Anal. Chem 1980, 52, 1 7 5 R - 1 8 5 R
Mossbauer Spectroscopy John G. Stevens” Department of Chemistry, University of North Carolina at Asheville, Asheville, North Carolina 288 14
Lawrence H. Bowen Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27607
INTRODUCTION This review is the eighth in an on-going series on Mossbauer spectroscopy being published biannually by ANALYTICAL CHEMISTRY. This current one covers the literature surveyed since the last review (297), so articles from the latter part of 1977 to the end of 1979 have been considered. Almost 3000 different scientific articles on the Mossbauer effect were examined as we attempted to select less than 15% of them for inclusion in this review. The applications of Mossbauer spectroscopy have continued to be developed during the past two years; especially rapid growth has been seen in industry. Unfortunately, however, much of this work does not get published. Also the number of countries in which Mossbauer spectroscopic research is being planned or begun continues to increase. A few newcomers include Cuba, Peru, Turkey, Thailand, and Saudi Arabia. A significant number of papers are now also coming from China, as the rebirth of the basic sciences takes place there. The observations of several new Mossbauer transitions have been reported in the literature since the last review. These include the 42.9 keV transition in 240Pu(158), the 78.7 keV transition in li3Yb (52),and 10.1 keV transition in 13’La (112). While most of the transitions in the past five years have been of little interest because they require low temperatures and have extremely small intensities, the most recent ones reported show more promise. 240Puallows for studies of an element not before possible. T h e vast majority of the papers continues to deal with investigations using 57Fe, far and away the most popular isotope, and its runner-up, l19Sn. A number of other isotopes are becoming more established as the information gleaned from studies on them using Mossbauer spectroscopy is now more easily understood. These isotopes include IgSAu,‘j1Eu lZ9I,‘“Sb, and 125Te. Combined they provided topic for -206 articles during the reportin period of the review. Articles in which 161Dy,237Np,170Yb,%+Ru, ‘j5Gd, and lslTa are topics appearing far less frequently. A major book entitled “Mossbauer Spectroscopy and Transition Metal Chemistry” (133) has recently been published. The authors, Gutlich, Link, and Trautwein, have done a n excellent job covering the material indicated in its title. “Mossbauer Isomer Shifts“, edited by G. K. Shenoy and F. E. Wagner, is a major contribution to Mossbauer spectroscopy, containing almost 1000 pages of contributions from -25 leading scientists (283). The proceedings of two international Mossbauer conferences held during these past two years have been published. The first, held in Kyoto, Japan, during the summer of 1978, had over 250 papers presented (320). In September of 1979, another conference was held in Portorui, Yugoslavia, the proceedings of which should be available by the time this review goes to press. The second and final volume of the international conference held in Romania in 1977 was also published during the period covered by this review (22). One other major conference published was “Workshop on New Directions in Mossbauer Spectroscopy”, held at Argonne National Laboratory during 1977 (251). As well as these larger international conferences, there has been a marked increase in national or regional meetings. T h e Mossbauer Effect Data Center, having replaced the annual “Mossbauer Effect Data Index” with a monthly information journal, the Mossbauer Effect Reference and Data Journal, is now totally self-supporting as it supplies the 0003-2700/80/0352-175R$O 1 .OO/O
Mossbauer community with its bibliographic and data needs. The first two volumes of the Journal have been successful and well received. We have used the facilities of the Data Center extensively in the preparation of this review. Because of the great variety of applications, there are a large number of reviews published. These applications-oriented reviews using Mossbauer spectroscopy center on topics such as hydrogen systems (335),polymers (192),ferric oxides and hydroxides (41), catalysts (33),corrosion (285), organoiron (240), surfaces (155),and coatings (34). Numerous Mossbauer biological reviews have appeared in the literature including ones on hemes and hemoproteins (201,202,226),peptides and proteins ( 8 1 ) ,and iron porphyrins (273). Two good general reviews on metallurgy have appeared (100, 153) as well as reviews on radiation defects (281),amorphous magnetic rare earth alloys (48), backscattering applications in metallurgy (259), and impurity host force constant changes (123). The aspects of mineralogy which have been reviewed are silicate minerals (18)and thermal and radiation effects on minerals (68). Topics dealing with Mossbauer methodology include source materials (296), double resonance (212),high magnetic fields (49),and spin crossover (129). The 1976 literature has been extensively reviewed by Donaldson and Tricker (86). T h e most difficult task is always that of selecting only a small percentage from all references available for inclusion in this review. I t is never an easy task to limit the number of papers on which we wish to report. It is unavoidable that this review is heavily weighted towards those areas in which we ourselves are interested. As in previous years the very active research in biological materials and magnetic hyperfine studies has not been included because we felt too limited in our understanding of these areas to give adequate coverage.
INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES Improvements in precision and reliability have accounted for most recent developments in spectrometers. Dunham describes a spectrometer (88)which is designed specifically to maximize available information. It has the capability of varying both the sample temperature (2-350 K) and the applied magnetic field (MT). Genand-Riondet and others (110) have developed an improved electromagnetic velocity drive by suppressing the fundamental resonance frequency in the moving rod. A constant velocity spectrometer which is free of long-term instrumental and radioactive decay drifts in the count rate is described by Sarma et al. ( 2 7 4 ) . Taragin gives description of an inexpensive digital triangle wave generator which can be used in a Mossbauer spectrometer without any internal connections to the multichannel analyzer (304). An alternative approach to the conventional Dopplerscanning method is suggested by Baldwin (16). He shows that the ?-ray frequency spectrum can be determined by observation of the time distribution of delayed coincidence counts of 7’s that are transmitted by a stationary resonant filter. He also discusses several possible experiments. In another paper discussing spectrometer improvements, Delyagin (76)describes a double Mossbauer spectrometer for measuring small line shifts. Minicomputers and microprocessors are becoming ever more useful, mainly as they expand the capabilities of the spectrometer. Although some of these are used simply as data acquisition systems, e.g., PDP-4 (336) or PDP-9 (with a CAMAC system for interfacing) (2531,others are inexpensive and C 1980 American
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flexible enough to offer potential for future applications and developments (318,254, 319). One system functions as a data processor (120),a development which will undoubtedly grow both in use and sophistication. Microprocessors will provide more and more unique services for spectrometers in the future. One present example is the use of a microprocessor to separate t h e elastic scattered intensity from the inelastic one in the vicinity of Bragg reflections in a Mossbauer diffractometer (271). Kobeissi and Hohenemser (169) describe a high precision Mossbauer furnace capable of reaching temperatures up to 1100 K with a long-term stability of 0.02 K and inhomogeneity of better than 0.05 K. Chappert’s group in Grenoble has developed a high field system using Bitter magnets (305). A maximum field of 16 T is obtained with a current of 15000 A. The glass cryostat described by Pasternak and Shamai (244) avoids the difficulty of sealing mylar windows to a ground open-end glass surface by using a special lower metal section that is sealed to the glass. Other Dewars that have been described are those for varying the temperature (54),using a magnet system (36), and changing high pressures (193). Lowering the temperature down to 20 K in a conversion electron Mossbauer spectrometer has been reported by Sawicki e t al. (277). A relatively inexpensive high counting system is described by Viegers and Trooster (329). By using the integrating counting technique, they are able to obtain high quality lg7Au Mossbauer spectra. Pawlowski and Cudny (246)have designed a multiwire proportional counter which is excellent for >’Fe studies because it has count rates up to 1.5 x lo6 cps with a gas amplication of 100 in the energy range of 3-15 keV. An improvement for proportional counters is accomplished by a rise time discriminator (164). Jaggi and Rao (151)have developed a combination detector-spectrometer system which can be used for the quantitative measurement of the ratio of ferrite to austenite in stainless steels. T h e procedure is rapid, nondestructive, inexpensive, and reproducible. Improvements in conversion electron Mossbauer spectroscopy are demonstrated by Carbucicchio (44). He uses a @-rayspectrometer to differentiate the electron energies and can, therefore, study materials a t different depths. The advantages of this particular apparatus are that studies can be made in the temperature range of 74-800 K and the sample can be rotated. Massenet (210)has extended the temDerature down to 4.2 K. Liljequist, Ekdahl, and Baverstam (187)have computed for the total transmission as well as the transmission into various angular and energy intervals of electrons from different depths in conversion electron Mossbauer spectroscopy (CEMS) of 57Feabsorbers. In a later work they discuss the interpretation a n d some practical analysis techniques for depth selection using CEMS (186). Mbllen and Stevenson (225) describe a conversion electron detector using microfoil in which they are able to obtain a high signal-to-noise ratio and high efficiency. Owens, Chien, and Walker have investigated depth profiling of the magnetic hyperfine field with ultrathin films of iron (238). T h e analysis of depth-selective Mossbauer effect data is described by the group at the University of Sofia (40,124,125). An empirical method of quantitative analysis is given. Other topics related to instrumentation which have been described in the literature include a low-temperature mechanical press for high pressure experiments (3381, a special cell for studying catalysts and small particles in the temperature range of 78-725 K ( 5 5 ) ,and an apparatus for investigating single crystal surfaces by Mossbauer effect emission (11).
