefore, the papers included in this review are from the period 1983-1985. During the last 2 years a total of over 2000 papers on Mossbauer spectroscopy were processed by the data center. From these 2000+ papers approximately 300 were selected to be discussed in this review. Because of our own individual interests, there are good contributions to the field of Mossbauer spectroscopy that we probably have overlooked. Despite the flaws in our process of selection, the review represents a ood summary of what has been happening in the field of dossbauer spectroscopy during the last 2 years. 1983 was a year of celebration in the field of Mossbauer spectroscopy, recognizing the 25th anniversary of this unique area of science. Two conferences, one in Europe and one in the United States, were held to celebrate the first 25 years. The European event was the second Seeheim Workshop, which was held the week of May 23rd, 1983. The lectures that were presented at this workshop were collected and edited by P. Gutlich and G. M. Kalvius (96). This particular publication includes a collection of 22 lectures on a variety of current topics in Mossbauer spectroscopy. In the following year on the other side of the Atlantic, in the United States, a special symposium of the American Chemical Society (ACS) was held in St. Louis in the early part of April. This gathering was organized by R. H. Herber who subsequently edited a book titled “Chemical Mossbauer Spectroscopy”, which includes the papers that were presented at the Symposium (107). The book contains 15 chapters. In contrast to the papers at the Seeheim meeting, which in most cases focused on the most recent findings, the papers that were presented at the ACS Symposium were mostly of a review nature. There have been several other important books recently published, One of these is the “Proceedings of the 6th International Conference on Hyperfine Interactions”, which was held in Groningen, The Netherlands, July 4-8, 1983 (183). The editors of the resulting two-volume conference proceedings, Hendrik de Waard and his two colleagues, L. Niesen and F. Pleiter, were also the organizers of the conference. A total of 208 articles are contained in the two volumes, which include not only papers on Mossbauer spectroscopy but also applications of other hyperfine interaction techniques. 250 R

Three other recent books are of noteworthy interest. One of these volumes, entitled “Mossbauer Spectroscopy Applied to Inorganic Chemistry (Vol. 1)”by G. J. Long, includes 18 chapters featuring a variety of topics of interest in inorganic chemistry (247). Besides the introductory chapters, Mossbauer application topics include coordination chemistry of iron (W. M. Reiff and G. J. Long), spin transitions in iron complexes (P. Gutlich), biological systems (D. P. E. Dickson), heterogeneous catalysis (F. J. Berry), silicate minerals (J. M. D. Coey), ceramics and archeological materials (G. Longworth), structure and bonding in tin compounds (R. B. Parish), characterization of gold compounds (R. B. Parish), and onedimensional magnetism (C. E. Johnson). A 16-chapter book edited by B. V. Thosar et al. is titled “Advances in Mossbauer Spectroscopy, Applications to Physics and Chemistry and Biology” (269). This large volume (over 900 pages) discusses a wide variety of topics such as dynamic and static crystal-field effects in Fez+ Mossbauer spectra, calculation of charge density, electric field gradient, and internal magnetic field with a nuclear site using molecular cluster theory, theory of zero field splitting spin-lattice coupling, and constants in nuclear quadrupole interactions. Another major contribution is a book titled “Dynamic Property of Solids, Vol. 5, Mossbauer Effect Structural Phase Transitions” (108). This particular book contains 2 chapters, one by E. Cook titled “Studies of Dynamic Properties in Solids with Mossbauer Effect” and the other chapter by Y. Yamada titled “Lattice Instability and Structural Phase Transitions in Coupled Systems”. During the last 2 years the most active area of the application of Mossbauer spectroscopy is the study of amorphous materials. Over 200 papers were written during the last 2 years on this topic, Two areas having over 100 Mossbauer papers are studies of minerals (including soils and meteorites) and of catalysts. In the case of the studies of amorphous materials and catalysts, Mossbauer spectroscopy is one of the few and in some instances the only technique that allows for the study of molecular bonding and structure, because often there is no long-range ordering in these materials. Another active area of study is ion implantation experiments. One of the Mossbauer techniques used to study many of these materials and others is conversion electron Mossbauer spectroscopy (CEMS). There are over 100 papers on CEMS that have been published during the time period covered by

0003-2700/86/0358-25OR$06.50~0 0 1986 American Chemical Society

M~SSEAUERSPECTROSCOPY

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amorphous materials have been extensively studied by use of M h b a u e r spectroscopy. They are the topic of several reviews, which are authored by U. Gonser and R Preston (83, A. K. Bhatnagar and R. Jagannathan (17). and M. D. Dyar (68). Other materials studies include catalysts (267,271), mrrosion (53,liquid crystals (2,221, and silicate materials (219, 263). Other reviews are on diffusion studies (173) and ion implantation (239). Two different reviews have been published on M h b a u e r spectroscopy of the actinides (66,120). Other reviews have been on relaxation (99, 114), minerals (270),"m ' Mossbauer spectroscopy (891, Massbauer spectroscopy in China (258).and high field Mossbauer spectrosCOPY (221). During the last 2 years there have been two major conferences. One was a special symposium titled the Industrial Applications of the MBssbauer Effect. This symposium was part of the Chemical Congress of Pacific Basin Societies. A t the symposium there were over 100 papers on a large variety of applications of Mossbauer spectroscopy in industry. The other conference was the 1985 International Conference on the Application of the Mossbauer Effect, which was held in Leuven, Belgium. This particular conference was the largest one in a long series of international conferences. In particular, there were over 400 participants. Both the International Conference and the Sympasium on the Industrial Applications will have their proceedings published sometime during the year 1986. A new M h b a u e r transition has recently been reported in Ni by H. Mutbig e t al. from the Technical Univenit in Miinich (178). They report a very narrow transition for LNi' using the y source %produced by the "Ni(t, a)%o reaction. Because of the long lifetime of the excited nuclear energy state, the natural line width is smaller than that of the isotope which is commonly used for nickel, 61Ni. The observed line width is approximately 0.02 mm/s, which is broadened about 5 times the minimum resonance possible for this particular transition. As in previous reviews, the facilities of the M h b a u e r Effect Data Center have been used. The center continues to publish the Mossbauer Effect Reference and Data Journal, with subscriptions going to over 40 different countries. A complete listine of all the moers that were considered for this review can h i iound in the I984 and 19% volumes of the Mmbauer Effect Reference and Data Journal. ~

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this review. This is a marked increase from past years. Another area in which there has been an increase in activity is the various Mirurbauer scattering experiments. Most of these are at the diffraction angles. Recently, there have been both experimental and theoretical developments in these areas. The vast majority of the re rted Mirurbauer spectroscopy research continues to be on &e. Following this isotope in pularity remains "%n. Of the remaining isotopes l5'Eu is far the most important Mossbauer isotope. In particular. there are over 50 papers on lslEu M h h a u e r spectroscopy from the last 2 ears. While this is considerably less than the number of 719Sn papers, it is much more than the number of papers on any one of the remaining isotopes. MBssbauer isotopes, in declining order of the number of pagers, but on which there are at least ten a ers, are Iz1Sb, 7Au, 16'Dy, 155Gd, 1291, W7Np, 16,Tm,andp7'&b. It should be noted that there has been a diminished interest in the main group isotopes from previous years. For example, in the last review period there were 35 papers on 'WTe while during the period covered by the current review there were only 8 papers! In general there has been an increased interest in many of the lanthanide isotopes. Among the reviews we have surveyed, several commemorate the 25 years of M c d a u e r spectmseopy. These include reviews by F. J. Berry (16),S. J. Campbell (35).R. S. Raghavan (213). and R. V. Parish (196,197'). As has already been mentioned,

One of the most important components of a MBssbauer spectrometer is the multichannel analyzer (MCA), which provides a multiscaling system for the collection of the data. With the advent of the microcomputer it is possible, and even more economical, to replace the multichannel analyzer with a microcomputer system. Some of these new systems were reported in our last review (257).There are two recent reports in which these popular microcomputers have been used to provide for the multiscaling. An Apple I1 has been used by Linares and Sundqvist (144) and a Commodore personal computer has been used by Miiller (174). In the case of the Apple I1 microcomputer the interface collects two simultaneous spectra and consequently allows for the use of a double ended spectrometer. The interface uses ten low-cost, lowpower integrated circuits. One of the major limitations to the Linares and Sundqvist system is that the minimum channel dwell time is given hy the execution time of the data-collecting routine; for example this limits the vibrator frequency to about 15 Hz when using 512 channels. In the Muller paper much detail is given on the operation of their spectrometer including complete descriptions of the software. They also describe a scheme by which the dead time between any two channels is only one clock cycle, e&. llms, and there is no dead time between two count pulies. Wallingford and Bordeleau (288)describe a micmcomputer for data acquisition using a TMS9900 CPU (Texas Instruments), which operates at 4-MHz clock frequency. Their system is built on an S-100 bus and includes 64K of memory and two 8-in. floppy disks. Their system uses a 16-bit microprocessor, which has the advantage over the &bit microprocessor of not having to operate in an "interrupt" mode, which is very time-consuming. The dead time between channel advances is zero. Their system also provides for pulse height analyses, which is a necessity for adjustments needed for obtaining good Mossbauer spectra. In another use of ANALYTICAL CHEMISTRY. VOL. 58. NO. 5. APRIL 1986

