Anal. Chem. 1984, 56, 199R-212R (11DD) Kettrup, A,; Stenner, H.; Weber, H. Ber.-Dtsch. Ges. Mlneraloelwlss Kohlechem. 1982, 268. (12DD) Kolb, B. LaborPraxls 1982,6, 156. (l3DD) Kolb, B. Chromatographla 1982, 15, 587. (14DD) Larsson, L. Anal. Chem. Symp. Ser. 1983, 13, 193. (15DD) Leggett, D. C. Report 1981, CRREL-SR-81-26; Order No. ADA108345. NTIS from Gov. Rep. Announce. Index 1982, 8 2 , 1540. (16DD) Llebl. M.; Seeleltner, G. Mlit. Versuchssfn Qaerungsgewerbe Wien 1982, 36. 81. (17DD) Ott, U.; Llardon, R. Flavour 81 Weurman Symp. 3rd 1981, 323. (18DD) Pausch, J. B.; McKalen, C. A. Rubber Chem. Technol. 1983, 5 6 ,
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(19DD)- Safersteln, R.; Park, S. A. J. Forensic Scl. 1982,2 7 , 484. (20DD) Sakane, Y.; Nltanda, T.; Shlmoda, M.; Osajlma, Y. Nlppon Shokuhln Kogyo Gakkalshi 1983,3 0 , 108. (2100) Shaw, D. A.; Anderson, T. F. Ind. Eng. Chem. Fundam. 1983,22, 79. (22DD) Termonla, M.; De Meyer, A.; Wybauw, M.; Jacobs, H. J. High Reso/ut.Chromafogr. Chromafogr. Commun. 1982, 5, 377. (23DD) Varner, S. L.; Breder, C. V.; Fazio, T. J. Assoc. Off. Anal. Chem. 1983, 6 6 , 1067. (23DD) Vlerke, W.; Gellert, J.; Teschke, R. Arch. Toxicol. 1982, 5 1 , 91. Sampllng (IEE) DI Pasquale, 0.; Vallatl, A.; Capaccloli, T.; Galli, M. J. Chromafogr. 1982,243, 357. (2EE) Duenges, W.; Bode, J.; Hoffman, H.; Mueller, M.; Soitau, B. Proc. Int. Symp. Caplllary Chromafogr., 4th 1981, 765. (3EE) Gauthler, M.; Pllon, R.; Kutschke, K. 0. J. Chromatogr. Scl. 1982,2 0 , 283. (4EE) Greter, J.; Staahle, 0. Anal. Chem. 1982,54, 1646. (SEE) Grob, K. Jr., Comm. Eur. Communltles, 1982, Anal. Org. Mlcropolluf. Water, 57. (6EE) Grob, K., Jr.; Mueller, R. J. Chromafogr. 1982, 244, 185. (7EE) Hlltunen, R.; Laakso, I.; Hovinen, S.; Derome, J. J. Chromafogr. 1982, 237, 41. (BEE) Hlnshaw, J. V. Jr.; Felnstein, P. L. Am. Lab. 1983,Sept., 116. (9EE) Hlnshaw, J. V., Jr.; Yang, F. J. J. Hlgh Resolut. Chromatogr. Chromafogr. Commun. 1983. 6 , 554. (IOEE) Hosklka, Y. Anal. Chem. 1982,5 4 , 2433. (1 IEE) Jacobsson, S.; Berg, S.J. High Resoluf. Chromafogr. Chromafogr Commun. 1982,5, 236. (I2EE) Kovarich, E.; Munarl, F. J. High Resoluf. Chromafogr. Chromafogr. Commun. 1982,5 , 175. (13EE) Pankow, J. F.; Asher, W. E.; Isabelle, L. M. Anal. Chem. 1983, 5 5 , 1451. (14EE) Proske, M. G.; Bender, M.; Schomburg, G.; Hueblnger. E. J. Chromafogr. 1982,240, 95. (15EE) Schomburg, G. Proc. Int. Symp. Capillary Chromafogr., 4th 1981, 371.
(16EE)- Schomburg, G.; Husmann, H.; Schulz, F. J. High Resolut. Chromafogr. Chromatogr. Commun. 1982,5 , 565.
(17EE) Spencer, H. J. J. Chromatogr. 1983, 260, 164. WEE) Yang, F. J. J. High Resoluf. Chromafogr. Chromafogr. Commun. 1983, 6 , 448. Multldlmenslonal (IFF) Bellabes, R.; Granger, R.; Vergnaud, J. M. Sep. Scl. Technol. 1982, 17, 1177. (2FF) Broeteli. H.; Rletz, 0.; Sandqvist, S.; Berg, M.; Ehrsson, H. J. High Chromatogr. Chromatogr. Commun. 1982,5 , 596. (3FF) Herkner, K.; Swoboda, W. Roc. Int. Symp. Capillary Chromatogr ., 4th 1981,429. (4FF) Huber, J. F. K.; Kenndler, E.; Nyiry, W.; Oreans, M. J. Chromafogr. 1982, 247, 211. (5FF) Mueller, F. Am. Lab. 1983, i 5 , 94. (6FF) Phillips, R. J.; Knauss, K. A,; Freeman, R. R. J. High Resoluf. Chromafogr. Chromatogr. Commun. 1982, 5, 546. (7FF) Welton, B.; Goedert, M.; Lyons, T. Chromafogr. News/. 1981,9 , 56. Hlgh-Speed GC (IGG) Berezkln, V. G.; Mallk, A.; Gavrichev, V. S. J. High Resoluf. Chromatogr. Chromatogr. Commun. 1983, 6, 388. (2GG) Duarte, P. E.; McCoy, B. J. Sepf. Sei. Technol. 1982, 17, 879. (3GG) Jonker, R. J.; Poppe, H.; Huber, J. F. K. Anal. Chem. 1982, 5 4 , 2447. (4GG) Leclercq, P. A.; Scherpenzeel, G. J.; Vermeer, E. A. A.; Cramers, C. A. J. Chromafogr. 1982, 241, 61. (5GG) Schutjes, C. P. M.; Vermeer, E. A.; Rijks, J. A.; Cramers, C. A. J . Chromatogr. 1982,253, 1. (6GG) Schutjes, C. P. M. Ph.D. Dissertation, Eindhoven University of Technology, 1983. Vapor-Phase and Supercrttlcal-Fluld Chromatography Board, R.; McManiglll, D.; Weaver, H.; Gere, D. CHEMSA 1983. Fleldsted, J. C.; Rlchter, B. E., Jackson, W. P.; Lee, M. L. J. Chromafogr..l983, 279, 423. (3") Fjeldsted, J. C.; Kong, R. C.; Lee, M. L. J. Chromafogr. 1983,279, A.A.-. P (4") Futrell, J. H.; Wahrhaftig, A. L.; Randall, L. G. Report 1982, EPA6OOf3-82-061; Order No. PB82-249178. (5") Parcher, J. F. J. Chromatogr. Sci. 1983,2 1 , 346. (6") Peaden, P. A.; Fjeldsted, J. C.; Lee, M. L., Springston, S. R..; Novotny, M. Anal. Chem. 1982,54, 1090. (7") Randall, L. G., Bowman, L. M., Jr., Eds. Sep. Sci. Technol. 1981, 17, No. 1. (8") Randall, L. G. Chem. Eng. Supercrn. Fluid Cond. 1983,477. (9") Shafer, K. H.; Griffiths, P. R. Anal. Chem. 1983, 5 5 , 1939. (10") Smith, R. D.; Fjeldsted, J. C.; Lee, M. L. J. Chromatogr. 1982, 247, (1") (2")
231.