Several laboratories are involved with proposing or experimenting with the use of synchrotron radiation as a source for Mossbauer spectroscopy. T h e group a t the Kurchatov Atomic Energy Institute in Moscow has roposed the following experimental procedure for observing 7p Fe resonant transition (13): After a preliminary monochromatization of the synchrotron radiation using Ge monocrystals, the coherent background is suppressed by using a purely nuclear reflection followed by a reduction of the noncoherent background using a n analyzing crystal. Some preliminary experimental results have been obtained. They also discuss the time dependence 176R
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of delayed radiation which undergoes resonance Bragg scattering (157). In another paper Cohen, Miller, and West ( 5 7 ) report to be the first to observe nuclear excitation using synchrotron radiation A very small resonance effect was observed by scanning their X-ray monochromator over the nuclear excitation energy. From theory Trammel1 and Hannon (311) discuss the quantum beats that will be produced when highly collimated pulses scatter on very small enriched single crystals a t the Bragg angle. Mossbauer spectroscopy continues to be a widely used technique for investigating implantation. Recently, using 57Fe, these included studies on implantations into Si, Ge (177, 1781, Cu-Fe alloys (152, 197), Ag-Fe alloys (198),and A1 (276). A number of other isotopes have also been used. Xe has been implanted into diamond (323),Mo, W, T a (264),and Fe (263). An on-line Mossbauer system studying for 83Kr into A1 at low temperatures is described by Gutlich and others (131). Cs has been implanted into diamond (324)and iron (245). Others include T e and D into iron and nickel (79, 342), I’IEu into iron (231),and Sn into T e and fcc metals (236, 266). Mossbauer spectroscopy studies of matrix isolated species provide data which can be used with good available models to obtain nuclear parameters and/or electronic state information. Dyson and Montano (91)have studied isolated FeMn molecules. The type of molecules formed was identified along with their electronic ground states. In another study monomer and dimer halide molecules of iron, tin, and europium were formed and investigated (190,191). Montano and others (221) have measured the effective internal magnetic field at the jSFe nucleus in monovalent iron in its 6D state. Specifically they were able to uniquely determine the ground-state Krammer’s doublet. While the usual matrix for isolation is one of the rare gases, Pasternak and Barrett (243) have used the matrix isolation method to study the phase transition in methane. They warn that one must be careful, however, that chemical reactions do not take place between the Mossbauer isotope and the matrix where rare gases are not used (23). Other additional information can be obtained by using single crystals. For example, Gibb (113)discusses procedures for determining the orientation of the electric field gradient (EFG) tensor. Only the sign of the EFG is sought in more simple situations (108);other situations are more complicated because both the magnetic and EFG axes are sought (127). Often magnetic fields are applied to the single crystal to assist in the determination of the magnetic structure (2, 160). Spiering (293) describes a systematic procedure for evaluating the EFG from single crystal measures and large magnetic fields. M$rup and others (223) have observed the effect of external fields up to 85 kCr on spin-spin relaxation in a single crystal. In another case the behavior of domain structure during magnetization of thin single crystals has been studied (94). Polarization effects have been reported in single crystals of FeC!2-4H20 (294). While most of the work has been done using 5‘Fe, Ernst et al. (97)have determined the EFG’s of Os in metals of Os and Re using the 137 keV resonance transition in lE60s. Scattering experiments in which the scatter does not contaln Mossbauer atoms are possible and they provide useful data. For example, Soltwisch et al. (291) have determined. the structure factor for glycerol. They also have obtained information on molecular motion of glycerol as a function of temperature (95). Other substances studied have been polyethylene (307),copper and aluminum (2081, and alkali halides (209). Kovalenko et al. (172) report observing experimentally, for the first time, a connection between the polarization characteristics of coherent Mossbauer scattering and the magnetic structure of the crystal. One of the simple ways of studying phase transitions of any sort is by using a constant velocity spectrometer set on an appropriate absorption peak. When the temperature is varied and the counts are plotted vs. temperature, a phase transition will cause an abrupt change in the plot. Chien et al. (61) have used this method to determine the magnetic ordering temperatures of several amorphous iron-boron samples. Gupta et al. (128)have used the technique effectively to observe the magnetic transition in Fe2Mo04. Descriptions of a number of miscellaneous experimental set-ups have appeared in the literature during the last two years. One of these is a proposal and demonstration of a