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microprocessors, Obenhuber et al. (190)describe a Mossbauer spectrometer for the frequency modulation technique using piezoquartzes. The system has been used for precision determination of hyperfine splittings and specifically used for the very high resolution 67ZnMossbauer spectroscopy. The spectrometer has proved to be quite stable for running times of 2 weeks. Their frequency modulation method is essentially free of systematic errors, which occur in the normal Mossbauer measurements. Another contribution of the microcomputer in the development of Mossbauer spectrometers is reported by Purcell et al. (212). They describe the design, construction, and operation of a time-resolved emission Mossbauer spectrometer. The computer allows for the collection of the data in which both the time and energy information are recorded. The time and energy data are not independent but are related by a Fourier transform. Photon counting can, therefore, be visualized as resulting in a three-dimensional surface of the number of photons as a function of the time and of the energy. The experimenter then selects the time period and then can plot the resulting spectrum. Another time Mossbauer experiment is described b Vapirev e t al. (282). In their experiment, they use the YgmSnsource and obtained time resolutions of 2.5 ns. In still another time experiment, Zhang et al. (300)vibrate the source at a frequency of 10 MHz. The resulting oscillating spectra is often referred to as “quantumbeat spectra”. The results can be analyzed and explained using optical theory. The approach that usually is used in the development and design of the electrochemical drive part of the Mossbauer spectrometer is on the basis of electrical and mechanical analogues. These efforts have been limiting. Reddy et al. (218) proposed a mathematical model for examining the design characteristics of velocity transducers. This approach allows quantitative examination of design parameters for improving the capability of the drive unit. Several papers have been published that provide techniques to assist with the analysis of the data. Gittsovich and Golubev (85) describe an automatic program for preprocessing Mossbauer spectra. They describe an algorithm for finding the peak intensities, positions, and widths. In another paper, Otterloo et al. (193) use a (Al,Ga)As laser diode and Michelson interferometer to provide for the Mossbauer velocity Calibration. One of the major advantages of the system is its small size. The laser housing itself takes up only 1 cm, which compares to the often used He-He laser which requires a length of about 35 cm. One of the consequences of the smallness is the relative ease in aligning the optical system. In another paper, Rajan and Nigam (215) describe a technique for the determination of the zero velocity channels. The principle is based on the fact that the zero velocity drive corresponds to the turning points of its motion which can be detected by an electrical method. Specifially one can measure the capacitance between two plates. It is possible in such an experimental setup to obtain zero velocity within 5 min, which compares with hours for other techniques that have been suggested in the past. The ability to obtain zero velocity channels in a short period of time allows one to monitor a spectrometer’s stability. Double magnetic nuclear y resonance is observed when there is both a constant magnetic field and at the same time a high frequency magnetic field, both acting on the resonance nuclei. Cheremisin and Dudkin (37)describe an experimental instrument that performs the double magnetic resonance in lEITa. Another special type of Mossbauer spectrometer is described by Toriyama et al. (272). They have designed a retarding-grid electron spectrometer for CEMS spectrometer. They use a pair of spheroidal grids as a high pass filter to increase transmission. Energy resolutions of the order of 0.3% a t 7.3 keV with a transmission of 1.2% are obtained. This counter resolution allows for good depth selectivity. High efficiency resonance scintillators for l19Sn Mossbauer spectroscopy are reported by Mandyukov et al. (156). The scintillators consist of layers of polycrystalline anthracene with fipely dispersed SnOz or CaSnO, in it. An application of this detector is the Rayleigh scattering of recoilless 23.8-keV y-rays from an aluminum scatterer. Petrikin et al. (205) describe a gas-discharge detector for CEMS. In another paper low-temperature depth-selected CEMS is described by Sato et al. (2,33).They use a cylindrical 252R

ANALYTICAL CHEMISTRY, VOL. 58, NO. 5, APRIL 1986

mirror type electrostatic electron spectrometer to obtain the necessary energy resolution. Another system for depth selective CEMS is described by Pancholi et al. (194). Their system is structured in such a way that a miniorange magnetic filter focuses conversion electrons emitted by the moving of the absorber onto a thin window silicon surface barrier detector. The major advantage of this system is the large reduction in the measuring time for obtaining the full data. In still another study, Takafuchi and Kobayashi (265)describe a method for improving the surface of selectivity, using a proportional counter. For some Mossbauer experiments it is important to be able to have nuclear equipment that is capable of handling high intensity sources. The usual procedure has been to measure the current coming from the detector as opposed to counting the number of pulses. Potzel and Halder (208)describe a fast pulse amplifier capable of high count rates. They report a count rate of 800000 counts/s. Their amplifier was used for obtaining spectra for 67Znin which the number of counts a t a particular velocity is of the order of lo9 Bressani et al. (29)describe the use of lead-loaded plastic scintillators which reduce by a factor of 7 the measuring time for obtaining a Mossbauer spectrum. Examples are given of spectra obtained in 5 min. A proportional electron counter is described by Chumakov et al. (41). The energy resolution of this detector is approximately 30%. One of the more interesting experiments reported is that of obtaining Mossbauer spectra using the visible light emitted from solids following the resonance absorption of the Mossbauer y-rays. This has been reported by Belozerskii (14) for a metallic iron foil. In still another paper using CEMS, Andreeva et al. (8) studied surfaces based on a new method by using the conditions of total reflection.

MOSSBAUER DIFFRACTION In recent years the most interesting area of fundamental Mossbauer research is the diffraction experiments. Although much of the theory on Mossbauer diffraction was developed over 20 years ago, there are still developments that are taking place. The most noteworthy advances were not previously possible, because of the instrumental limitations. Two recent papers from the group at Munich and the Kurchatov Institute of Atomic Energy in Moscow have used nearly perfect FeBO, single crystals to study nuclear resonance diffraction in the Bragg geometry (161,277). They examined both the reflection and the transmission of this crystal under the Bragg conditions of the (111)and the (222) reflection planes. Because of the various competing absorption processes, it might be concluded that the probability of resonance diffraction would be close to zero. But in fact, the theory shows that the reflection under the diffraction condition should be almost 100%. It is shown experimentally by the Munich/Moscow group that there is a very high reflectivity and very little absorption when the Bragg condition is met. In addition, a shift of the resulting spectra line is seen experimentally when the y-rays are either slightly above or below the Bragg angle. In another paper the time dependence of coherent scattering of resonant y-rays are measured for the first time by Smirnov et al. (252)using single crystals of FeBo3 Also they report an acceleration of the decay of an excited nuclear state along the direction of the Bragg angle. In another Mossbauer diffraction experiment, the magnetic structure of Fe3BOBis investigated by Kovalenko et al. (134). The results obtained confirm those of neutron diffraction studies of the magnetic structure in this compound. The advantages of the Mossbauer diffraction technique are discussed in the Kovalenko paper. In another interesting experiment, nuclear Bragg diffraction of synchrotron radiation in yttrium iron garnet is observed. This experiment was performed by Gerdau et al. (82). There are several reports in the literature of elastic and inelastic diffraction of Mossbauer y-rays with several different non-iron-containing materials. Wittmann and Jex (296) have studied y diffraction from the (111)to (555) Bragg reflections. The energy resolution of their experiments is 28 neV. The elastic scattering dies out rapidly due to a large Debye-Waller factor. In the case of the higher reflections the inelastic scattering is larger in magnitude than the elastic one. Kashiwase and Minoura (126)have determined the integrated reflective power of silicon crystals by Mossbauer diffraction.


They were able to separate the elastic and inelastic scattering. Specifically, they examined the thermal diffuse scattering, which is a major portion of the inelastic scattering, In another diffraction experiment, Krec and Steiner (135)report a further improvement in this particular experiment. They also discuss in further detail their results and expand the theoretical interpretations. In addition to examining silica, Krec et al. (136) have examined a single crystal of LiNbOB by Mossbauer diffraction in order to separate out the elastic and inelastic scattering of the y radiation. The microscopic study of the motions in supercooled 1,3-butanediol quasi-elastic Rayleigh scattering of the 14.4-keV Mossbauer y-ray. A report by Soltwisch et al. (253) compares the results of the Rayleigh scattering of Mossbauer radiation with three other techniques: NMR, optical digital correlation spectroscopy, and ultrasonic relaxation. Two other materials that have been studied by Rayleigh scatteing of Mossbauer radiation are Ectothiorhodospira Shaposhnikovii (137) and L-Dopa Melenin (4). Important information is gained on these two species including the ability to study their motion. Zolotoyabko et al. (302) describe the experimental setup of the Mossbauer diffraction on a rotating crystal. Their report includes both experimental and theoretical results. There are many experiments in the future that need to be performed using Mossbauer diffraction and scattering. Mossbauer diffraction experiments are one of the prime areas for new developments taking place in the field of Mossbauer spectroscopy.

LINE SHAPES Several studies are reported on the obtaining of Mossbauer parameters from the line shapes of Mossbauer spectra. Ratner (217) discusses a method for the precise measurement of small parameter changes of single lines. The method he uses is the residual-area-analysis method (RAAM). This particular approach has the advantage that it does not necessitate curve fitting or even a knowledge of the line shape. Precision of the method is unaffected by the deviation of the line shape symmetry. Massiot (158) describes a method of comparing line shapes of a group of related spectra in which there are small changes, He applies his method to a series of Mossbauer spectra with success. In another paper, Blaes et al. (22) describes a method for obtaining analytical expressions of the 57FeMossbauer line shape in the presence of mixed hyperfine interactions. The method uses a superoperator formalism to yield the closed form expressions. Kansy et al. (123) have studied the effect of nonhomogeneity of sample thickness on the determination of absolute Mossbauer fractions. As would be expected, nonhomogeneity lowers the intensity of the lines and deforms the spectra shape. Using their method, they have obtained improved Mossbauer fraction data for sodium nitroprusside, source in Cr. pyrite, and the 570 Two different methods for solving the transmission integral, which is necessary when there are saturation effects and overlapping lines in Mossbauer spectra, are discussed by Jernberg (117). Comparisons of the computing times for the different methods of treating the transmission integral are reported in this paper. An improved derivative technique for the analysis of the Mossbauer spectra is described by Kobayashi (129). He applies his approach to several cases, resolving two or more overlapping lines.

THEORY Many papers have appeared in the recent literature dealing with quantum mechanical calculation of Mossbauer parameters for specific inorganic and organic compounds or groups of compounds. Relaxation effects continue to be of interest for theoretical studies. Interpretations of electronic spin transitions have been given in a number of papers. Cser et al. (49) analyzed the changes produced in isomer shift and hyperfine field by 3d metal impurities in iron. They report that the isomer shift data are well described by a Friedel model of charge screening, although the hyperfine field data require additional considerations. Dubiel and Zinn (65) have also considered this problem of substitutional effects in iron metal, although they treat primarily non-transition-metal substitution and larger substitutions. A model based on work by Miedema and van der Woude gives better agreement with