(11iiH)- Smlth, R. D.; Felix, W. D.; Fjeldsted, J. C.; Lee, M. L. Anal. Chem. 1982, 5 4 , 1883. (12") Wllsch, A.; Felst, R.; Schnelder, G. M. FluidPhase Equlllb. 1983, IO, 299.
Mossbauer Spectroscopy John G . Stevens* Department of Chemistry, University of North Carolina at Asheville, Asheville, North Carolina 28814-8467
Lawrence H. Bowen Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Mbssbauer spectroscopy has just completed its 25th year.
For this Analytical chemistry biannual review, it is the 10th in the series (20th year). Many of us have watched with great
interest the development of this field of research. There are now nearly 20000 articles written by research groups from over 70 different countries. Approximately 1200 articles have been published during each of the last 10 years. To commemorate the 25th anniversary of the Mossbauer effect, Editors (Deutch, Kaufmann, and de Waard) published a special issue of Hyperfine Interactions (76). After the Foreword written by R. L. Mossbauer, the volume contains the following chapters:
"Mossbauer Spectroscopyin Physical Metallurgy" (U. Gonser), "Mossbauer Spectroscopy in Magnetism" (J. Chappert), "The Impact of Mossbauer Spectroscopy on Chemistry" (T. C. Gibb), "The Understanding of Nuclear Structure Through Mossbauer Experiments" (L. Grodzins), "Mossbauer Spectroscopy of Implanted Sources" (L. Niesen "Mossbauer Studies of Valence of Fluctuations (I. Nowik), "'&n Mossbauer Spectroscopy" (T. Katila and K. Riski), "Mossbauer Spectroscopy with 191Jg31r"(F. E. Wagner), "Mossbauer Spectroscopy with Actinide Elements" (W. Potzel, J. Moser and G. M. Kalvius), "Experimental Techniques for Conversion
OO03-2700/84/0356-l99R$06.50/ 0 0 1984 American Chemical Society
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Electron Mossbauer S ectroscopy" (J. A. Sawicki and B. D. Sawicki), and "Isomer lhift Reference Scales" (J. G. Stevens). Included in this review for Analytical Chemistry are papers that have been received and surveyed by the Mossbauer Effect Data Center since the last one in the series (304). Consequently, the literature will include years 1981 through 1983. During this time there has been a dramatic increase in the number of papers being received from China. The trend noted in earlier reviews toward industrial applications of the Mossbauer effect has continued particularly as they relate to surface studies, catalysts, electrodeposits, zeolites, steel slags, and amorphous substances. Especially active research on amorphous metallic solids is evidenced by the upswing in publications related to that area. Little change in instrumentation has been noted except in the use of the channeltron for detecting the Mossbauer radiations. Conversion electron Mossbauer spectroscopy continues to gain wide acceptance. There are more experiments using geometries different from the most common transmission geometry. There has been only one new Mossbauer transition reported since 1979. This new transition is from manganese, an element that has not been used before in Mbssbauer s ectroscopy (285). The actual transition is the 126-keV y inq5Mn which has been Coulomb excited with 1.0-MeV protons. Although this particular element is of great interest to chemists, the difficulty of doing the experiment and the poor data obtained make it doubtful that there will be much future experimentation. As has been characteristic of Mossbauer spectroscopy, the vast majority of the pa ers discuss investigations of the 57Fe isotope, followed by I1 Sn. Typically, there is a fairly large gap between these and the next most active isotope 151Eu,for which approximately 50 papers were published during the last 2 years. Other active isotopes which have a combined number of publications of a proximately 200 are in descending order: 126Te, lZgI , 121Sb,l9'%u,161Dy,and 237Np.Mossbauer studies using B7Znhave increased during the past 2 years. To perform 67ZnMossbauer spectroscopy special ap aratus and care are needed but with the encouraging corngination of its high resolution and strong interest in the element itself, researchers have had renewed interest in it. As well as the special publication of Hyperfine Interactions mentioned at the beginning of this review, one other eneral book has been written on Mossbauer spectroscopy: "A vances in Mossbauer Spectroscopy, Applications to Physics, Chemistry and Biology", edited by B. V. Thosar and P. K. Iyengar is part of the Elsevier Scientific Publishing Company series on "Studies in Physical and Theoretical Chemistry" (311). This particular volume contains 16 chapters prepared by a large variety of experts in the field of Mossbauer spectroscopy. During this reporting period the Proceedings of the Conference on the Applications of the Mossbauer Effect,from the international meetin held in Jaipur, India, in December 1981, has been publishe (318). This large volume contains approximately 1000 pages and over 270 papers. The most recent international conference on Mossbauer spectroscopy was held in Alma Ata, USSR, this past fall. It was v e r y y l y attended because of international tensions that existe then. Many scientists were either prevented from attending or found making arrangements too difficult. It is always disappointing when governments actually prohibit or create unsurmountable barriers to scientific and personal exchanges among their citizens as was the case for this particular meeting. It would have been nearly impossible to have put this review together if not for the facilities of the Mossbauer Effect Data Center. The Center continues to publish the Mossbauer Effect Reference and Data Journal ten times per year to keep researchers up to date on work and activities in the field. To assist in the collection, evaluation and dissemination of data and information, the Center continues to publish a series of specialist handbooks which cover topics such as minerals, corrosion studies, conversion electron Mossbauer spectroscopy, ion implantation, theory, ca"t"lzc)"t" surface studies, amorphous materials, instrumentation, I, Ikt'e, and 121Sb. The Center continues to provide assistance for many specialized information needs of Mossbauer researchers. In the 25 years of Mossbauer spectroscopy over 1200 reviews have been published. One of the main reasons for this large number of reviews is the diversity of application problems to which the Mossbauer effect can be applied. Almost every one
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of the reviews published during the last 2 years focuses on a specific application area. The reviews covering industrial applications include those on catalysts (147),coal (I77,209, 2111, and amorphous metals (55,93,110,53). Other materials studies include ion implantation of alloys (188), phase analysis (335),and ternary superconductors (88). There have been two different reviews on certain aspects of semiconductors, one on chalcogenide vitreous semiconductors (287) and the other on laser annealing of semiconductors (171). Chemical systems reviewed include the spin crossover phenomenon (120), bioinorganics (314), intercalation compounds (128), organotin (253), and a aper on a specific area of organotin, thiosulfur in five-coodnated Sn(1V) (18). One interesting paper is a review of the history of the attempts to elucidate the structure of triiron dodecacarbonyl, Fe3(CO) 2. This is interesting because of the many changes in unaerstanding and because Mossbauer spectroscopy has played a significant role in determining the structure for the species (75). Two very extensive reviews have appeared on y-ray lasers (13,12). The fist one of these is written by Baldwin and Solem (Los Alamos Laboratory) and Gol'danskii (USSR Academy of Sciences). Although an extensive amount of research is still taking place in the Soviet Union and the United States, and probably elsewhere, it is unlikely that there will be any further reports in the open scientific literature because of the military nature of this area of research. Other reviews have as topics highpressure techniques (170),the published literature since 1975 on relaxation (123),solid-state chemistry (327), and chemistry (59). A general treatment for chemists using Mossbauer spectroscopy has also been published (305). A collection of useful data tables is in the most recent volume of the "CRC Handbook of Spectroscopy" (303).