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John G. Stevens is a professor of chemistry at the University of North Carolina at Asheville He received his €3 S (1964) degree in chemistry and Ph D (1969) in physical chemistry from North Carolina State University. He has soent three summers at Aroonne National Laboratory, a leave of absgnce at MaxPlanck-Institut fur Festkorperforschung and a year-leave of absence in addition to two summers at the University of Nijmegen His main research interest is Mossbauer spectroscopy and its applications to the study of antimony compounds and minerals His additional interests are with the problems of evaluation and dissemination of scientific data and information He is the head of the Mossbauer Effect Data Center from where he co-edits the Mossbauer Effect Reference and Data Journal He is on the executive board of the International Commission for Applications of the Mossbauer Effect (ICAME) He is a member of the American Chemical Society American Physical Society Sigma Xi Sigma Pi Sigma and the Chemical Society Lawrence H. Bowen received his 6 S from Virginia Military Institute in 1956 and the Ph D in physical chemistry from the Massachusetts Institute of Technology in 1961 He has been at North Carolina State University since September of 1961 and is now professor of chemistry His research interests include radiochemical techniques in analytical chemistry and applications of Mossbauer spectroscopy, in particular to inorganic and organometallic compounds of antimony and to iron oxides in soil He is a member of the American Chemical Society, American Physical Society, and Sigma Xi
troprusside. They have studied the effect of absorber thickness and polarization of the absorber. Other investigations on line shapes include the effect of an external periodic perturbation of ultrasonic frequency on a source (316), the effect of the dispersion parameter in high precision data of 237NpMossbauer spectroscopy (14),and the effect of inelastic scattering of the order-parameter fluctuation near phase transitions (346). In recent years there has been an increased interest in texture, an observable phenomenon of Mossbauer spectroscopy which results from preferred orientation of microcrystals. Nagy (228) considers texture caused by deformation and also suggests a method for preparing texture-free absorbers. All too often in Mossbauer spectroscopy errors are underestimated because individuals use computer calculated errors, the programs for which w e a least-squares method with weighting factors that are inversely proportional to the dispersions of the value being measured. Gavrilov and Zemskov (109) consider this problem and suggest an alternate treatment of the data which considers the velocity errors due to nonlinearity. velocity noise, etc. Two papers on computer fitting of Mossbauer data are worthy of mention here. Lagunov and Polozenko ( I 75) describe an algorithm and program written in .ALGOL-60 which is very flexible allowing for a sum of Lorentzian or Gaussian lines. A more sophisticated program is reported by Ruebenbauer and Birchall (268) which has very general applicability. Its main features include the use of transmission integral procedures, a full Hamiltonian diagonalization for arbitrary spins, a magnetic field distribution analysis to the first order, a single or polycrystalline intensity calculation for arbitrary transitions, and the options for including Goldanskii-Karygin or texture effects.
THEORY method for cbtaining Mossbauer spectra when the source and the absorber are a great distance from each other. That is, Pound and Vetterling (255)have used guides for ?-rays which nearly completely reflect -,-rays when the grazing is quite small. Other experiments have included a Mossbauer polarimeter (1381, ultrasonic excitation (219, 220). and an ultrafast shutter for resonant ?-rays (12). In another experiment, called "quantum beats of recoil-free radiation", Perlow (252) frequency modulates a source of 5 i C ~ by vibrating it with a piezoelectric crystal so that one of the lines of the resulting multiple emission spectrum is absorbed. The spectrum remaining is time dependent and its harmonic composition and relative phases are sensitive to small energy shifts.