the data they consider than does the Friedel model. Menil (166) considers isomer shift data for a large number of compounds with Fe-0 and Fe-F bonds. In addition to verifying the expected decrease in 6 with increased oxidation state and with decreased coordination number, the data show clear evidence for a marked inductive effect by the second nearest neighbor in Fe-0-X coordination, especially when X = S or P (sulfates and phosphates). Iron-ligand bond distances show little correlation. Liu (146) reports equations that describe the variation of area ratio for a doublet spectrum of a single crystal using linearly polarized y radiation. These equations are used to predict area ratios for pyrite and marcasite as a function of angle and of the plane of polarization. Corson and Hoy (46) report low-temperature Mossbauer data for K2Fe04(down to 0.16 K) showing appreciable spin relaxation. They analyze the results in terms of a stochastic relaxation model with an abrupt slowing down of the spin fluctuations around the critical temperature of 3.6 K. Eibschutz et al. (72) studied amorphous FeF, prepared by vapor deposition. They analyze the 4 K spectrum to obtain detailed information about the hyperfine parameter distributions and the variation of iron environment in this glassy material. It should be noted that crystalline FeF, has only a single iron site. In the most recent of a series of apers on erovskite solid solutions, Gibb (84) reports new P7Fe and lEu Mossbauer data on the series E U F ~ ~ - ~ M , where O , , M = Al, Mn, Ni, and other cations, and discusses the details of the superexchange interactions. He concludes that Fe-0-Cr superexchange in iron-chromium perovskites is antiferromagnetic. Nagel (180) has used the MSX, cluster technique to investigate the electronic structure of hematite, a-Fe203. From comparison of calculations with the measured quadru ole interaction, a new value of the quadrupole moment for .p"Fe is suggested, Q ii: O.1lb. Significant deviations from an ionic structure were observed. Dormann et al. (61) studied fine particles of Fe203 dispersed in alumina and compare the temperature variation of the Mossbauer spectra with the model of superparamagnetism. They conclude that the noncompensated magnetic moment of the particles is prowhere n is the number of iron atoms, and portional to nl/*, that this influences the observed relaxation rates. Srivastava (254) discusses the contribution of orbit-lattice interactions to the field gradient of Fe2+in MgC0,. These interactions prove to be important in explaining the observed decrease in quadrupole splitting with increasing temperature. Cianchi et al. (42) report calculation of spin-lattice relaxation times for Fe2+in MgC03 and ZnCO,. This effect is observed only at lower temperature where the orbit-lattice interaction becomes small. They find better agreement with the data from the assumption that relaxation is an Orbach rather than Raman process. Dainyak et al. give a general method for interpreting the quadrupole splittings of Fe3+in dioctahedral layer silicates (521, based on field gradient calculations, and in following papers apply the model to the spectra of nontronite (51) and celadonite (50). They note that according to electron diffraction these minerals have only cis-OH octahedra and thus the usual model based on cis and trans sites is incorrect. In nontronite the two major Fe3+ doublets are in agreement with the calculation based on the relative proportion of A13+substitution in neighboring tetrahedral sites. In celadonite, which has little AP+, but appreciable Mg2+and Fe2+in octahedral sites, a high degree of ordering of the Fe3+ ions is required to interpret the spectra. Stadnik (255)reports field gradient calculations for rare-earth iron garnets. The monopole contribution explains the sign and changes of coupling constant at two of the three sites. Dipole contribution is important; however, these two effects do not account for observed magnitudes. Guenzburger et al. (91) report molecular orbital (MO) calculations on Fe(COI5 and its photodecomposition fragments. These calculations are compared with earlier experimental Mossbauer data on the fragments isolated in polyethylene. General trends appear consistent between calculated and experimental values for Mossbauer as well as optical spectra. Ikuta et al. (113) use MO calculations to interpret the decrease in quadrupole splitting observed in ferrocenophanes relative to ferrocene. This decrease is attributed to a lesser nuclear contribution at 57Ferather than electronic redistribution among the 3d orbitals. Cohn et al. (44) studied

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Mijssbauer spectra of mixed-valence biferrocenium salts in external magnetic fields. They find effects of spin relaxation on the observed internal hyperfine field. They have simulated Mossbauer spectra of several delocalized mixed-valence species using an S = 1/2 spin Hamiltonian. Bill et al. (19) report MO calculations used to interpret Mossbauer spectra of (FeIm6)[HFe(C0)4], Im = 2-ethylimidazole. The anion site shows relaxation effects due to diffusion-hopping of CO. Sanner et al. (232) give Mossbauer results on the dilution effect of Co in the high-spin-low-spin and (HS-LS) transition of Fe(2-pi~olylarnine)~Cl~~EtOH compare these results to the predictions of theory, in particular the elastic interaction model of Spiering et al. Wei and Jean (289)report the first evidence of a particle size effect on Fe(I1) HS-LS transition in carbon or MgO-supported Fe(Nphenyl-2-pyridinaldiamine)z(NCS)z.Low iron loading promotes the low-spin phase. Maeda et al. (151)give a number of new examples of Fe(II1) complexes exhibiting HS-LS transition and discuss a model for interpreting their results in terms of intramolecular electron transfer. A number of papers have appeared dealing with calculations for biological compounds and comparison of these calculations with Mossbauer data. The iron-sulfur compounds have been prominent in this regard. Christner et al. (40) develop a spin coupling model for the interaction between the 4Fe-4S cluster and the homoprotein subunit in E. coli reductase. The model gives agreement with Mossbauer and EPR data for a number of complexes. Trautwein et al. (275) report MO calculations on mononuclear Fe"S4 and binuclear FeSzMo cores and compare these calculations to Mossbauer spectra analyzed by a spin Hamiltonian. They concIude that Fe(SPh)?- is a good model anion for the electronic structure of reduced rubredoxin. Noodleman et al. (187) have also made calculations on the FeS4 cluster as well as other clusters with up to four irons. They used an X a method for the calculations. Again, comparison with Mossbauer isomer shifts and quadrupole splittings of model compounds is an important part of their analysis. In an interesting but rather different biological application, Ofer et al. (191)used a diffusion model to calculate rotational and translational motions of the FesOI particles in magnetotactic bacteria. These motions account for an extremely broad Mossbauer resonance above freezin temperature. The calculations imply the particles are fixecfin whole cells, and the viscosity of the medium is about 15 times that of water. Van der Velden and Stadnik (278) report MO calculations for gold(1) tertiary phosphine cluster compounds. They find an empirical correlation between the 6p population of the metal and the lg7Auquadrupole splitting. Barbieri and Silvestri (11) calculated partial changes at the tin nucleus in five-coordinate Sn(1V) using an orbital electronegativity equalization procedure and present correlation dia rams between these partial charges and isomer shifts for ?lgSn in a large number of series of compounds. Winkler and Mehner (295) interpret the l19Sn isomer shifts by MO calculations on Sn(1V) halides and related compounds. Their analysis yields a revised value for the nuclear radius parameter, AR/R = 0.94

x 10-4.

Peev and VBrtes (202) derive a simple model for relating changes in particle size due to chemical reaction at the surface with relative Mossbauer peak areas of the original and reacting species. Du Marchie van Voorthuysen et al. (64) measured the quantum beat and sideband Mossbauer spectra as function of frequency and report good agreement with optical theory.

CHEMICAL STUDIES Many applications of Mossbauer spectroscopy fall into this category. As is customary in this review, we emphasize those papers in which the Mossbauer results are primary in the conclusions reached. Although 57Fe is represented in this section, the large literature dealing with that isotope has been divided into many other sections of this review. Thus, Mossbauer spectroscopy of other isotopes is emphasized here. Ambe et al. (6) report emission Mossbauer spectra of 67C0 adsorbed on a-Fez03,Cr203,and A1,0,. For the A1.Q and Cr O3 the spectra are doublets down to 78 K, both Fe and Fe3+ being observed. Broadened sextets are superimposed at 4 K, more so for the Cr203 The adsorption on Fez03gives a clear sextet up to 297 K. This sextet exhibits the Morin transition just as the bulk material does, indicating the ad254R

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sorbed ions are incorporated into the substrate structure. Pasternak et al. (198) also used W o emission, in this case to study the interaction in solid H2 They present clear evidence for the formation of a single molecular species characterized by quadrupole splitting of 2.4 mm s-l and isomer shift 0.59 mm s-l. This species is attributed to FeH, not observed in Mossbauer absorption studies. Pinkerton and Dunham (206) report Mossbauer spectra for a number of rare-earth iron-boron alloys of the type R2Fe14B, important new permanent magnet materials. The hyperfine fields vary with Curie temperature but indicate essentially the same iron configuration independent of the rare-earth component. Buschow et al. (31) have studied the compound of this stoichiometry with R = Gd by the Mossbauer effect in 155Gd. The two Gd sites have both strong magnetic and quadrupole interactions at 4 K, and a full Hamiltonian was necessary for data analysis. An ambiguity occurred in that equally satisfactory fits were obtained with the assumption that the field gradient was parallel or perpendicular to the c axis. However, the authors conclude that the second-order crystal field parameter differs in sign between the two sites. Gubbens et al. (90)used lslDy to study DyzFe14B. They also analyze the spectra in terms of two sites with strong magnetic and quadrupole interactions, both consistent with the -15 2 ground state spin. These authors present evidence that t e reported difference in crystal field parameter from the 155Gd studies is too large. Bicker et al. (18) and Lukas et al. (148) report continuing studies of ferrocene complexes. The first of these discusses stabilization and electron transfer in differocenyl cations and the second carbenium ions and ketones of [3]ferrocenophanes. Moore and Hendrickson (168) used Mossbauer, EPR, and optical spectroscopy to investigate the electronic structure of the mixed-valence cation [(%acetylfulvalene)(fulvalene)diiron]+ and compare with that of the nonsubstituted symmetric cation. All the data are shown to be consistent with a delocalized electronic structure for both cations. A number of papers report Mossbauer studies of biological species or related model compounds. Gunter et al. (92) discuss the preparation and properties of a cyanide-bridged Fe(I1I)-Cu(II) porphyrin complex. Mossbauer data indicate a single low-spin Fe(II1) species. Huyuh et al. (110)have characterized a sulfite reductase from D. vulgaris, using Mossbauer, EPR, and other techniques. Mossbauer spectra are similar tothose of the sulfite reductase from E. coli and indicate an exchange coupling between the siroheme and (4Fe-4s) unit. Kanatzidis et al. (122) report synthesis and characterization studies of cubanes with [Fe4S4 cores having DZdgeometry. These compounds all have oublet Mossbauer spectra, fitted to two components with similar isomer shift but different quadrupole splitting. Kennedy et al. (127) present Mossbauer, EPR, and optical studies on unfolded aconitase from beef heart, which has a [3Fe-4S] cluster. The Mossbauer data are interpreted as due to high-spin Fe(II1) in tetrahedral coordination with sulfur and exchange-coupled. These authors analyze the data with a spin coupling model. Lukas and Silver (149) report Mossbauer studies on protoporphyrin IX iron(I1) frozen solutions after exposure to oxygen and discuss the similarities with oxyhemoglobin, especially when the fifth ligand is 2methylpiperidine. The rare-earth Mossbauer isotopes have been used increasingly for chemical studies. Of these 151Euis most PO ular. Felner and Nowik (78) used this isotope as well as "'Gd, P61Dy, and 57Feto study the rare-earth compounds RM2Si2. For M = Ru the Eu is predominantly trivalent, while for M = Rh the Eu is divalent, both compounds having a mixed valence component. Two distinct antiferromagnetic ordering transitions occur for the Rh compounds, one for the rare earth and another for the Rh. The ternary hydride Eu21rH5has been studied by Stadnik and Moyer (256). The divalent Eu orders ferromagnetically at 20 K, and its hyperfine field follows a spin 7/2 Brillouin function. Stucky et al. (262) used 151EuMossbauer studies as well as luminescence, EXAFS, and EPR to study the electron transfer properties of Eu2+and Eu3+in zeolites. Abd-Elmeguid et al. ( 1 )redetermined the 151Eu Mossbauer spectra for EuCuzSi2 and EuPdzSizas a function of temperature in order to explore in particular the behavior of the satellite component present with relative area from 10 to 40%. Sanchez et al. (230)report Mossbauer, structural, and magnetic data for the europium