INSTRUMENTATION Several papers report the use of microprocessors to control Mossbauer spectrometers. These typically contain 1024 channels and have storage capacities of 1.6 X lo7 counts per channel. Benedetii and Fernandez (23)describe a fully proammable eight-bit microprocessor in which wave forms can e selected as well as the frequencies of the drive. With the addition of an ADC, their unit can operate also as a PHA. De Grave et al. (69) describe the use of a microprocessor in which interferometric calibration is performed during the collection of the Mijssbauer spectroscopic data. In addition to collecting these data, the microprocessor is also able to perform curve fitting of the data. It is possible to graphically display raw unfitted data on the terminal. An additional capability is that the data collected in the microprocessing unit can be transmitted to a large external main-frame computer for complex spectral data fitting. Another microprocessor unit is described by Dai and others (65). Nolle et al. (236) designed a microprocessor-controlled spectrometer for thermal scanning Mossbauer spectroscopy. The system they describe is low cost and it sets up the temperature, takes up the spectra, and evaluates the ordering temperature from the sequence of measurements. Two different papers describe the use of the Apple 11+ microcomputers with 48k of RAM. Sisson and Boolchand (298)report the use of the Apple I1 primarily for the analysis of Mossbauer spectra. The run time for a six-line iron spectra is 1to 4 h. While this is considerably longer than the time on the main-frame computer, the actual wait time is approximately the same. Several suggestions are given for shortening the run time to 20-60 min. Sundqvist and Wappling (307) also use the Apple I1 microcomputer for Mossbauer data acquisition. The microprocessorsas reported in these papers decrease the cost of the Mossbauer spectrometer and have several advantages over many spectrometers, particularly the increased counts per channel and the possibility of software selecting wave forms and frequencies for the drive unit. In the future there should be further improvements. Most of the detector improvements for Mossbauer spectroscopy have been those associated with conversion electron Mossbauer spectroscopy (CEMS). It is now quite easy to obtain such spectra at temperatures as low as liquid helium, primarily through the use of the channeltron. This new device offers great promise for Mossbauer spectroscopy. It is able to detect Mossbauer conversion electrons at low temperatures. Measurements at 4 K can be performed with relative ease
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because of the compactness and high efficiency of the channeltron for detecting secondary electrons. M h h a u e r measurements that were difficult or impossible before are now possible. The actual experimental setup usin the channeltron is quite simple because the channeltron, t e abeorber, and the source are mounted into a small vacuum chamber which then is inserted into the cryogenetic liquid. The device does not call for any special Dewars with windows. Instead, only a container of the cryogenetic liquid is needed. Such devices have been used to obtain CEMS spectra for 5'Fe, "%n lslEu, and lanW(278). In another paper Sawicki and others (279) describe this device at low temperatures using a 100-mCi source. They are able to obtain spectral intensities of up to 400% and counting rates of Zoo0 counta/s for 57Fe. Further descriptions on channeltrons are given hy Sato et al. (275) and Atkinson and Cranshaw (IO). Other improvements for CEMS in which proportionaldetectors are used include the technique of coating the% tector cone with MgO coating. This coating increases the efficiency by over a factor of 3. Nakagawa et al. (224)report on the use of a gas-flow proportional counter for studying depth elective CEMS. Mizui (206)used a film plastic scintillator for CEMS of %n at temperatures down to 78 K. Bressani et al. (42) compare plastic scintillators with sodium NaI(TI). They show that there is a much better performance from the plastic scintillators when there is a high rate of single puke transmissions. Several other proportional detectors used for CEMS include one described by Isozumi et al. (237) detailing a proportional detector g o d a t temperatures down to 77K. Sato (273)also describes a proportional y detector which is operative down to 77 K. Its isometric design allows it to count both scattered and transmission gammas a t the same time. A unique ring detector reported by Gaubman et al. (103) is able to count Rayleigh scattering from a MBsshauer experiment. The improvement in the counting rate in such a detector is a factor of 20. Medvedeva et al. (200)describe a specially constructed low background proportional resonance counter for Mhshauer spectroscopic studies in which the detector is shielded from
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the normal electron background produced by the chamber walls. Rather than having the usual metal or alloy as the resonance material, this detector uses thin coatin of sodium ferrocyanide Na,Fe(CN)610HZ0 enriched with%Fe. Since sodium ferrocyanide is soluble in water, it is quite easy to prepare thin coatings of homogeneous composition. Using the detector, the investigators examined a number of different layer thicknesses and gas compositions. They have been able to obtain over 400% linea which, is a substantial improvement over most other resonance detectors. During this reporting period no substantial improvements in either heaters or cryostats for Mljasbauer spectroscopy are found in the literature. Nikolaev and Potapov (232) report the only improvements in the experimental techniques for high-pressure experiments. In their paper they describe a comparatively uncomplicated high-pressure chamber which is capable of providing pressures up to 17 khar for temperatures in the range of 78-450 K. The area of the samples used is approximately 20 mm2;this is an improvement over most other high-pressure devices. A general computer program for fitting M h b a u e r spectra up to 24 Lorentzians using a minicomputer is described by Verhiest (328). It is written in Fortran and has been used on both PDP 11and Vax 11 Digital computers. Typical run times are from minutes to several hours on a PDP 11/34. In another paper Cai (47) examines the problem of fitting corn lex spectra and suggests a modified Gauss-Newton method. &dically. they not only take the first approximation from the Taylor expansion but also add a correction term that is described in their paper. Another variation from the normal Gauss and/or Newton method(s) is suggested by Aramu et al. (7). They describe a procedure in which special weight distributions are introduced. By using rectangular or Gaussian distributions with proper widths they have been able to minimize the