SPECTRAL ANALYSIS In two recent papers (70, 71) Dattagupta develops the stochastic theories of line shape for interpreting data from time-dependent hyperfine interactions. In particular. he treats the case in which the frequency of the radiating system is comparable to the rate of fluctation in the surrounding bath. The theory developed is much more general than previous ones. The solutions are given in terms of a transition matrix which contains information about the random properties of the Hamiltonian. In another area of relaxation, Belozerskii (28,29) has continued his experimental and theoretical studies of microcrystals and superparamagnetism. In these current papers he considers the effects of the particle size distribution on the spectra. Also considered are two determinations, Le., the anisotropy energy and the diffusion coefficient of the vector of magnetization. T h e effect of diffusion on the Mossbauer line shape is calculated analvticallv bv Bender and Schroeder (32). Thev have considered the particular case of diffusion via vacancies. In another paper Kucera and Kucera (173) evaluate previous Mossbauer line diffusion broadening by an exponential fit. As Mossbauer spectroscopy continues to mature, the wide variety of phenomena that affect the line shape are being more clearly understood. For example, Pearson and Williams (247) have considered the result when a quadrupole-split powdered absorber is situated in a larger external magnetic field. Spiering and Witzgall (295) recently considered sodium ni-
One of the major concerns of recent theoretical papers has been what to expect for line shapes when the -/-ray is polarized. Most of these are scattering type geometries. Banerjee (20) investigates the theoretical results when there is relaxation and polarization. He proposes that such cases will result in additional Mossbauer lines, the analysis of which can be useful in obtaining information on the dynamical processes in an atomic system. The discussion of this newer paper builds on an earlier paper in which Banerjee and Blume (21) consider polarization analysis of emission lines in the presence of relaxation. Daniels (67) considers the more specific problem of determining the spin arrangements in magnetic materials by using polarized y-rays. Another specific applications area is that of conversion electron Mossbauer spectroscopy in which the y's are polarized ( 2 4 2 ) . Hartmann-Boutron (137) continues her work on the use of Liouville relaxation supermatrix R derived by the method of the equation of motion of the density matrix and the supermatrix R' obtained by the resolvent method. The range in which both of these are valid is explored and an analytical expression is derived for the particular case of I7'Yb. Spinspin relaxation and its effect on the Mossbauer spectral line shape is further investigated by Afanas'ev and Onishchenko ( 5 ) . They have obtained a single function for systems of cubic symmetry and 2-0 nuclear transitions in which it is possible to obtain the frequency dependence of the relaxatior? functions. More attention continues to be placed on the understanding of Mossbauer scattered radiation. Several developments have been discussed at the beginning of this section, but two others are also noteworthy. Meisel and Keszthelyi (213) have determined relations for the hyperfine fields from perturbed angular distributions of the scattered Mossbauer radiation. As an application they discovered that the quadrupole interactions are the main causes for the line broadening in stainless steels. In the other study Bashkirov and his group (24, 25) investigated y-ray scattering in which double resonance was considered. The double resonance is the combination of Mossbauer resonance with the nuclear magnetic or quadrupole resonance. Particular problems arise when the frequency of the rf field is equal to the Larmor frequency of the nuclear spin in the excited state. Several papers consider the effects of lattice anharmonicity. Goldstein and Peierls (115)have developed a model for low temperature. Matsushita and Matsubara (21I ) consider ANALYTICAL CHEMISTRY, VOL. 52, NO. 5, APRIL 1980
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metallic fine particles. Roy et al. (267) investigated the expected effect of anharmonicity of the second-order Doppler shift. Other theoretical developments discuss the determination of the interference effect in the Mossbauer spectra by E2 transitions (250), the result of the acoustic modulation of y radiation (270), t h e effect of inhomogeneous ultrasonic modulation of y radiation (218), the effect of magnetic Brownian particles on the line shape ( 9 2 ) ,and the development of a method to determine the sign of the hyperfine interaction through combination of the Mossbauer effect and time-dependent perturbed angular correlations techniques (3).