b

d2+


dihalides. They observe a linear correlation between isomer shift and the saturation hyperfine field. Because the ordering temperatures are below 2 K, the saturation fields were estimated from spectra at higher temperature in an external field. Nowik et al. (189) studied the 151Eu Mossbauer effect in EuNi2P2,EuPdzSi2,and EuPd6B4,in particular to investigate the temperatures where both isomer shift and line width vary markedly. They associate the line broadening with valence inhomogeneities in the local electronic structure. Malik et al. (154) report on the unusual valence changes and magnetic behavior of Eu,Rh3B2. For x = 1 the Eu is divalent, while mixed states appear for lower values of x , with accompanying changes in magnetic behavior. In the same issue of Physical Review Letters, Shaheen et al. (246) report their independent study of the stoichiometric compound with x = 1. Both these papers note the extremely large quadrupole splitting observed for the stoichiometric compound and rationalize its value in terms of the structure. Malik et al. (155) present 161Dy and ls5GdMossbauer results on RCoQB2,R = Dy or Gd. The Mossbauer results supplement magnetic measurements and show large crystalline electric fields at the rare-earth site, which reduce the hyperfine field observed. Pszczola et al. (211) report both 57Feand 161Dy Mossbauer results for @,Fey intermetallic compounds. These compounds have reduced moments at iron and excess hyperfine fields at the dysprosium sites. Mechanisms for this magnetic behavior are discussed. The 156GdMossbauer effect was used by Sanchez et al. (231) to study hyperfine magnetic interactions in the antiferromagnet GdS. The lZISbMiissbauer spectra of a number of iron-containing spinel antimonates are reported by Grandjean et al. (88). For those spinels with magnetic M2+cations, a su ertransferred hyperfine field is observed at the Sb site. The 8Fe Mossbauer spectra were also obtained and are analyzed in terms of site distributions. Pebler and Dehnicke (201) present 12'Sb data for a number of octahedral Sb(V) oxyhalides and show that the data give a linear relationship between isomer shift and average electronegativity difference. The effect of ring substitution on the cation for a series of pyridinium bromoantimonate(II1)compounds was studied by Hedges and Bowen (100) using 121SbMossbauer and Raman spectroscopy. They report a correlation between isomer shift and several lowfrequency Raman bands for the SbBr4- series. Thiosemicarbazone and semicarbazone complexes of Sb(II1) were studied by Saxena et al. (241),using the lzlSb Mossbauer effect as well as IR and NMR. They note distinct trends in both isomer shift and quadrupole splitting as compared to alkoxide complexes. Alonzo et al. (5) report 12'Sb Mossbauer studies of catecholatoantimony(II1) halides. The data are discussed in terms of a point charge model assuming a constant contribution from the lone pair, with the bridging halogens providing variation in e2qQ and 0. A number of chemical studies using the lg7AuMossbauer resonance have appeared recently. Al-Sa'ady et al. (3) report studies of Au(1) thiolates including several drugs used in arthritis treatment. The ig7A~spectra indicate linear coordination by sulfur. Braunstein et al. (28) present le7Au Mossbauer results on some new metal-metal bonded Au(1) complexes of general formula (M-Au-M)-, where M is a metal carbonyl. These complexes also have linear coordination at the Au(1) site. Calis and Hadjiliadis (32) discuss structure and bonding in Au(1) and Au(II1) nucleosides, as deduced from the Mossbauer spectra. Shaw et al. (248) used lg7Au Mossbauer effect as an important part of an extensive characterization of the complex between bovine serum albumin and sodium gold(1) thiomalate. The Mossbauer data show two very similar sites with S-Au-S coordination and rule out a number of proposed bonding models involving N or 0 ligands. A few papers should be mentioned in which more exotic Mossbauer isotopes were used for chemical information. Magnetic effects in a number of nickel spinels were studied by use of 61Niby Gutlich et al. (97). Both sign and magnitude of the hyperfine fields were determined, using an externally applied field. Ligand field theory is used to explain the variations observed. Lukasiak et al. (150)used both 57Feand 61Nito stud long range ordering in Ni3Fe. Calogero et al. (33) report 95Fe Mossbauer data for a number of Te(I1) and Te(1V) thiourea derivative complexes and discuss the results in terms of bond hybridization. Birchall et al. (21) report 1271

Mossbauer studies of phenyl-iodine compounds, including Mn porphyrin complexes with iodosylbenzene. Their data are interpreted in terms of distortion of eometry at the iodine site by a nearby fourth ligand. The k7Np Mossbauer transition was used by Friedt et al. (79) to study the low-temperature phase transition in NpO,. The Mossbauer data indicate that the structural change occurring at 25 K competes with magnetic ordering.

AMORPHOUS MATERIALS Since the last review a vast number of papers have been published on various aspects of the Mossbauer spectroscopy of amorphous materials. Many of these papers were concerned with the analysis of the distribution of hyperfine fields that exist in the Mossbauer spectra of amorphous alloys. Campbell (34) presented an outline of the problems encountered in such spectra and described several experimental approaches that can be useful in simplifying spectra and in the devolution of broad distributions of hyperfine fields. Eibschutz (71) analyzed Mossbauer Zeeman spectra from room temperature measurements on six different boron and phosphorus metalloid glasses. The analysis was performed by focusing on the coarser aspects of the hyperfine field distribution, such as the mean value and the variance, so as to retain all relevant parameter correlations. Approximations that are made in 57Fe hyperfine field distribution calculations are discussed by Le Caer et al. (140). They present a diagram that gives an immediate indication of the validity of a given hyperfine field distribution obtained using first-order perturbation theory. Eickelmann et al. (73) performed high-pressure Mossbauer studies of amorphous and crystalline (Fe0.25Ni0.75)75B7bat 4.2 K. They found that the pressure dependence of the hyperfine parameters enables the formation of conclusions on the local structure which tend to be model independent. Local structure in metallic glasses is important for the understanding of these substances which are metastable and amorphous. Results of the study led to the conclusion that the amorphous and crystalline phases of (Fe0.25Ni0.75)75B75 have quite similar local structure. Pinkerton and Dunham (207) were able to deduce the hyperfine parameters of the iron sublattices in the ternary phase of Nd2Fe14B. This compound forms the basis for a new class of high-performance light-rare-earth-iron permanent magnet materials. This study reported the 57Fe Mossbauer spectrum of the ternary phase of Nd2FellB for the first time. Numerous other papers were concerned with the quadrupole s litting of amorphous alloys. Deppe and Rosenberg (58) used &Fe Mossbauer spectroscopy to study the quadrupole splitting at the Fe site and its dependence on temperature in the paramagnetic range of ferromagnetic Fe-Mo-B and Fe-Cr-B amorphous alloys. The goal of the investigation was to compare the EFG in amorphous alloys with that of noncubic metals, which have been extensively studied. The intrinsic isotropy of an amorphous structure does not imply a local isotropy of the electric field. Short range order can lead to local noncubic surroundings for the Fe atoms. In two papers (130,131) Kopcewicz et al. introduced and investigated a new method for direct determination of the quadrupole splitting of amorphous alloys in the ferromagnetic state. This method makes use of the radio-frequency-induced collapse of the magnetic hyperfine structure. On exposure to a radio frequency magnetic field, the magnetic hyperfine field is averaged to zero, and the effects of the electric monopole interaction (isomer shift) and the quadrupole interaction remain. However, it was found that exposure to radio frequency magnetic fields can induce crystallization caused by magnetostatic vibrations. In other papers concerned chiefly with the analysis of the Mossbauer spectra of amorphous materials, Nikolov (184) proposed a mathematical model for decomposition of symmetrical Mossbauer spectra of amorphous magnetic materials, and Billard et al. (20)simulated an amorphous structure using a soft-sphere relaxed model and proposed a model for analyzing the spectra from such a structure. The study of amorphous alloy ribbons is of much current interest. Bourrous et al. (25)found a correlation between the magnetic texture and the thickness of a selected zone of such ribbons. Henry et al. (104) also investigated the magnetic properties of ribbons, in this case iron-rich amorphous alloy quenched ribbons, 2 mm wide, containing from 0 to 8 atom ANALYTICAL CHEMISTRY, VOL. 58, NO. 5, APRIL 1986

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% chromium. Chromium addition in these alloys greatly increases the resistance to corrosion, but it also has sizable effects on the magnetic properties of the alloys. Henry et al. found that the Curie temperature and the magnetization were decreased by the chromium additions but that the low-field permeability was increased. In another ribbon study, Wagner et al. (286) used both transmission Mossbauer spectroscopy and CEMS to investigate the crystallization of amorphous transition-metal-metalloid alloys. They were able to show that the crystallization starts a t the ribbon surface and that crystallization temperature depends on the atmosphere used during annealing. Wagner et al. (287) performed Mossbauer measurements on amorphous ZrFe and NiZr(Fe) alloys. Similar spectra were measured, and the Fe environment was postulated to be the same in both cases. Boolchand and Grothaus (24) studied broken chemical order in melt-quenched GeSez and GeSz glasses and found that there was higher broken chemical order in g-GeSzthan in g-GeSez. l19Sn Mossbauer spectroscopy was used by Cremers et al. (48) to consider the role of tin in 18 chalcogenide glasses of different compositions in the system Se-Sn-As. V6rtes et al. (283) discovered that liquids and solutions containing Mossbauer-active isotopes, when trapped in the capillaries of porous silicate glasses, show the Mossbauer effect not only a t cryogenic temperatures but a t room temperature as well. In an effort to uncover information about the moon, Dyar and Birnie (69) investigated the microstructures in a synthetic analogue of an Apollo 15 green glass. Glasses are thought to occur in major proportions in the lunar soil because they are formed as products of both volcanic fire fountaining and meteor impacts.

MATERIALS STUDIES Because Mossbauer spectroscopy is a sensitive quantitative probe of broken chemical order, it is a most useful tool for the study of structural phase transitions. Yamada (298) gives the theoretical background of structural phase transitions. Jernberg et al. (118) combined Mossbauer spectroscopy, magnetic susceptibility measurements, and X-ray diffraction to study the lamellar semiconductor FePS This compound was found to become antiferromagnetic $elow 120 K in a magnetic transition of first order. A structural transition, characterized by a discontinuous change in lattice parameters, also occurs at the Nee1 temperature. In two papers, Stevens et al. (259, 260) report the results of a combined RamanMossbauer spectroscopic study of structural phase transitions on Gel,Snx(S or Se)z alloy glasses. The existence of a universal transition from a heterogeneous to a homogeneous network morphology was postulated from the spectroscopic evidence presented for the case of the easy-glass-forming chalcogenides. Stevens et al. were also able to demonstrate that internal surfaces play a central role in stabilizing the glass network against crystallization. Kostikas et al. (132) obtained information on the phase composition and the origin of magnetic hardening in FeNd-B alloys of various compositions with Mossbauer spectroscopy. Such studies have applications in the development of new permanent magnetic materials. Sukharevskii et al. (264) used Mossbauer spectroscopy to investigate pretransition phenomena (anomalous changes in the lattice parameters and volume and in the specific heat of a material) that occur just before the first-order structural phase transition in solid solutions based on CuSiF6-6H,0. The pretransition phenomena considered occurred in the neighborhood 0.9Tc < T < T, of the phase transition temperature T,. The investigators were able to show that the pretransition phenomena are due to the cooperative motion of the elements of structure in preparation for the phase transition. Structural defects in metals and alloys play an important role in determining their material properties. Wichert (293) gives a discussion of the current state and of possible future developments concerning defect studies by nuclear methods, including Mossbauer spectroscopy, that use the hyperfine interaction. The hyperfine interaction has a high degree of sensitivity to the presence of defects. Damage produced by the self-implantation of tellurium was investigated by Wu et al. (297). Gupta et al. (94) used Mossbauer spectroscopy to study the plastic deformation of magnetic transition-metalmetalloid metallic glasses. Results indicated that structural defects are generated throughout the volume of the specimen, and the investigators concluded that the elastic stress field 256R