hyperiime parameter fluctuations and get better results from the Gaussian weights. An alternative to the method of least squares has been developed by Urwank (329). The concept of a probation calculation is used to develop a method of self-consistent fit of experimental spectra. The advantages of the method are that it does not contain any solution ambiguities for suitable basis functions above the limit of resolution power and that it decouples the single problems (single peak fit)in a dynamic matter. A transmission M h b a u e r polarizer which can be attached to any standard M h b a u e r spectrometer is described by Barb e t al. (26). They describe its use in an experimental arrangement where the same transmission geometry for the Mbbauer spectrum is used for the Malus curve correaponding to the different energies of the polarized y-ray. With such an apparatus, it is possible to investigate the magnetic structure of thick absorbers. Hirvonen et al. (230)examined the transmission of polarized y-rays in media with an isotropic nuclear resonance absorption. They note that the incidence polarized beam decomposes into two components which are attenuated differently. In two recent papers of interest on the scattering a t Bragg reflections, Ti e t al. (322) report their study on V,Sn and Kashiwase et al. (253)theirs on KCI. In both of these papers the authors report they they have been able to place energy resolution between the elastic and inelastic scattering within 4X eV. In the Ti paper measurements are reported as a function of temperature for two different orthogonal orientations of the crystal. The Kashiwase group reports that their experimental apparatus includes a position-sensitive detector. Renard et al. (262) warn that for comoarisons between Mosshauer spectra using a correlation link the technique of Aramu (8)is only practical when the effective thickness does not differ by more than 0.25. This is because the deviations from the correlation line can arise from thickness effects as well an from differences in the nature of the samples. Unfortunately, this unually eliminates the possibility of discrimination between twn unknown samples. Lung et al. (187) present tabular data and a simple pro. cedure for determining ideal ahsorher thicknesses. On the basis of the strrhantic theory of nuclear events Shgh et al. (2971 show that the background contribution to the Mbsbauer absorption can he reduced by several orders of magnitude by using an AC dual Doppler modulation techANALYTICAL CEMISTRY. VOL. 56, NO. 5. APRIL 1984
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nique. They describe an experimental arrangement using stainless steel which was used to demonstrate the results of their theory. One of the most active applied research areas of Mossbauer spectroscopy in recent years has been cultivated through the use of the conversion electrons that result from the Mossbauer resonance. Scattering geometry is used in these experiments rather than the transmission geometry typical of almost all other Mossbauer spectroscopic measurements. This technique, usually called CEMS has been known for some time. It has only been during the last 4 to 5 years that a great variety of experiments have been reported. One use for CEMS is to obtain magnetic and electronic information at various depths into the surface. This experiment is extremely difficult, so several groups have examined the problem very carefully. Staniek et al. (302) investigated the depth sensitivity of this technique by using experimental results obtained from a series of thin iron films. In their experiments they have been able to note changes in the hyperfine fields of the surface layers and that these hyperfine fields are different from that of the bulk material. In another paper, Itoh et al. (138) used a high-resolution electron spectrometer, necessary for most of the depth selective Mossbauer measurements, to obtain information at various depths. In this study they measured the ener y distributions of the 7.3-keV electrons emitted from a thin 5780 source covered with iron films of various thicknesses. From these distributions they have been able to derive the accurate weight coefficients for depth selective CEMS. The results compare quite favorably with the theoretical calculations. Smit and Van Stapele (299) propose an alternate method for obtaining depth selective Mossbauer spectra in which only a simple proportional counter is necessary. They report experimental results with depth resolutions of about 20-30 nm. To obtain the depth selectivity,they removed thin layers from the sample and measured the conversion electron Mossbauer spectra. The layer spectra (that is, the Mossbauer spectra associated with the removed layers) are determined by calculating the number of electrons which originate for each layer and reach the surface of the sample. They apply their technique to the examination of YIG films in which they report the change in magnetization as a function of the depth. A spherical electrostatic electron spectrometer is described by Yang et al. (337). With their spectrometer they are able to obtain excellent spectra within 1day or even within several hours by using a 50-100 mCi source. The signal to background ratio is often in excess of 800%. The instrument was used specifically to study the dependence of the magnetic hyperfine interactions as a function of depth from the surface intoa bulk sample of Fez03film. They are able to determine differences in the hyperfine field of less than 100 G, Le., 0.02%. Also, they are able to determine extremely small changes of the quadrupole interactions a t the surface of foils. With the capability that this instrument has for measuring such small changes in the Mossbauer parameters, a number of new problems can be examined. Another scattering geometry that is sometimes used is associated with resonance detectors. The detectors have a selective sensitivity to resonance y-rays and, consequently, a unique resolution. Discriminating efficiency of resonance detectors is due to the fact that the resonant absorption effect is used in the detection process. Resonance absorbers, called converters, are actually placed inside the detector. These converters need to possess a high fraction of absorption without recoil energy loss. They should have a single-unbroaded absorption line, and the line should coincide with the emission line of the source. While the first two requirements can be satisfied easily, the third re uirement is quite difficult and can be practically met only inXgSnand 12TeMossbauer spectroscopy. Irkaev et al. (136) describe an experimental setup which meets all the requirements for the resonance detector. To et the emitted y-ray to have the required frequency for aisorption by the converter, the source is given a specific Doppler velocity. Such a device is appropriate for any Mossbauer isotope.