LATTICE DYNAMICS T h e temperature dependence of the various Mossbauer parameters may be related to the dynamic properties of the solid state lattice. These include primarily isomer shift and recoil-free fraction, f, variations. de Grave et al. (74) studied these in two compounds of the spinel series Fez(]- ,Mgl+,Ti O4 having Fe3+ in both octahedral and tetrahedra( sites. TAey found the Debye temperatures for the two sites were identical within experimental error, but differed for the two compounds with y = 0.8 and 0.9. Kobeissi et al. (168) used both source and absorber experiments to search for isomer shift anomalies around the Curie point of iron. In contrast to some earlier work, no magnetization effects were observed in the isomer shift. Friedt et al. (106) studied the 237NpMossbauer effect in ternary oxides of Np(VI1). In addition to isomer shift and quadrupole coupling data, they obtained the detailed temperature variation of peak area for CsNpO, and fit this to a Debye model. Friedt et al. (104) also studied 237Npemission spectra following a decay of 241Amin a variety of matrices. A single resonance was observed in the metallic hosts, whereas several different valence states occurred systematically in oxide hosts. The temperature dependence off in Am metal obeyed the Debye model. Sitek et al. (287)determined the Mossbauer parameters for lI9Sn in Nb3Sn, Nb6Sn5, and NbSn, for application in quantitative analysis of superconducting Nb-Sn systems. The NbaSn had an appreciably higher Debye temperature. Harrison et al. (136) related the fvs. T data to X-ray structure for a tetraphenylporphinato tin(1V) complex. The crystallographic data were used to obtain absolute f values. A substantial Goldanskii-Karyagin effect was observed and analyzed in terms of vibrational amplitudes. Herber and Katada (139)studied lattice dynamics in iron chloride-graphite intercalation compounds. The Debye temperature was lower than in the pure iron chlorides. The ferrous compound had two distinct doublets, one of which did not vary with angle in single crystal experiments. LaPrice and Uhrich (179)obtained unusual temperature dependence off in a nematic glass and found evidence for rotational diffusion of diacetylferrocene in the supercooled nematic phase. Weiss and Langhoff (337) used the 58 keV Mossbauer transition from Ij9Tb to probe the one phonon localized modes in TbO,. Using an ultracentrifuge for velocity scan they found sharp resonances a t 12.3, 14.1, and 16.0 meV, and fit these to a model with localized Einstein terms in addition to the Debye lattice. Herber and Smelkinson (140) prepared and characterized some Sn(I1) complexes with crown ethers. The compounds with C1- or NCS- have two tin sites, but with C104- a very positive isomer shift and sharp singlet was found with 18crown-6, indicative of a n almost bare Sn2+ ion. The temperature variation off for this latter compound was much more drastic than for the compound without the crown ether. Viegers et al. (330)studied lattice dynamics of gold complexes and described a model for the data in terms of inter- and intramolecular vibrational modes. The difference between Au and S n lattice dynamics in molecular complexes was indicated, as was the difference between Au(1) and Au(II1) complexes. Von Eynatten and Bommel (334) studied the f vs. T behavior of iron microcrystals. Two possible explanations for the shape of the curves have been proposed-either oscillations of the mcrocrystals or changes in the surface phonon spectrum. Darlington et al. (69)measured elastic and inelastic scattering of j7Fe Mossbauer y-rays from ammonium fluoroberyllate to show that the phasons of the incommensurate phase do not make large contributions to the DebyeWaller factor. Elastic scattering disappears above the phase transition a t 181 K. Vereshchak et al. (327) studied iron 178R
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oxalates irradiated with y and n. The f value of ferrous oxalate produced by radiolysis from ferric oxalate was higher than the chemically prepared compound. These results are independent of the nature of radiation. Nascimento and Garg (230) studied the lattice dynamics in FeS04.4H20 single crystals as function of both temperature and crystal orientation.