associated with the structural defects results in deterioration of the soft magnetic properties and a change in the alignment of atomic spins in the sample. Silicon nitride has remarkable properties in nuclear and industrial applications, properties which are strongly influenced by the presence of small amounts of iron. Sat0 and Motoi (234) measured Mossbauer spectra of iron in p-silicon nitride under various conditions. Iron was introduced into the crystal as an impurity. The spectra changed in a complicated manner, and when the samples were exposed to high doses of y-rays or heated at high temperature (as might occur in nuclear or industrial applications) a sudden change in the spectra occurred. The samples also demonstrated significant aging a t room temperature. In this paper the authors described the experimental results and gave explanations for some of them. Several papers used '19Sn Mossbauer spectroscopy to investigate the effects of defects introduced by ion implantation. Weyer et al. (291) studied the formation and annealing of impurity-defect complexes in iron implanted with ll9In with an energy of 60 keV at temperatures ranging from 100 to 500 K. In a systematic study, Petersen et al. (204) utilized l19Sn emission Mossbauer spectrosco y and CEMS to stud the implantation behavior of lI9In, l1 Sn, ll%Sn, l19Sb,and 9mTe ions in Sic. The Mossbauer spectra could be decom osed into two general groups of lines, one originating from l1 Sn atoms on substitutional Si sites and the other from various Sn-vacancy complexes. Annealing experiments revealed a strong dependence of the structure of the defects and the formation and annealing kinetics on the chemical nature of the impurities. Semiconductors continue to be a subject of interest in Mossbauer spectroscopy and in many other branches of chemistry and physics. The effects of X-rays on multilayer single-crystal semiconductors were investigated by Bashkirov et al. (12). Mossbauer spectroscopy yields information on the effects of ionizing radiation on the electron structure of crystals and on the nature of the defects induced by the radiation. Many other studies dealt with the effects of doping on semiconductors. The optical and electrical properties of semiconductors are influenced by the presence of impurities. In two papers (279,280) Van Rossum et al. investigated group 111-V semiconductor compounds implanted with 125mTeor lZgmTeusing lZ5Teor lZ9IMossbauer spectroscopy in order to study the nuclear quadrupole interactions of these compounds. Neilsen et al. (181) were also interested in group 111-V semiconductor compounds. They used l19Sn Mossbauer spectroscopy to conduct a systematic experimental and theoretical study of the lattice vibrations of substitutional Sn impurities in these compounds. The Debye temperatures of impurities on the group I11 and V sites in GaP, GaAs, GaSb, InP, InAs, and InSb were determined. The Debye temperature gives a measure of the strength of the impurity's coupling to the host lattice. Lantratova et al. (139) also used 9Sn Mossbauer spectroscopy to study Sn impurity atoms, but in AsSe, a glassy semiconductor. Photostructural transformations, thought to underlie the phenomena of a reversible change in the optical and physicochemical properties of chalcogenide glassy semiconductors under the influence of light, were considered. The general advantages of Mossbauer spectroscopy in surface studies, particularly the study of chemical surface reactions and phase transitions, are summarized by Meisel (164). Scherson et al. (242)summarize the status of Mossbauer spectroscopy as it relates to problems in interfacial electrochemistry, especially in situ. Applications of in situ Mossbauer spectroscopy in emission and transmission modes and of in situ CEMS are given. Shinjo et al. (249) studied extremely thin oxide layers on Fe metal surfaces with thickness on the order of 10 A using surface selectivelyenriched samples. They were able to investigate the initial stage of the oxidation of the Fe surface. The oxidation of Fe surfaces coated by Sb layers was also studied. Magnetic and Mossbauer measurements were combined by Ayoub et al. (10) in the study of the superparamagnetic behavior of a magnetic fluid containing iron particles. Small particles will have a significant fraction of atoms residing on or near the surface. These surface atoms might be expected to experience a very different magnetic environment than those in the body of the particle and thus may exhibit different magnetic behavior. Measurements reported on small iron particles with substantial surface oxi-

B

r

P


dation indicated that the surface behavior was determined primarily by the oxidation. Two papers serve to represent the surface studies under way on iron and steels. More investigations are reported in the section on industrial applications. Dos Santos et al. (62) combined CEMS and X-ray photoelectron spectroscopy to investigate the precipitation of carbonitrides by successive implantation of carbon and nitrogen into pure iron. The order of implantation of the two ionic species was varied in two samples, and the as-implanted surface compositions of the samples were found to be quite different. Nomura and Ujihira (186) samples CEMS to analyze black layers produced on the surface of iron and steel by their treatment in hot alkaline solutions. The black layer is of interest because it aids in preventing corrosion and resists mechanical working more strongly than other coatings. Much information about magnetic systems can be gained from a study of their response to applied magnetic fields. Reiff (220) reviewed the use of zero and high field 57FeMossbauer spectroscopy in the study of magnetic ordering phenomena. Thomas et al. (268) measured the Mossbauer spectra of single-crystal samples to which magnetic fields were applied along specific axes. Responses of the system observed and discussed included spin reorientation transitions, enhancement of saturation magnetization in quasi-one-dimensional antiferromagnets, an increase or decrease of the ordering temperature in antiferromagnets, and field-induced ordering. Mahesh (152) described a procedure to determine the magnetic anisotropy constants in single crystals of iron and dilute alloys of iron using Mossbauer spectroscopy in an external applied magnetic field. The procedure was useful in investigating the effects of temperature, pressure, and impurities on anisotropy constants. In a more specific study, Townsend et al. (274) combined neutron diffraction with Mossbauer spectroscopy to study Minnesotaite, a 2:l layer sheet silicate, in an external magnetic field of 4.0 T. Minnesotaite is antiferromagnetic below 30 K. A spin flop or metamagnetic transition occurs in a small applied magnetic field. In a very interesting paper, Sakai and Sekizawa (224)report for the first time the measurement of Mossbauer spectra of 57Femetal with a new resonance technique, one that utilizes a magnetically modulated paramagnetic Pd(57Co) source without using Doppler modulation. They found that resonance can occur if the energy levels of the source nuclei undergo hyperfine splitting due to the presence of an external field and the resultant energy of the emitted y-ray is equal to one of the energy differences between the ground and excited states of the absorber. Advantages of this method due to the elimination of Doppler modulation are discussed. In a final note, Sakai and Sekizawa pointed out that the P d ( T o ) source is limited in the resonance region. An experiment is now in progress that involves another paramagnetic source which extends the resonance range above 5 mm/s. Several investigations considered the effect of high pressure on Mossbauer spectra parameters. Sauer et al. (235) studied the magnetic hyperfine fields of europium monochalcogenides EuX (X = S, Se, Te) and of the magnetic dilution system Eu,Srl-,S using 151EuMossbauer spectroscopy and simultaneously applying pressures up to 19.6 kbar and magnetic fields up to 13 T. The aim of this study was to clarify the relations between characteristic magnetic quantities. Bouzabata et al. (26) considered the Mossbauer effect in several ferromagnetic metallic glasses at high pressures. The Curie temperatures were observed to decrease with pressure. The pressure dependence of the hyperfine field distributions was also observed. Kapitanov and Yakovlev (124) obtained experimental results of the hydrostatic pressure influence on Mossbauer parameters for Sn using a high-pressure chamber made with polycrystalline cubic boron nitride anvils, which are transparent to radiation in the Mossbauer and X-ray ranges. Static pressures up to 200 kbar on 1 mm2 samples were achieved. Mossbauer spectroscopy can be utilized over a large range of temperatures. Bonville et al. (23) performed a very low temperature (between 0.09 and 4.2 K) study of the Mossbauer emission spectra of the 170Yb3+ion in Au(170Tm) sources. Willgeroth et al. (294) observed and explained the temperature dependence of the isomer shift in fcc iron alloys in the temperature range 13-1010 K. Several new sources for Mossbauer spectroscopy have been developed since the last of these reviews was published.

6 )

Weschenfelder et al. 290 reported on 174Yb.This new isotope of Yb is superior to l7 Yb m resolution and f factor and because of its high isotopic abundance (32%). Pasternak et al. (199) developed two new iodine sources, Mg,Te06 and Ca3Te06. These sources yield single narrow emission lines with a recoilless fraction at room temperature of 0.55,lO times larger than that of the conventional telluride sources. '%IMossbauer spectroscopy should now be possible at room temperature, Mossbauer spectroscopy at low and the performance of 1271 temperatures should be significantly improved. In addition, it was found that these compounds can be repeatedly irradiated with no perceivable radiation damage and with negligible residual activity from the other constituent elements. Pasternak et al. recommend the adoption of Mg3Te06as a standard iodine Mossbauer source. Herber (105) gave a brief review of the fundamental equations describing the temperature dependence of the recoil-free fraction. Hedges and Bowen (201) performed Mossbauer spectroscopic studies as a function of temperature for selected antimony compounds and found that the Debye model gives a reasonable approximation for the temperature dependence of the recoil-free fraction in these compounds. In other studies, De Grave et al. (55) considered the effect of small A1 substitutions in hematite as determined from the 57Feisomer shifts. Roy and Bhattacharya (223) presented a simple theoretical analysis of the dynamics of an impurity atom in a general harmonic solid using a double time Greens function method. They found that the presence of impurities causes a broadening of the Mossbauer line, which increases with the increase in the mass of the impurity. Gupta (93) proposed a model for the dynamics of a Mossbauer atom in some biological systems. In these systems the observed spectrum contains a wide component spread over an energy range orders of magnitude larger than the natural line width. According to the model, the wide component is due to the damped harmonic motion of the Mossbauer atom and is called the quasi-elastic component. Predictions of the model were found to agree qualitatively with the observed temperature dependence of the width and intensity of this quasi-elastic component. Herber and Eckert (106) provided a detailed overview of the application of Mossbauer spectroscopy to the study of intercalation compounds. They present a comparative and critical survey of the available data. The limitations of different techniques (including Mossbauer spectroscopy) applied to the study of graphite intercalates was discussed by Schlogl et al. (243). In an example of the application of Mossbauer spectroscopy to the study of intercalation compounds, Courant et al. (47) were able to determine the structure of phosphate-based glasses containing transition-metal oxides and the structure of the glasses obtained after the intercalation of Li and Na.