THEORY Some theoretical results are discussed in other sections. Most of the papers covered in this section deal with isomer shifts or quadrupole splittings, although a few papers dealing 202R
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with other topics are included. Concerning isomer shifts, Bansal and Shrivastava (15) estimate the nuclear radius change in 40Kfrom Hartree-Fock calculations on KX halides. Lie and Taft (178) use the X a method to estimate electron densities in the FeSf cluster and relate these to isomer shift calibrations for 57Fe. Parish (243) evaluates literature data on isomer shift and quadrupole coupling correlations with electron coflipation for the main group sequence l19Sn,lZISb lZ5Te,and I. Some revised correlation equations ard presented. Makariunas (192) evaluates electron ca ture data and reports new isomer shift calibrations for 7pI and lZ9I. Varnek et al. (326)use CNDO calculations for SnC14and some of its organic complexes to compare with isomer shift data and relate these results to the interactioin between organic ligand and tin. Hartmann and Rygavjl (122) use the X a method to calculate electron densities in tellurium compounds and show that their results give agreement with earlier estimates of the nuclear radius change for lZ5Te. Obara and Kashiwagi (239)report ab initio molecular orbital calculations for various iron(I1) porphyrin complexes and relate these to Mossbauer isomer shifts and quadrupole splittings. Czyzek (64) discusses the distribution of electric field gradients in amorphous solids and reviews experimental data in terms of the unique information obtained from Mossbauer and NMR on local angular distributions as opposed to the radial distributions from XAFS measurements. Zimmermann and Doerfler (340)present a method for thicknesss correction of quadrupole splitting data on single crystals. Litterst et al. (185)derive an analytical solution for electric field gradient fluctuationsin the case of jump diffusion in an octahedral cage. Kohn and Lee (162) interpret low-temperature valence fluctuation phenomena on the basis of a quantum mechanical model and oint out the error of interpreting these effects by an assumettime dependence. Riseborough and Hanggi (263) analyze theoretically the Mossbauer spectra of particles undergoing surface diffusion on fiiite surfaces. Rad0 and Walker (257) interpret results on spontaneous magnetization of ferromagnets as a function of distance from the surface and discuss the effects of surface anisotropy. M0rup (214)presents approximations for the reduction in magnetic field at low temperature observed due to microcrystallinity. He relates the observed magnetic splitting to the particle size and magnetic anisotropy. Knapp et al. (160)discuss the theoretical basis for the unusual double line Mossbauer spectra of proteins and their temperature variation. The dynamics of protein structure are emphasized.
RELAXATION An active area of research during these last 2 years has been on the spin crossover transitions which occur in a number of Fe(I1) and Fe(II1) complexes. The spin transitions are the 5Tz+ lAl and the 6Al + 2Tz. Burnett et al. (45) studied several series of Fe(I1) and Fe(II1) complexes of 2,2’-bi-2imidazoline and other related ligands. As for many of the studies, they used magnetic susceptibility in addition to the Mossbauer spectroscopy measurements to determine at which temperature the spin crossover occurs. Kambara and Sasaki (150) report on a theoretical approach to Fe(I1) + Fe(II1) transitions that are induced by molecular distortion in complexes which have spin crossover for both Fe(I1) and Fe(II1). Another study includes the effect of low pressures (up to 150 barr) (202). In another theoretical study pressure-induced high spin-low spin transitions in compounds of Fe(II1) are described (272). The effect of metal dilution on the spin crossover behavior in (FexM1,(phen)2(NCS)2)(M = Mn, Co, Fe, Zn) was investigated by Ganguli et al. (99). They report that in the host lattice, when the ionic radius is less than that of ferrous iron, the amount of rest paramagnetism and the region well below the transition temperature increase with metal dilution, whereas in the host lattice with a radius less than the radius of Fez+,the residual dimagnetism in the region is well above the transition temperature decreases with metal dilution. These observations are interpreted quantitatively in terms of “negative” and “positive” local pressures which change the Fe-N bond length. Konig et al. (163) report report an extremely sharp high spin-low spin transformation. They also note the simultaneous change of the spin and the lattice characteristics in addition to the order-disorder phenomenon of the parachlorate ion. An unusual spin transition which includes a
MOSSBAUER SPECTROSCOPY
two-step spin conversion in the crossover region in the system [Fe(B-pic) ]Cl,(EtOH) is reported by Koppen et al. (166). Sasaki and Kambara (271) examine the effect of an induced magnetic field on high spin-low spin transitions in both ferrous and ferric compounds. In another paper, Konig et al. (164) note an anomalous behavior when there are differences in effective thickness. The explanation for this particular behavior is that the Mbssbauer fractions of the two spin states of a spin crossover transition are not the same. Muller et al. (217) studied the effect of domains in the high spin-low spin transition in (dithiocyanato)bis(2,2’-bi-2-thiazoline)iron(II). Examples of electron hopping include a report by Coey et al. (611, Litterst et al. (182), and Merrup and Topsere (215). Coey et al. give evidence for a spontaneous,thermally activated charge transfer in cronstedtite, an iron-rich 1:llayer of silicate. Litterst et al. calculate the effect on the Mossbauer spectra of tetrahedral cage hopping for 67Fewith correlated jumps of an axial electric field gradient. Marup and Topsere discuss slow electron hopping in 60-A particles of Fe30e Shaitan and Rubin (288) describe a model of a Brownian oscillator with very strong attenuation for which the results agree well with experimental data. In another paper these later authors (288) describe their investigation of Mossbauer absorption in overly damp, harmonical bound particles which have Brownian motion. This kind of modeling is close to descriptions of a variety of different biological systems. With their model, they have been able to obtain excellent fits of experimental data. In another biological application, Knapp et al. (160) used Mossbauer spectroscopy to study protein dynamics. The spectra they obtained were analyzed by three Brownian oscillator modes which are able to account for the protein-specific motion. Two of the modes are extremely over-damped and a third mode has a diffusion-like character. There continues to be considerable interest in the literature in spin relaxation. There are constant improvements in the analysis of the spectra. Cianchi et al. (58) used asymmetry to simplify the analysis in such a way that they only need to invert a 40 X 40 matrix rather than an 80 X 80 matrix, used in most other previous theories. In another paper Hoy et al. (133) evaluate the nonadiabatic, stochastic model on experimental data of the paramagnet ferrichrome A at temperatures down to 115 mK. They have determined that the dominant relaxation mechanism is the spin-spin interaction. Basically they obtained good theoretical fits to all their data. Recently there have been a number of relaxation experiments in which the material is a glass substance. These include materials such as Ba0-2Ti02-2Fez0 (139),YFez and Fe16N&2B14Si8 (2341, DyAg (541, and Fe(NO& in water (333). Other studies include the nuclear motion in an octahedral change (184),spin-lattice relaxation in TmAlz(116), and slow electronic spin relaxation in high-spin hexakis(pyridine N oxide)iron(II) perchlorate (48). Gol’danskii and Stukan (109) discuss tunneling chemical relaxation by using emission Mossbauer spectroscopy. An excellent paper published by Bonville (35) describes the most frequently encountered paramagnetic relaxation methods (phonons, conduction electrons, exchange, or dipolar interactions) in condensed matter.