AMORPHOUS MATERIALS The Mdssbauer effect, dependent primarily on short-range order, is useful in the study of amorphous substances. Among these, materials of particular interest have been metallic glasses and silica glasses. Among the former, magnetic properties have been of primary concern. Chien and Hasegawa (50) studied the temperature dependence of the magnetic field in a number of iron-containing glassy ferromagnets and found the T3/* coefficient to be especially large compared to crystalline materials, indicative of spin-wave excitation. Bayreuther and Lang (27) used conversion electron detection to study the order-disorder transition in thin films of Fe-Co. Commercial metallic glasses were studied with Mossbauer spectroscopy by Franke and Rosenberg (102) and by Gonser et al. (116). In both these, the distribution of hyperfine fields was noted. The former work also studied the effect of heat treatment near the crystallization temperature. The latter authors noted the similarity of field distribution to a liquid structure analog. Vincze and co-workers also studied several aspects of the magnetic field distribution in amorphous metallic glasses. The comparison between Fe,oNi,oB20and F e d , , (332) shows the effect of Ni substitution is different from that of crystalline alloys. The temperature dependence of field in a commercial sample was determined and interpreted (17). Also Vincze (331) has given a general method to evaluate field distributions in such samples, using the polarization of a small magnet. Litterst et al. (188) determined fields in noncrystalline iron metal as function of temperature for various methods of perparation and noted the nonrandom effects on the hyperfine field from heterogeneity. Tseng e t al. (317 ) determined the effect of annealing temperature on the hyperfine field of a metallic glass, finding a strong polarization perpendicular to the ribbon plane a t high annealing temperatures. Fischer et al. (99) studied the polarization effect of an uniaxial tensile stress, using polarized y-rays from a-Fe. In the general area of silicate glasses, Jach and Nabatian (148) studied the reaction between iron filings and sodium disilicate glass at various temperatures. They found that the reaction in air shows distinct effects of the glass on the oxidation of iron. Iwamoto et al. (147) made a detailed Mossbauer study of calcium-iron-silicate glasses, including a determination of the structural sites of iron as function of CaO content and O2 pressure. Mysen and Virgo (227) used Mossbauer spectroscopy to study melt structures in Na(A1,Fe)Si206as functions of pressure, temperature, and composition, and suggested geochemical implications of their results. Varret and Naudin (325) proposed and tested a method to assay quantitatively the Fe2+/FeJ+ratio in silicate glasses if the iron is dilute enough to cause paramagnetic structure in Fe3+ a t 77 K. A number of studies have been reported on borate glasses. Raman et al. (258) measured room temperature spectra of a large number of glasses with Bz03,Fe203,and varying amounts of Na20 and A1203, as well as other alkali metal oxides. Strong variations in isomer shift were correlated with changes in coordination for Fe3+. Also the isomer shift vs. T was used to estimate local specific heats. Sekhon and Kamal (279) studied Bz03and Na20.2B203glasses with small amounts of Fe203. In one of the B203glasses an Fez+ state was found. The magnetic properties of the Fe203differed in the two types of glass. The Pb0.3B2O,-Fe2O3 system was studied by Burzo and Ardelean (43). In this case all iron was in the matrix even for 50% FezO . Both tetrahedral and octahedral coordination were observed for Fe3+, independent of concentration. Syono et al. (302) measured Mossbauer spectra to 4 K in the BaOFe203system with a small amount Bz03. Low temperature magnetic ordering was observed and interpreted as short-range antiferromagnetism. Among other glasses without B20, or Si02, Laville e t al. (181) found low temperature magnetic ordering in a new homogeneous glass of the system BaOFe203-Na20.
van Diepen and Popma (321) studied the temperature dependence of the field in amorphous Fe203. Unlike the metallic glasses, a very slow change was found which fits a spin Brillouin function, even though the field at 0 K was more than expected from spin s/2. Gyorgy et al. (134)report the Mossbauer spectra of amorphous Y3Fe5012. The 4 K spectra show antiferromagnetic ordering with a single field different from that observed in crystalline Y3Fe5012.Among amorphous systems not containing iron should be mentioned the tin oxide reported by Collins et al. (63)and As2Te, Isex studied by Jones and Tse (156). Also, Bowen and Long (42) used orbital population analysis of the lZ1SbMossbauer parameters to postulate structures for some amorphous stibonic and stibinic acids.