MINERALS, SOILS, AND SEDIMENTS Application of Mossbauer spectroscopy to environmental samples has become a very active area. Many papers dealing with well-characterized minerals have appeared in this latest review period, and an increase in the studies of complex soils and sediments is apparent. All these papers use the 57Fe Mossbauer resonance. This section discusses first the minerals and then the soils and sediments. As in other sections of this review, the papers discussed are a limited sample of the literature, although we have attempted to include many papers in which Mossbauer spectroscopy contributed a large part to the significance of the study. The layer-structured aluminosilicates continue to be of considerable interest to Mossbauer spectroscopists. One of the most studied, because of its high iron content, is biotite. Townsend and Lon worth (273) show that the coupling of Fe3+-Fe3+and of FeE+-Fe2+in biotite is ferromagnetic, using results from application of external magnetic fields to a single crystal at 4 K. Another important related mineral is nontronite. Some recent results for that mineral are discussed in the Theory section. In addition, the work of Johnston and Cardile (119) should be mentioned. They compare spectra of Ca- and K-saturated nontronite with the untreated sample to show the effect of the interlayer cation interactions on the Fe3+ doublets. Van Wonterghem et al. (281) consider the nature of the singlet peak produced in nontronite and other iron silicates by H2 reduction at high temperature. Their ANALYTICAL CHEMISTRY, VOL. 58, NO. 5, APRIL 1986

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experiments, using externally applied fields, show that this singlet is due to y-Fe, produced in about a 1:4 ratio to the more stable a-Fe. A large number of natural glauconite samples have been studied by DeGrave et al. (54). These samples required two Fez+and three Fe3+ doublets for fitting. The third Fe3+ doublet is assigned to interlayer iron species. Mixed valence iron silicates, which show thermally activated electron transfer, are the subject of several investigations. Coey et al. (43) report detailed studies on the magnetic and electrical properties of ilvaite. Kan and Coey (121) present Mossbauer and other studies on the mixed valence olivine laihunite, a mineral only recently discovered, from Liaoning province in China. Ma netic ordering appears at different temperatures for the Fe2 5 and Fe3+in this mineral. Amthauer and Rossman (7) give results of Mossbauer and optical spectroscopy on a large variety of mixed valence minerals. They relate the structure and the distribution of iron to the optical properties and to the ability of these minerals to undergo thermally activated electron transfer. Annersten et al. (9) report a Mossbauer study of the Fe2+-Mn2+distribution between M1 and M2 sites in synthetic and natural iron-manganese olivines. The iron favors the M1 site, but partial disorder occurs during quenching at higher temperatures. Ericsson and Nord (77) have studied the olivine-related (Nil-,Fe,)3(P04)2 with the sarcopside structure and find a strong site preference of Ni2+ for the M1 site. Although magnetic ordering of Fe2+occurs at low tem erature, only M2 sites participate. Mattson and Rossman (16Oypresent Mossbauer data for a ferric tourmaline showing line narrowing a t low temperature. The magnetic ordering in the garnet andradite, Ca3Fe2(Si04)3,has been studied by Murad (175). Below the ordering temperature of 11.5 K two sextets are observed with similar hyperfine fields but widely different quadrupole interaction, corresponding to two angles between the magnetization and the field gradient axes. Murad and Taylor (177) report a study of the relatively unstable iron-aluminum hydroxycarbonate green rusts, using Mossbauer spectroscopy to determine the changes in the site populations with oxidation. DeGrave and Vochten (56) present a detailed Mossbauer study of a natural ankerite, CaM2+(C0J2,with M2+being primarily Mg and Fe. This mineral exhibits strong relaxation effects at low temperature and an unusual, almost linear temperature dependence of the quadrupole splitting. Vochten et al. (285) report Mossbauer and other studies on the iron uranyl phosphate bassetite, both before and after oxidation. The titanium minerals perovskite and titanite have been studied with Mossbauer spectroscopy in work reported by Muir et al. (172). Site occupancies for iron in both natural and synthetic samples are discussed. The vacancy distribution in natural pyrrhotites Fel,S has been studied by Gupta et al. (95) using Mossbauer spectroscopy and X-ray diffraction. They observe the presence of both the hexagonal and monoclinic forms in varying relative amounts. Prasad et al. (209) used Mossbauer spectroscopy to study the kinetics of oxidation for pyrite roasted at about 600 OC. A number of papers deal with various iron oxides. McCammon and Price (163) present Mossbauer data for wustite, Fe,O, with x > 0.95. The low density of vacancies simplifies the spectra obtained, compared with samples having greater nonstoichiometry, and the room temperature spectra are fitted to two Fe2+doublets, one Fe2+and one Fe3+singlet. A new model is presented for the defect structure. Murad and Schwertmann (176) note the effect of crystallinity on the Mossbauer spectrum of lepidocrocite, y-FeOOH. Two samples of different crystallinity had marked difference in quadrupole splitting at room temperature, different ordering temperatures, and different magnetic field distributions at 4 K. Schwertmann et al. (244) also studied the effects of crystallinity on the Mossbauer parameters of goethite, a-Fe00H. The magnetic hyperfine field variations are apparently surfacecontrolled. Morris et al. (169) report Mossbauer and other data for submicrometer particles of a- and y-FezO3, FeOOH, and of magnetite Fe304. The main concern of this paper is optical properties of small particles. It should be noted that the particle size of the goethites, at least, must be large, compared to the goethites studied by Schwertmann. Although structural iron in layer silicates has been well studied, Helson and Goodman (103) report one of the few studies on interlayer iron in clay minerals, namely, mont258R


morillonite and hectorite. Ferrous iron gives a doublet at 77 K, which disappears at room temperature due to hydration effects. A ferric doublet with splitting of about 0.6 mm s-l is apparent at both temperatures, while an additional ferric doublet with much larger splitting is seen only at 77 K for montmorillonite. Mortland et al. (170) used Mossbauer spectroscopy together with other techniques to study the interaction of flavomononucleotide with smectite. A significant lowering of the Fe3+ quadrupole splitting by the nucleotide is reported. Mention should be made of a seven-laboratory test study of reproducibility and precision in Mossbauer spectral determination of several sample minerals. Dyar (67) reports this broad-ranged study and discusses the statistical results, which seem quite encouraging to those interested in the quantitative applications of Mossbauer spectroscopy to mineralogy. Bowen and Weed (27) review the Mossbauer literature on soils and sediments through 1983. A variety of papers have appeared since that review period. Ibanga et al. (111) used Mossbauer spectroscopy to estimate aluminum content of goethite and hematite components in petroferric (iron oxide rich) soil minerals. Jeanroy et al. (116) compare Mossbauer determination of the various iron minerals in example soils with chemical extraction data. Agreement is generally good except when the soil contains large amounts of organic matter. McBride et al. (162)characterize with Mossbauer spectroscopy the iron extracted from organic-rich podzols by EDTA. At pH 9 the extract is a polymeric hydrous iron oxide associated with organic matter, with Mossbauer spectra and X-ray diffraction pattern related to ferrihydrite. In contrast, ironEDTA complexes predominate in the extracted material at pH 7. Childs et al. (38) report 4 K Mossbauer data for a number of ferrihydrites, including several substituted with Si or AI. Substitution reduces the magnetic hyperfine field by 1-2 T. Ericsson et al. (76) used Mossbauer spectroscopy to study the effect of DCB (dithionite-citrate-bicarbonate) extraction on a soil, vermiculite, and smectite. They observe that the extraction of hematite from the soil was incomplete and the Fe2+/Fe3+ratio was increased by extraction. Gessa et al. (83) report Mossbauer and chemical extraction studies of iron distributed in Cambisol clays. They note that oxalate treatment may extract Al-substituted goethite. The hematite content of the soils studied was highest in those with low hyperfine fields for both hematite and goethite. Sersale et al. (245) studied the Fe2+/Fe3+distribution in four representative Italian tuffs, using Mossbauer spectroscopy to determine the relative amount of the two ferric and single ferrous doublets observed at room temperature and 80 K. The geologically important banded iron formations are the subject of a Mossbauer study by Vieira et al. (284). An inverse relation between iron oxidation and age is reported, the younger rocks being most oxidized. Cole (45) reports a study of the nature and origin of smectite in the sediment of the Bauer Deep, southeastern Pacific, using Mossbauer spectroscopy and many other techniques. Examination of Mossbauer spectra of magnetite from volcanic ash leads Daniels et al. (53) to propose that the variation in Mossbauer parameters may be a useful diagnostic relating to the source of the ash and to crystallization temperature. An extensive Mossbauer study of sediments from Danish oil wells has been carried out by Merrup et al. (171). Variation of iron components with depth is presented and an interesting correlation between the occurrence of ankerite, readily distinguished by its Mossbauer parameters, and oil or gas formation in the sediment layer is proposed. Finally in this section, we mention two papers dealing with meteorites. Zhang et al. (301) report Mossbauer investigation of the Lunan meteorites, which fell in 1980 in Yunnan province, China. Two Fez+doublets and two magnetic components, attributed to Fe-Ni and Fe-S species, are observed. Dyar et al. (70) used 57Fe-doped analogues to study the Mossbauer effect in hibonite, CaAlI2Ol9,and relate their results to the possible origin of the blue color in meteoritic hibonites.

INDUSTRIAL APPLICATIONS Interest in industrial applications of the Mossbauer effect is indicated both by the considerable number of papers published on such applications and by the Symposium on the Industrial Applications of the Mossbauer Effect held at the 1984 Chemical Congress of Pacific Basin Societies. For the


purpose of this review, the industrial applications of the Mossbauer effect are divided up into six general areas: coal, steels, catalysts, analysis, corrosion, and cement. Huffman and Huggins (109) performed an analysis of lower-rank coals by Mossbauer spectroscopy, computer-controlled electron microscopy, and EXAFS spectroscopy. They observed distinct difference between the inorganic phase distributions in these coals and those in bituminous coals. The iron-bearing minerals present in some British Columbian coals were characterized by 57FeMossbauer spectroscopy in a study performed by Taneja and Jones (266). For two of these coal samples, the chemical changes that occur on ashing coal in air were also investigated. Many other papers discussed the analysis of coal ash with Mossbauer spectroscopy. The combustion of coal in power plants leads to the production of bottom and fly ash. This ash has an impact on the environment either by the release of fumes during combustion or by its deposit in the environment through disposal. Patil et al. (200) studied fly ash from coal-fired power plants and bottom ash from lignitenatural gas combustion. In the fly ashes, the magnetic fraction was found to range from 1.1%to 7.3%. No magnetic fraction was found in the bottom ash, but it was found to contain trivalent iron ions, suggesting its formation under higher temperature conditions than bottom ashes previously analyzed. Aikin et al. (2) used Mossbauer spectroscopy to study the iron phases present in an electrostatic precipitator ash, and uncooled ash deposit, and a cooled superheater ash deposit from a power station burning Morwell brown coal. The fireside ash deposits formed during the combustion of Monvell seam brown coal seriously affect boiler heat transfer in the power plants. Separate determination of the mineral formation processes that occur durin combustion and deposit consolidation is made possible y the characterization of the ash-forming constituents present in the coal and the mineral phases present in the fly ash and the fireside deposits. CEMS has been used to investigate the composition and thermal evolution of nitrogen-implanted steels. Dos Santos et al. (63) performed a systematic study in terms of the carbon concentration in the steel matrix and the nitrogen implantation dose. Kaplow et al. (125) review with analysis of the Mossbauer spectra of virgin martensite and follow the tempering process. Fe-N martensite systems were also investigated in this study. The sensitivity of Mossbauer spectroscopy to variations in the atomic environment makes it an especially useful technique for such studies. A strong dependence of the magnetic induction of rotor steels on the cooling rate from the austenitic state was confirmed by Sigut et al. (250). This sensitivity is not due to any changes of the microscopic magnetization of the ferromagnetic matrix but seems to be due to a consequence of the alteration of the content of a-iron. Nomura and Ujihira (185)analyzed the thermal decomposition products of phosphate and oxalate coatings on steels using CEMS. Mossbauer spectroscopy is particularly useful in the study of initial decomposition products which are frequently amorphous and, therefore, cannot be studied with X-ray diffraction. The influence of phosphoric acid on steel and its corrosion products studied by Meisel et al. (165) has applications in industrial steel coatings (phosphoric acid is used as a pretreatment to the surface), in rust transformers (which are primarily composed of phosphoric acid), and in the removal of radioactive corrosion products from surfaces in nuclear power plants. Abrasives move along the surface of a material and produce wear debris. Liu et al. (145) studied the characteristics of selected wear debris produced from various steels. Many papers dealt with Mossbauer spectroscopic studies of catalysts. A few representative examples are mentioned below. Maksimov et al. (153) devised a gradient-free Mossbauer catalytic reactor, which permits the simultaneous study of the conversions of a solid and the kinetics of the catalytic reaction. Christensen et al. (39) combined Mossbauer emission and absorption spectroscopy and were able to characterize both the Co and Fe atoms in silica-supported Fe-Co catalysts with respect to their magnetic and chemical properties. Resulta imply that alloy formation has taken place but that the alloy composition is not homogeneous; the surface seems to be enriched with iron. Pannaparayil et al. (195) used results from Mossbauer spectroscopy, magnetic measurements, and IR spectroscopy to characterize zeolite-based catalysts;