CHEMICAL STUDIES (IRON) In addition to the specialized subjects of the other sections, it is proper in a review for Analytical Chemistry to discuss chemical applications of Mossbauer spectroscopy. Many more papers have appeared than could possibly be mentioned here, however. As has been our custom, we have generally omitted those papers in which straightforward Mossbauer measurements in conjunction with other techniques have given useful structural information, and concentrated on those in which Mossbauer studies are the primary focus. Even so, the references discussed below must be considered incomplete and somewhat arbitrarily chosen. Both electron spin resonance and Mossbauer spectroscopy were used by Beardwood et al. (21)to ident the one-electron reduction product of FezSz[(SCH2)zC6H40]z The reduction product of this 2Fe2S protein analogue has proven difficult to stabilize. Their 4 K Mossbauer spectra were obtained with external field and show the two irons in the trianion are antiferromagnetically coupled. Moura et al. (216)studied a 3Fe-3S cluster from a natural ferredoxin, also by ESR and Mossbauer techniques. Their main emphasis is on the con-
5t.
version to 4Fe-4S clusters by chemical treatment. Spectra were also obtained in an external field to characterize the magnetic structure. English et al. (84) obtained Mossbauer spectra for a number of oxidized iron porphyrin complexes with differing axial coordination and find a marked change in Mossbauer parameters depending on the bridging atom, which they interpret as a change in site of oxidation from metal to ligand. Boso et al. (39) have studied the Mossbauer spectra of a synthetic iron(1V) porphyrin complex as function of temperature and externally applied magnetic field. They present a theoretical interpretation based on tight spin coupling. Pecoraro et al. (248) report Mossbauer studies on the pH dependence of Mossbauer spectra from ferric enterobactin. Although a variety of changes in the spectra were observed, the iron remained Fe(II1) in aqueous media. Reimer et al. (261) obtained low-temperature Mossbauer spectra in applied magnetic fields for carbonyl complexes of biddimethylglyoximato)iron(II). They find a large field gradient effect both cis and trans to the substitution when CO replaces amine as ligand. Turning from biological systems and analogues to organometallics of nonbiological interest, we mention here several papers of interest. Muller et al. (218) discuss both absorption and emission spectra they obtain for square-planar complexes of Fe(I1) (the latter showing essentially no Mossbauer effect). They fiid rather low isomer shifts, presumed due to covalance. Neshvad et al. have reported a number of studies on ferrocene complexes. These include a study (227) of substituent effects on ferrocenyl-carbenium ions in which a decreasing quadrupole splitting was correlated with increasing electron donation from the substituent Nicolini and Reiff (228)discuss magnetic interactions in the linear chain complexes M(2,2’bpy)(.HzO)zSO,, where M is Fe, Ni, or Cu and bpy is bipyridine, both from low-temperature magnetic susceptibilities and, in the case of the iron compound, Mossbauer spectra. Blomquist et al. (30)compare their extended Huckel molecular orbital calculationswith Mossbauer and ESCA results on iron dithiolate complexes. Mossbauer spectra at 4 K were also obtained in an external field to evaluate the sign of the field gradient. Moore et al. (213) report Mossbauer studies of mixed-metal bimetallic complexes of pentacyanoferrate(II), in particular with Co(II1) and Rh(II1). Some differences are observed in the remote effects from the two metal ions on Mossbauer spectra, but these do not appear to explain the photochemical behavior previously observed.
CHEMICAL STUDIES (TIN) Following the success of previous Mossbauer studies of matrix-isolated atoms and small clusters, Shamai et al. (289) report a double Mossbauer experiment on matrix-isolated clusters of Fe-Sn. Using a special cell and both 57Feand ll%n sources, they were able to detect all the dimers and most trimers of this mixed system. The Sn-Fe-Sn trimers were more abundant than FeSnz. Korecz et al. (167) use isomer shifts and quadrupole splittings to determine coordination number and geometry in some 35 stannatrane complexes. They compare Mossbauer results with available X-ray, NMR, and IR data and find good agreement. Mahieu et al. (191) give results on some tetrahedral organotin compounds containing the Sn-Mn bond. Changes of Mossbauer parameters with substitution of the organic groups are emphasized. A study of cis-octahedral complexes of SnX with urea and thiourea ligands was made by Calogero et al. (51). These complexes have no quadrupole splitting observed but exhibit a marked decrease in isomer shift from the original tin(1V) halides. The variation with temperature of the Mossbauer peak area indicates noninteracting, discrete molecules. Cusack et al. (63) use l19Sn Mossbauer effects as well as IR to characterize new inorganic tin derivatives of amino acids or esters. Cusack et al. (62)have also reported spectroscopic studies on crown ether complexes with SnC1, and SnBr,. Low isomer shifts are observed, consistent with strong complexation. The complexes with 18crown-6 and 15-crown-5 exhibit appreciable quadrupole splitting, while those with 12-crown-4 have none. This is interpreted as due to cis-oxygen coordination for the latter. CHEMICAL STUDIES (OTHER ISOTOPES) As example of the chemical information from the lzlSb Mossbauer effect, the work of Pebler et al. on organoantimony ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
203R
MOSSBAUER SPECTROSCOPY