CATALYSTS AND SURFACES T h e field of catalysts is quite active and Mossbauer spectroscopy continues to make important contributions in this area. The kinetics of CO reduction of NO with an Fe203/A1203 catalyst has been shown by Sokol'skii et al. (289) to be first order in Fez+concentration, using the Mossbauer effect for determining Fez+. Amelse et al. ( 8 )studied the carburization of SO2-supported iron catalysts at various stages of oxidation-reduction and after use in hydrocarbon synthesis. Dezsi e t al. (80) studied Si02-supported Pt-Fe catalysts with different iron concentrations with respect to redox behavior. Ludwiczek et al. (200) studied ammonia catalysts of the A1203-promoted type by a variety of techniques, including Mossbauer spectroscopy. Prasada Rao and Menon (257)also used many spectroscopic and other physical techniques, including Mossbauer, to study the multielement molybdate catalysts for ammoxidation of propylene. Carbucicchio (45) reports Mossbauer studies on supported Fe203-Mo03catalysts with emphasis on the change in iron state due to interaction with the support. Maksimov et al. (204)determined the effect of bismuth on structural changes in Co-Mo-Fe catalysts used in oxidation of propylene. In a series of papers, Raupp and Delgass- report Mossbauer spectroscopic results on their studies of supported Fe and Fe-Ni catlaysts. First the effect of pretreatment on particle size (260) was studied primarily by the magnetic properties of the alloy. Then two papers followed dealing with Fischer-Tropsch synthesis: a study of the types of carbide phase formed on the catalyst (261) and a paper dealing with the use of constant velocity Mossbauer spectroscopy to determine rates of carbiding of the catalyst during the synthesis reaction (262). Birchall and Sleight (38)reinvestigated the catalytic oxides USbO, and USb3010by lZ1Sb Mossbauer spectroscopy. In contrast to previous work, they found no evidence of unusual covalency in the Sb-0 bonds. However, the two compounds did exhibit a measurable difference in isomer shift. If scattered conversion electrons are counted, Mossbauer spectroscopy becomes a technique to study solid surfaces. In addition to the more common iron and tin isotopes, this technique was recently applied by Salomon et al. (272)to the study of tantalum metal surfaces using lalTa. Foils had broader lines than single crystals due to adsorbed gas. Huffman and Dunmyre (143) used lI9Sn to study surface properties of tinplate, showing that quantitative thickness determination can be made for oxide coatings up to several hundred angstroms. Sedunov et al. (278) used scattered radiation to study surface composition when steel is carburized or carbonitrided: depth selection was effected by counting the scattered y-rays (-1 fim depth) and separately the scattered electrons (-0.1 fim depth). Huffman and Stanley (145) used similar techniques to study Si-sheet steel during annealing in Hz-Nz-HzO atmospheres. Among other types of experiments related to surface properties, Anderson et al. (10) d e ~ o s i t e d . ~ ~on C oan ultraclean Ni surface and obtained Mossbauer emission spectra, using Auger electron spectroscopy to determine surface contamination. Shechter et al. (282)used transmission Mossbauer spectroscopy to study (in stacks) adsorbed monolayers of butadiene iron tricarbonyl on graphite. Measurements perpendicular and parallel to the film plane were made, and the variation in peak area with temperature was analyzed in terms of diffusion within the film. Among the more applied areas of surface studies by Mossbauer techniques is corrosion. Blesa et al. (39)used room temperature spectroscopy to characterize the iron oxides in synthetic analogues of nuclear reactor corrosion products.
Ensling et al. (96)used conversion electron detection to study corrosion of steel under boiler conditions. The kinetics of oxide growth were determined. Peev et al. (249) found two distinct iron corrosion products, magnetite and goethite, in the anion exchanger of an industrial desalinizer. Shibuya et al. (284) studied acidic corrosion on tinplate both by conversion electron and transmission 119Sn Mossbauer effect. There has been a variety of Mossbauer studies of thin films. Some of these have been mentioned earlier. Others of particular note include the study by Bando et al. (19)of iron oxide films prepared by evaporating iron in low pressure oxygen environment, and the work by Haneda and Morrish (135)on surface oxidation of small particles of iron prepared by an aerosol method. van Diepen et al. (322)also studied the oxide layer on iron particles, in this case passivated in 0 2 / N 2 mixture. Several papers have appeared dealing with conversion electron scattering experiments on iron oxide surfaces. Graham et al. (119) studied magnetite films on iron and quantitatively related the spectrum to film thickness in the range up to -3000 A. Tricker (312) reevaluated the calibration for thickness determination in iron oxide layers, based on attenuation studies of conversion electrons. Tricker et al. (313) have presented a method to distinguish signals from substrate and surface overlayers in back-scattered 57Feradiation. They used a He/CH, flow counter and evaporated a thin layer of gold on the surface to enhance the photoelectron signal from deep-scattered y-rays. Duncan et al. (87) studied the variation of magnetic field in thin iron films as function of temperature. For the thinnest films