fl

these have applications in the conversion of coal-derived syngas (CO HJ to hydrocarbon fuel. The bulk phases of Fe-Mn catalysts after calcination, reduction, and synthesis were investigated by Jaggi et al. (115). Niemantsverdriet et al. (182) looked at small particle effects in the Mossbauer spectra of a carbon-supported iron catalyst (5 wt % Fe) at 295,77, and 4 K. The Mossbauer spectra at 295 and 77 K show substantial contributions of superparamagnetic a-Fe. The magnetic hyperfine fields of a-Fe in the spectra at 4 K are enhanced due to the influence of the demagnetizing field in the small particles. The effective Debye temperature of the catalysts was found to be much smaller than in the iron bulk compounds. This study illustrated the usefulness of in situ Mossbauer spectroscopy at low temperatures in the investigation of systems made up of small particles. Stewart et al. (261) performed CEMS studies of carbon deposition on iron foils by the catalytic decomposition of an acetone-carbon dioxide mixture and proposed a mechanism for carbon deposition in such a system. In the area of analysis, Bateev et al. (13) proposed a method for calculatin the concentration of the Mossbauer element and the probahty of resonance absorption from the electronic nuclear-y-resonance spectrum of a multicomponent sample, taking into account the photoeffect of atoms of all the elements that appear in its composition. Comparison of the calculations with experimental data yields a relative error of 10-20%. Mossbauer spectroscopy is a very valuable and powerful tool in corrosion research, as evidenced by the number of papers in this area. Most papers involved Mossbauer spectroscopy studies on the formation of rust. Peev and V6rtes (203) investigated the qualitative composition of corrosion products in a sample from a water-softening exchange-plant and in laboratory-prepared samples and were able to deduce a scheme for the formation of corrosion products. Leidheiser and Czako-Nagy (141) were interested in the early stages of the rusting of iron and compared rust formation during simulated atmospheric corrosion with rust formation during exposure to actual atmospheric conditions. Another study of corrosion under laboratory conditions was conducted by Hanada et al. (98). They used CEMS to study the formation of corrosion products deposited on steel under HC1 atmosphere and also to analyze the corrosion products. Spectra were measured at both room temperature and 80 K. CEMS is a particularly useful tool in the analysis of corrosion products due to its depth sensitivity. In another CEMS study, Dorik et al. (60) investigated the effect of nitrogen implantation in the dose range (1-3) X 10l6ions/cm2 and subsequent annealing on the oxidation kinetics of iron foils. The as-implanted samples were found to exhibit enhanced oxidation compared to virgin iron foils. Ion implantation is known to produce point defects and compressive stresses in the implanted layers. Dorik et al. were able to conclude that the point defects, which enhance the oxidation rate, were eliminated by annealing. Annealing also stabilizes the compressive stresses, which inhibit the oxidation rate in iron foils, and blocks short circuit diffusion paths due to the migration of nitrogen atoms toward dislocations. Mossbauer spectroscopy is a relatively recent addition to the techniques and methods used to characterize cements. Eissa et al. (75) studied the states of iron in cement clinker raw mixture (87% limestone, 12% clay, 1%pyrite ash) at various temperatures up to the clinker forming temperature of 1500 "C. They established a method for assessing the quality of manufactured clinker using Mossbauer spectroscopy. In a later study, Eissa and Hassaan (74) investigated the differences between the states of iron in ordinary portland cement clinker and clinker doped with sodium carbonate.

+

MISCELLANEOUS APPLICATIONS In addition to the applications of Mossbauer spectroscopy reviewed above, there are numerous additional categories of studies that are currently under way. Kishimoto et al. (128) studied the orientation of particles in magnetic recording tapes. Raj and Kumar (214) used 57FeMossbauer spectroscopy in an environmental study to characterize the chemical composition of iron aerosols collected from remote, urban, and industrially active areas. The Mossbauer spectra of the various samples gave evidence of the environmental differences in the sources of the samples. Medical applications of Mossbauer spectroscopy include ANALYTICAL CHEMISTRY, VOL. 58, NO. 5, APRIL 1986

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a study by Berg et al. (15) of polysaccharide iron complex (PIC), an oral hematinic that has been effective in the treatment of iron deficiency anemia but whose chemical nature and physical structure had not been investigated. Mossbauer spectroscopy has also been applied to the study of the structure of proteins. Nadler and Schulten ( 179) suggested an algorithm for the Mossbauer spectra of an atom undergoing one-dimensional Brownian motion in an arbitrary static potential. Nowik et al. (188) calculated Mossbauer spectral shapes of proteinic matter based on the assumption that the degrees of freedom of the protein macromolecules can be described in terms of damped harmonic oscillators acted on by random forces. They were able to use known classical correlation functions for harmonically bound particles in Brownian motion. The calculated spectra agreed quite well with experimental data. The use of Mossbauer spectroscopy in the characterization of ancient pottery continues to expand. Maniatis et al. (157) studied Punic amphoras found in Corinth, Greece, with Mossbauer spectroscopy and with a variety of other techniques and found that the complete set of data obtained gave a full description of the amphoras. Gancedo et al. (80) also made use of Mossbauer spectroscopy as one of a number of techniques in the analysis of Iberian pottery found in a site in Seville. Background studies of clays necessary to obtain reference data for studies of ancient potteries are also progressing. Salazar et al. (225) considered results from firing studies of six Peruvian clays to obtain information on valence states and site symmetry of the iron atoms in the clay mineral lattice, on the oxide phase present in the clay, and on the atmosphere present during firing. Libyan Desert silica glass was the subject of a combined Mossbauer effect and X-ray fluorescence study by Sallam et al. (226). They concluded that Libyan silica glass is a type of tektite formed by the impact of a huge meteorite on the earth’s surface about 38 million years ago. There are numerous papers reporting on the use of Mossbauer spectroscopy in the field of ion implantation. DBzsi et al. (59)discuss the implantation of various dose values of lZ5Iinto silicon and germanium lattices. Spectra measured after thermal annealing suggest that the site populations of Te and I atoms after implantation and annealing are the same and indicate an off-substitutional site populatioa in the silicon lattice. Ion implantation into thick targets was the subject of a study by Kotlicki et al. (133). They found that the formation of a thick layer of carbon (due to the presence of diffusion pump oil and avoidable by heating the sample during implantation) results in the trapping of implanted ions in concentrations higher than the saturation density for the isotope in the metallic foil. Many of the studies on ion implantation involved the use of conversion electron Mossbauer spectroscopy (CEMS). CEMS is an excellent tool for ion implantation studies because it is surface-sensitive. Sawicka and Sawicki (238)and Sawicka et al. (237)investigated iron ion implantation in metals and metalloids and amorphous transition-metal-rare-earth alloys, respectively. In both cases they found that the Mossbauer spectra are sensitive to the volume of sites occupied by iron atoms through the isomer shift values. CEMS was also used to characterize the surface structure of nitrogen-implanted steels, which in general tend to have enhanced wear resistance and fatigue lifetime. Principi et al. (210)give a partial report of a systematic study of such steels, the aim of which was to correlate the chemical state of the modified surface with the composition and initial structure of the steel and with the implantation parameters. CEMS has become a valuable technique in a variety of study areas. Ogale et al. (192) studied ion-beam mixing at Fe-Si interfaces and found that the thin FeSi layer formed at the interface grows upon ion bombardment. In addition to the work in ion implantation, Sawicka et al. (236)used both transmission Mossbauer spectroscopy and CEMS to study metal deposits on the carbon limiters of the ASDEX Tokamak. CEMS studies of the 77.3-keV transition in Ig7Auwere found by Sawicki and Sawicka (240) to be 3 orders of magnitude more sensitive than those using the usual transmission technique. They were able to measure spectra of metallic gold films as thin as 10-100 pg/cm2. Yang et al. (299) developed a new high-transmission, high-resolution electrostatic electron spectrometer and used it to measure the 2s and 3s contribu26OR