compounds shows marked difference between Mossbauer results for Me3Sb0 and Ph3SbS (247). The former has large negative quadrupole coupling constant, while the latter has almost zero. These authors also obtained the crystal structure of Ph3SbS and show that the tetrahedral arrangement about Sb agrees well with Mossbauer results. Dehe and Behn (73) used 121Sb(V)substituted in spinel oxides to study the 3uper-transferred-hyperfine field. Double Mossbauer experiments indicate the magnetic ordering affects Sb at higher temperatures than Fe in the same spinel. Nagarathna et al. (222) report extended Huckel calculations on small clusters of Sb. These calculations are compared with Mossbauer results on matrix isolated samples to refine the value of A( r 2 ) for 121Sb. Matrix isolation studies are also reported for 151Eu by Montan0 (210). Magnetic field line broadening is observed for the isolated atoms. The methods for preparation of anhydrous Eu13 are examined by lSIEuMossbauer effect in the studies of Jenden and Lyle (144). None of the Eu(II1) iodide was observed after various treatments, which all gave Eu(I1). Double Mossbauer experiments with IS1Euand 57Fehave been used to characterize the magnetic interactions in perovskites of formula E U F ~ ~ - ~ C by~Gibb , O ~(1062. The observed supertransferred field at both 57Feand IEu depends on the substituent, Cr, having certain unique characteristics. Gerard et al. (105) report an unusually large magnetic hyperfine field for lSIEuin EuFe4P12.This is interpreted as due to f-d band coupling. Other recent studies of Mtjssbauer-activerare-earth elements include papers by Friedt et al. (92) on lelDy in Dy(I1) and Dy(II1) halides and oxyhalides and a study on magnetic and electric field interactions in RzFe3Si5compounds using lS5Gd,l@Er,and S7FeMossbauer spectra (Noakes et al., ref 235). Concerning the actinide elements, reviews have been reported by Friedt (90) and by Potzel et al. (255). A review of 9 7 AMossbauer ~ spectroscopy is given by Parish (244). A few recent examples of application of lg7AuMossbauer effect to chemical problems should be mentioned. Katada et al. (156) report results on mixed-valence compounds and chelates of Au(II1) with phenanthroline and related ligands. Evidence from the Mossbauer spectra is presented for metal-metal interaction in some of the com lexes studied. The effect of y-irradiation has been studie on several organogold complexes using lg7AuMossbauer effect by Sakai et al. (268). Evidence of reduction of y-irradiation is presented for one of the Au(II1) complexes. Hill et al. (129) report lg7Austudies on thiolates and their phosphine complexes of Au(1). Their results are discussed in light of the biological role some of these complexes have as antiarthritic drugs. The Mossbauer effect in 61Niwas used by Rummel et al. (267)to study the effect of Hz adsorption/desorption on LaNi,. Microprecipitates of nickel metal ap eared in the Mossbauer spectra after cycling. A review of 6:Zn Mossbauer effect is given by Katila and Riski (157). Potzel et al. (256)re ort new measurements on the quadrupole interaction in &Zn and discuss the use of Zn metal for calibration of drive units. Both lnI and l%e Mossbauer effects were used by Birchall et al. together with NMR data to characterize OTeF5- compounds (29). The relative electronegativities of OTeF6- and F- are discussed, the former being somewhat less electronegative on the basis of Mossbauer results. Berry et al. (25) report iodine Mossbauer studies on PbIz and the mixed valence lead oxyiodide. In both cases the iodine spectra were characteristic of I-. Several recent studies on iodine-doped polyacetylene have appeared. Kaindl et al. (148) report both 1, and 15-linear groups observed, with 1, decreasing as the doping level increases. Matsuyama et al. (199) also find evidence for the presence of I- and discuss its importance in the conductivity behavior of the polymer.
B
PHASE TRANSITIONS AND HIGH-PRESSURE EXPERIMENTS The Mossbauer effect is a useful technique to study changes in solid-state structure with temperature or pressure. Magnetic phase transitions are particularly amenable to study by this technique. In addition to the abrupt changes in Mossbauer parameters at a phase transition, gradual effects of increasing pressure are of interest in a number of the papers surveyed. Potzel et al. (254) report 67ZnMossbauer spectra from ZnS 204R
ANALYTICAL CHEMISTRY, VOL. 56, NO. 5, APRIL 1984
up to 33 kbar. Their main objective was to determine the effect of pressure on the isomer shift, and a linear increase of s-electron density at zinc was found. A substantial second-order Doppler shift correction was required for the very narrow 67Znresonance. Kapitanov et al. (152) studied single crystals of a-Te02up to 50 kbar in two orientations to determine the effect of pressure on the anisotropy of the recoilless fraction. A reversal in anisotropy occurs at about 15 kbar. High-pressure lg7AuMossbauer experiments are reported on the mixed valent CszAuzC1,and on CszAgAuC&and AuI by Stanek (301). In the former compound, the gold sites become crystallographicallyindistinguishable at about 50 kbar. However, Mossbauer results show the Au+ and Au3+ions are distinct up to 68 kbar at 45 K. Abd-Elmeguid and Micklitz (1)studied the 151E~ Mossbauer resonance in EuMo6SBup to 16 kbar. They show that th.e superconductivity observed at p > 7 kbar is not due to a valence change in the Ed+. Ratner and Ron (260) applied uniaxial tensile stress on face-centered cubic stainless steels and determined the change in isomer shift as a function of stress. Phase transitions in SnF2have been studied by using the '19Sn Mossbauer effect by Birchall et al. (28). Sharp discontinuities are observed at the first-order transition a y at about 425 K. The second-order y /3 transition has no discontinuities. Van Deen and Van der Woude (322) used Mossbauer spectroscopyto study the orderdisorder transition in Ni3Fe at 780 K and to map the phase diagram in this region. Single crystals of FeI, were studied in external fields up to 15 T by Calis et al. (49). The high-field magnetic transitions produce three different ferrimagnetic phases at low temperature. The exchange mechanism accounting for these transitions is presented in terms of a model consisting of eight sublattices. Dockum and Reiff (78) report Mossbauer studies of the reversible phase transition in bipyridyliron(I1) thiocyanate between 130 and 185 K. Unlike the high spin-low spin transition in the bis(bipyridyl)thiocyanate, the above transition involves no spin change, but a structural reorganization. Sato et al. (276) observed evidence of a second-order phase transition near the melting point in acetyl and carboxyl ferrocenes. The Mossbauer line width and recoilless fraction exhibit anomalous behavior associated with the transition. De Grave et al. (71) studied the Morin transition in hematite containing about 5% Al. Below 245 K two magnetic phases coexist. The disappearance of the antiferromagneticphase occurs gradually as the temperature is raised and is accompanied by spin reorientation. Pure hematite has been studied up to 53 kbar by Bruzzone and Ingalls (43). They explain the changes in quadrupole splitting and hyperfine field observed at the pressure-induced Morin transition in terms of the anisotropy energy density.
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DEFECTS Several studies have appeared on various aspects of the Mossbauer spectroscopy of wustite, Fel-,O. Chalabov et al. (52) report decomposition studies in inert atmosphere and show that cation vacancy concentrations can be estimated from the Mossbauer spectra of the decomposition products. Gohy et al. (107) report room temperature Mossbauer spectra of Fel,O and Fel,,MnyO and analyze these in terms of two doublets and a singlet, the latter due to Fe3+. In an independent study, Hope et al. (131) used neutron diffraction, magnetic susceptibility, and Mossbauer effect to study Fel-,Fel-,-yMnyO. They interpret the Mossbauer data as indicating three doublets due to Fe2+. Both these papers indicate the relation of the measured spectra t o the defect structure. Point defects in vanadium metal have been studied by Janot et al. (143) using both positron annihilation and Mossbauer s ectroscopy. The Mossbauer measurements were made with 7pCo in V as source up to temperatures of 1700 "C, unusually high for Mossbauer experiments. LATTICE DYNAMICS The temperature dependence of the recoilless fraction gives information on the dynamic properties of solids. As in the past, a variety of papers have dealt with applications of this type of measurement. Litterst et al. (186) measured temperature variation of the Mossbauer resonance in vinylferrocene polymers. The anomalous changes in recoilless fraction at temperatures from 80 K to 130 K are interpreted
MOSSBAUER SPECTROSCOPY
as due to hindered motion of the ferrocene grou Kandpal and Bhide (151)used diacetyl ferrocene as a Mossiauer probe to study molecular diffusion in nematic liquid crystals at low temperature. Several smectic liquid crystals containing tin in the crystalline matrix have been studied at 77 K by Ktorides et al. (169). Effects of the alignment angle on the lI9Sn recoilless fraction are reported. Molloy et al. (208) report variable-temperature '19Sn Mossbauer results on Sn(I1) and Sn(IV) amines. These results are interpreted in terms of structure. Arsenio et al. (9) present data on the temperature dependence of hyperfine field and recoilless fraction for single crystals of the linear chain compound RbFeS2. The results are compared to previous measurements on KFeSz. Shenoy et al. (291) have studied the superconductingLaFe4P12by variable-temperature Mossbauer spectroscopy, as well as in external field. Their results are related to the superconducting properties. Mossbauer diffraction from sin le crystal lead germanate was used by Gavrilov et al. (1047 to investigate the lattice dynamics around its ferroelectric transition. Rotenberg et al. (264) report recoilless fraction measurements on small particles of Fez03imbedded in Teflon. The behavior of the Mossbauer fraction is related to the dynamic properties of the polymer matrix. Picone et al. (252) have measured hyperfine fields and recoilless fractions for ultrafiie metallic iron and iron oxide prepared in a variety of ways. The behavior of the Mossbauer fraction indicates that the mode of recoil depends on temperature and on the supporting material. Petry et al. (250) report emission Mossbauer experiments on electron-irradiated 57C0 in aluminum. The decrease in intensity around 15 K is correlated with localized diffusion of the 57Fein an interstitial cage. High-temperature measurements of 5 7 Cin~ aluminum are reported as a function of crystal orientation by Mantl et al. (197). A theoretical interpretation of the diffusion process is given. A number of studies have appeared in the last few years on protein dynamics in metmyoglobin and deoxymyoglobin. Krupyanskii et al. (168)used Rayleigh scattering of Mossbauer radiation to study motions in metmyoglobin samples with varying water content. Parak et al. (242) report Mossbauer absorption of 57Fe in deoxymyoglobin and on K,Fe(CN), dissolved in the water of metmyoglobin. The protein dynamics are correlated with water mobility in the system. Bauminger et al. (19) also re ort 57Feabsorption measurements in metmyoglobin and &oxymyoglobin and discuss their interpretation in terms of protein dynamics.
KINETIC STUDIES Some studies related to kinetic phenomena have been discussed in previous sections. The use of Mossbauer spectroscopy is generally to evaluate phase composition at various stages of reaction. Because of the length of time required for obtaining such spectra, the application of the technique to follow time dependent changes in situ has been limited. Nevertheless, Mossbauer measurements sometimes prove crucial to explaining kinetic results. Phillips and Dumesic (251) report a Mossbauer study on the thermal decomposition of iron pentacarbonyl on Grafoil. In this paper the outgassing methods are emphasized. Kinetic measurements and Mossbauer results indicate an alteration of site reactivity with pretreatment. The interaction of NO with F e Y zeolite is reported by Segawa et al. (284). Infrared spectroscopy was used to follow the kinetics and Mossbauer spectroscopy to study the changes in iron sites. Corrosion is a kinetic process particularly susceptible to Mossbauer study. Peev and Rousseva-Plachkova (249) compare kinetic data and Mossbauer results on the effect of rust transformers in preventing corrosion by formation of phosphate layers. Belozerskii et al. (22) used depth selective CEMS to study the oxidation process on iron foils. The passivation effect of H20zis shown. Leidheiser and Music (175) report Mossbauer studies of long-term atmospheric oxidation of steel. The results indicate a predominant influence of sulfate on the corrosion products. Saragovi-Badler et al. (270) also studied long-term oxidation of steel in the atmosphere, but with emphasis on the chan es in oxidation products due to CuS04 pretreatment. Domie and Kyvelos (79) used CEMS to study oxidation of a thin crystal of iron. The various oxide phases detected are related to the kinetics of the process. Some depth-selective resulta are reported. Other papers on oxidation
of iron surfaces are discussed under the section SURFACE STUDIES.
ENVIRONMENTAL MATERIALS Both naturally occurring and synthetic minerals continue to be of wide-spread interest among Mossbauer spectroscopists. A number of papers have discussed results on goethite, a-FeOOH, and hematite, a-Fe203,which have been substituted by AI3+. Goodman and Lewis (114) have prepared a large number of samples with varying aluminum content up to -30% Al and report Mossbauer spectra at room temperature and 77 K. They emphasize the complications introduced in the spectra by the effects of substitution and small particle size. Johnston and Norrish report room temperature spectra of a series of natural goethites (146) and present evidence that other impurities in addition to A1 may have marked effect on the observed spectra. Murad (220)suggests that a distribution of magnetic fields may be used for such spectra to characterize the samples and presents data on a variety of samples both at room temperature and down to 4 K. Fysh and Clark (94) use the line width method to determine the recoilless fraction for pure and Al-substituted goethites at 4 K and room temperature. They find particle size to have little effect on lowtemperature spectra. In contrast, Murad and Schwertmann (221) report a multiple correlation of magnetic field with both A1 content and degree of crystallinity at 4 K. Fysh and Clark (95) also report recoilless fraction and magnetic field data for Al-substituted hematites at 4 K. They find marked differences between samples prepared at high temperature and those prepared at