tions to the core polarization at the nuclear site of 57Fein Fez03 films. The theory of CEMS including second-order effects and detection coincidence correction was discussed by Salvat and Parellada (228),and weight functions for integral CEMS, computed from a Monte Carlo simulation of electron transport, were tested for scaling properties by Salvat et al. (227). Depth profiling of solid surfaces using CEMS has become increasingly popular, particularly with 6.6-7.3 keV 57Fe electrons scattered from solid surfaces. Matsuo et al. (159) measured spectra of corrosion products in the surface layer of iron foils and other substances and were able to achieve much better depth-selectivity than with a proportional counter. Liljequist and Ismail (143)obtained by Monte Carlo simulation of electron scattering and energy loss the transmission probability as a function of electron starting depth at different electron-emission angles relative to the absorber surface normal and at different electron energies greater than 6.3 keV. Lilje uist et al. (142) measured and analyzed depth-selective %Fe CEMS spectra using a high-resolution electron-energy analyzer combined with a Mossbauer spectrometer and achieved good agreement with the above theory. Many papers reported on studies on the effects of irradiation on various materials. Ibragimov et al. (112)combined Mossbauer spectroscopy with X-ray diffraction to study the effects of neutron irradiation on Fe3A1alloy. In an interesting study, Brooks et al. (30)investigated the effects of y irradiation on organotin stabilizers within a PVC matrix. Such a matrix might be found in the packaging of food preserved by exposure to sterilizing y radiation. For the first time, the diffraction of synchrotron radiation by Mossbauer nuclei was measured. Chechin et al. (36)measured the temporal distribution of the Mossbauer y-rays diffracted by a single crystal and found it radically different from that of the decay of isolated nuclei. The decay of excited nuclei in the crystal was found to be directional and rapid. The use of polarized y-rays in 67FeMossbauer spectroscopy is the subject of several studies. Sharma et al. (247) report on a theoretical analysis of Mossbauer polarization using the Stokes parameterization technique. Explicit results for the M1 and the E2 transition are reported. Ullrich and Hesse (276) tested models used to understand the anomalies in physical properties of ferromagnetic fcc FeNi alloys in the concentration range 30-36 at. %. With 57Fe Mossbauer spectroscopy and polarized y radiation, they were also to obtain information on the magnitude and direction of the hyperfine field vectors at the iron nuclei. Results gave evidence for the existence of antiparallel magnetic moments of Fe atoms, and results of isomer shift measurements excluded the existence of different electronic states of the Fe atoms. Garg et al. (81)measured 57Fespectra in pyrite crystals as a function of the orientation of the crystal axis with respect to linearly polarized y-rays. Analysis showed the the Mossbauer fraction was isotropic and that the asymmetry parameter was zero. Mossbauer spectroscopy can be used to study the kinetics of chemical conversions, if the time of such conversions is comparable to the lifetime of the Mossbauer level. Gol’danskii (86)was able to show that the transformation of Turnbull blue into Prussian blue at 12-400 K involves electron-nuclear tunneling. Sanchez et al. (229) found that it is possible to modify the internal oxidation kinetics of AgSn alloys by controlling the size of tin oxide particles in the alloy. Mossbauer spectroscopy was used to study the oxide particle size. The results of various types of laser annealing of materials have been the subject of Mossbauer spectroscopic studies. X-ray diffraction and optical and scanning electron microscopy were combined with Mossbauer spectroscopy by Ramous et al. (216) to study the surface melting and annealing of a high-carbon alloy steel. Surface hardening was found to result from the treatment of such steel. Two papers (138, 292) considered the effects of laser annealing on amorphous metallic ribbons. Amorphous metals are interesting because of their structural and magnetic properties and because they have many technological applications. Lanotte et al. (138) found that continuous laser irradiation can be used to improve magnetic properties under appropriate conditions. The effects of contintlous laser annealing were reported to be due mainly both to the quenching stress relaxation (which produces a rearrangement of short range order) and to the formation of


crystalline phases, Whittle et al. (292)used X-ray diffraction and Mossbauer spectroscopy to show that under certain conditions picosecond laser annealing of metallic glass ribbons is also an important technique for the improvement of their structural and magnetic properties. The Miissbauer effect data reported show that laser annealing of ribbon surfaces also affects the bulk properties of the ribbons due to induced stresses from the surface layers. A comparison study of the accuracies of modulation techniques used in Mossbauer spectroscopy to measure small energy shifts between source and absorber line positions was performed by Helisto et al. (102). Modulation techniques studied included sinusoidal and constant-velocity Doppler modulation at low drive frequencies, sinusoidal and square wave-shaped phase modulation at high drive frequencies, and various modulations in the transient frequency range. In many cases, it was shown that the transient and high-frequency modulation techniques are more accurate than conventional methods in the determination of small energy shifts. The accuracies of the methods were found to depend strongly on the absorber thickness and additional line broadenings. Mkrtchyan et al. (167) and Smirnov et al. (251) considered time-dependent Mossbauer spectroscopy. The former paper studied the time dependence of the y resonance absorption line intensity in the case of modulation by acoustic waves. The latter paper proposed and put into practice a new method of time-dependent Mossbauer measurements in which the time at which the beam of radiation strikes the system under study is determined by the use of a magnetic resonance shutter. The fast response property of FeB03 crystals to remagnetization permits a fast turn on or turn off of the beam of Mossbauer y-rays, useful in the study of the time dependence of the resonance diffraction of y-rays.

ACKNOWLEDGMENT We wish to acknowledge Pam Newman and Joyce Witherspoon, who assisted in the preparation of this review. We also wish to note our appreciation to Richard White, Janet Gibson, and Niu Junning, who aided in the retrieval and organization of the literature. Finally, we wish to acknowledge the support of the National Science Foundation to L. H. Bowen (Grant No. EAR-85-0656). LITERATURE CITED Abd-Elmeguid, M. M.; Sauer, C.; Zinn, W. J . Phys. C 1985, 78, 345-35 1. (2) Aikin, T. L. H.;Cashlon, J. D.; Ottrey, A. L. Fuel 1984, 63, 1269-1275. (3) AI-Sa'ady, A. K. H.; Moss, K.; McAullffe, C. A,; Parish, R. V. J . Chem. Soc., Dalton Trans. 1984, 1609-1616. (4) Albanese, G.; Bridelli, M. G.; Deriu, A. Biopolymers 1984, 23, 148 1- 1498. ( 5 ) Alonzo, G.; Bertazzi, N.; Huber, F. Can. J . Spectrosc. 1985, 30, 21-24. (6) Ambe, F.; Okada, T.; Ambe, S.; Sekizawa, H. J . Phys. Chem. 1984, 88, 3015-3020. (7) Amthauer, G.; Rossman, G. R. Phys. Chem. Mlner. 1985, 7 7 , 37-51. (6) Andreeva, M. A.; Borisova, S.E.; Kuz'min, R. N. Sov. Phys.-Tech. Phys. (Engl. Transl.) 1983, 28, 675-679. (9) Annersten. H.; Adetunji, J.; Filippidis, A. Am. Mineral. 1984, 69, 1110-1115 . . .- . . .-. (10) Ayoub, N.; Kobeissi, M. A.; Chantrell, R. W.; O'Grady, K.; Popplewell, J. J . Phys. F1985, 75,2229-2235. (11) Barbierl, R.; Silvestri, A. J . Chem. Soc., Dalton Trans. 1984, 1019-1 025. (12) Bashkirov, Sh. Sh.; Llberman, A. B.; Tsarevskll, S.S.Sov. Phys.-Tech. Phys. (Engl. Transl.) 1984, 5 4 , 118-119. (13) Bateeu, A. B.; Vakar, 0. M.; Gruzin, P. L.; Petrikin, Yu. V. Sov. Phys. J . (Engl. Transl.) 1983, 26, 1001-1007. (14) Belozerskll, G. N. Sov. Phys.-Solid State (Engl. Transl.) 1983, 25, 1451-1452. (15) Berg, K. A.; Bowen, L. H.; Hedges, S.W.; Bereman, R. D., Vance, C. T. J . Inorg. Blochem. 1984, 22, 125-135. (16) Berry, F. J. Phys. Bull. 1983. 3 4 , 517-519. (17) Bhatnagar, A. K.; Jagannathan, R. "Metallic Glasses: Productlon. Properties and Applications"; Anantharaman, T. R., Ed.; Trans. Tech. Publications Ltd Switzerland, 1984; pp 89-114. (18) Blckar, D.; Lukas, 6.; Neshvad, G.; Roberts, R. M. G.; Silver, J. J . Organomet. Chem. 1084, 263,225-234. (19) Bill, E.; Blaes, N.; Fischer, K. F.; Gonser, U.; Pauly, K. H.; Preston, R.; Sed, F.; Staab, R.; Trautweln. A. X. Z . Naturforsch. B : Anorg. Chem., Org. Chem., Biochem., Blophys., Blol. 1984, 39b, 333-344. (20) Blllard, L.; Lancon. F.; Rodmacq, B.; Chamberod, A. J . Phys. F 1984, 74, 555-564. (21) Birchall, T.; Smegal, J. A.; Hill, C. L. Inorg. Chem. 1984, 23, 1910-1913. (22) Blaes, N.; Fischer, H.: Gonser, U. Nucl. Instrum. Methods Phys. Res. 1985, B9, 201-208. (1)

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Kinetic Determinations and Some Kinetic Aspects of Analytical Chemistry Horacio A. Mottola Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078

Harry B. Mark, Jr.* Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221

The organizational structure of previous reviews (1) has been, basically, retained in the preparation of this report. The papers reviewed have been selected from those that appeared since November 1983 and were received for the authors' consideration through approximately November 15, 1985. Highlights of the First International Symposium on Kinetics in Analytical Chemistry have been covered in a previous review (1) and it is rewarding to report here the anticipation of a second international gathering in September 1986. This Second Symposium on Kinetics in Analytical Chemistry is under organization with the support of the University of Ionnina (Ionnina, Greece), the Greek Chemists Association, and the Ministry of Culture and Science of Greece. It is scheduled to take place from September 9 through September 12, 1986, in Preveza, Greece. The symposium will consist of five plenary lectures, contributed papers, and poster sections on a variety of kinetic topics of interest to analytical chemists. Although catalytic methods continue their dominance in the number of methods published, a noticeable decrease in the use of inhibition and activation of catalytic systems has been observed in the past 2 years. Worth noting also is the increased use of electrochemical techniques in general and in flow systems in particular. Differential procedures have shown an increase in the number of papers dedicated to them in the past 2 years.

BOOKS AND REVIEWS Volume 16 of a series of monographs and textbooks on Clinical and Biochemical Analysis, entitled "Chemi- and 264 R

0003-2700/86/0358-264R$06.50/0

Bioluminescence", contains material of interest to kinetic methods, particularly its Chapters 11 (Analytical Applications of Gas Phase Chemiluminescence, by D. H. Stedman and M. E. Fraser) and 12 (Chemiluminescence Analysis in Solution, by M. L. Grayeski) (2). Kricka and Thorpe (3) reviewed chemiluminescent and bioluminescent methods in chemical analysis. Their review, after discussion of the most useful types of chemiluminescent and bioluminescent reactions, considers measurement of peak light intensity (differential kinetic measurements) or area under the light-emission-time curve (integral kinetic methods) and closes with a review of applications mainly in the bioanalytical area. The mechanism of electrochemiluminescent reactions in solution as well as their use for the determination of low concentrations of organic and inorganic species has been reviewed by Herejk and Holzbecher ( 4 ) . In an extra issue of the second volume of the publication Quimica Analitica (Spanish Society of Analytical Chemistry) have appeared the five plenary lectures presented at the First Symposium on Kinetics in Analytical Chemistry. They were: an overview presentation of the impact that kinetics has had in the development of analytical chemistry (5), a holistic view of chemical analysis with emphasis on kinetic methods (6), a discussion of kinetics and mechanism of metal chelation processes in solvent extraction (7), temperature, solvent, and salt effects in analytical applications of kinetics (8), and selectivity in catalytic methods (9). Historical accounts of the evolution of a given branch of chemistry are welcome contributions; they account for the present status of such a branch by recalling its roots and 0 1986 American Chemical Society

Moessbauer spectroscopy - American Chemical Society

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