Moessbauer spectroscopy

Anal. Chem. 1982, 54, 204R-216R

Mossbauer Spectroscopy

John G. Stevens* and Gljs H. M. Calls’

Department of Chemistry, University of North Carolina at Asheville, Asheville, North Carollna 288 14

Lawrence H. Bowen

Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27650

The field of Mossbauer spectroscopy will celebrate its 25th anniversary next year. Indeed, this particular area of science has had phenomenal growth since the original paper of Ru- dolph L. Mossbauer (203). The diversity of its applications has been reviewed extensively over the years; in the series published biannually by Analytical Chemistry, this particular review is the ninth. The results of Mossbauer spectroscopy continue to contribute to our understanding of general areas such as mineralogy, biology, physics, and, of course, chemistry. During these last 2 years new areas of study have included grafoih, intercalation compounds, and layer compounds. Also new has been the study of the effect of laser annealing.

This current paper covers the literature surveyed since our last review (280). Consequently, we have considered articles from the latter part of 1979 toward the end of 1981. Ap- proximately 3000 scientific articles on the Mossbauer effect were examined, and again, we have had the difficult task of selecting less than 15% of these published articles for inclusion here. We note during these past 2 years a rapid increase in the number of papers coming from both India and China. It is obvious that these two countries have selected Mossbauer spectroscopy as an area in which they can do significant research, in spite of the contrained resources available to their sciences. Elsewhere, internationally, the level of research in Mijssbauer spectroscopy has remained approximately the same during the last 2 years, although there continues to be an increase in the industrial use of the technique. This is especially true in the areas of its application to the studies of catalysts, surfaces, and alloys.

In all previous reviews, we have reported observation of new Mossbauer transitions in the literature; this is the first review in which we have none to report. As in the past, 57Fe continues to be the most widely used isotope for Mossbauer studies. There remains an unlimited number of potential experiments available to research groups throughout the world. While ll9Sn continues to be widely used, difficulties have been experienced recently in obtaining sources for this particular Mossbauer isotope. Three years ago, there were three principal companies supplying l19Sn; now, however, there is only one commercial company able to provide this source, and even with this there has been some difficulty in providing the source characteristics that are required for many experiments.

It is encouraging that a number of other isotopes continue to be widely used by a number of research laboratories. The two most actively used are 151Eu and lmI; close to 100 studies have been reported during the last 2 years using these two isotopes. The followin isoto es have had between 15 and 25 pa ers each: Ig7Au, 6 % lDy, Er, lSSGd, 237Np , 9 9 R ~ , 121Sb, and g5Te. There continues to be increased interest in Mossbauer lanthanide isotopes. Recently there have also been several papers using the very high resolution transition in 67Zn. Many of these isotopes are discussed in sections below.

Publishing books in Mossbauer spectroscopy has been an area of noticeable activity since the writing of the last review. I. P. Suzdalev wrote a 192-page Russian book titled “Dynamic Effects in Gamma Resonance Spectroscopy” (290). In German there is the 500-page book by D. Barb entitled “Grundlagen and Anwendungen der Mossbauer Spektroskopie” (30). The

Present address: Dutch State Mines Laboratories, Geleen, The Netherlands.

other five books are all in English and include a most interesting one entitled “Mossbauer Spectroscopy: The Exotic Side of the Method”, edited by U. Gonser (128). Two of the books are parts of continuing series. A. V6rtes wrote Volume 5 in the series “Studies in Physical and Theoretical Chemistry”, with the title of his volume being simply “Mossbauer Spectroscopy” (314). The other, edited by R. L. Cohen, is Volume 2 of the series on “Applications of Mossbauer Spectroscopy” (74). Stevens and Shenoy edited a book entitled “Mossbauer Spectroscopy and Its Chemical Applications”, a 642-page book containing selected papers from a symposium held during the 179th Meeting of the American Chemical Society in Houston (281). The last book to be noted is the proceedings of the International Conference on the Mossbauer Spectroscopy, held in Portoroz, Yugoslavia (303). During the most recent international Mossbauer conference, just held this past December in Jaipur, India, approximately 400 papers were presented. It is expected that these proceedings will be published sometime during the latter part of 1982.

The Mossbauer Effect Data Center is now into its fifth year of publishing a monthly information jounral entitled Mossbauer Effect Resonance and Data Journal. The Center continues to be supported almost entirely by the research groups in the Mossbauer community. There are a number of services that the Data Center provides, including a series of specialist Mossbauer handbooks on surface studies, amorphous substances, 12BI, instrumentation, and catalysts (282-286). The facilities at the Data Center were once again used extensively in preparing this review.

Mossbauer spectroscopy continues to be one of the most reviewed subjects in science. Since our last review in this journal, we have received approximately 150 reviews, most of them on very specific applications. It is not really possible to mention all of these in this paragraph, but we will mention some of the more extensive ones in which readers might be interested. Many of the reviews emphasize the industrial applications of Mossbauer spectrosocpy. These include reviews on surface-treated steels (149), catalysts and surfaces (296), electrochemistry (315), surface chemistry (242), and catalysts (95). Reviews on applications to materials include: diffusion in solids and liquids (116), phase analysis in metallic systems (131), oxygen transport and storage materials (276), impurities and ion implantation (92), and iron ion implantation (257). Reviews that pertain to Mossbauer methodology include scattering (67), grasers (126), y-ray polarimetry (129), and conversion electron spectroscopy (297). A variety of other applications reviewed include studies in the following fields: physiological and medical applications (98), clay minerals (133), and characterization of coal (152). Other subjects reviewed are Mossbauer studies of rare earths (32,270), orga- noneptunium compounds (87), mixed valence compounds (55, 59), synchrotron radiation (75), and magnetic microcrystals (201). Electron densities at the nuclear center and surfaces for Mossbauer atoms were calculated by using relative Di- rac-Fock equations (24). Other papers include a general one on materials science (192), the use of Mossbauer spectroscopy as a radioanalytical method (127), and an extensive review, primarily of the 1977 Mossbauer literature (100). We also would like to mention an excellent general paper on Mossbauer spectroscopy written by Friedt and Danon (121). Several other pertinent reviews are mentioned in the sections below.

204 R 0003-2700/82/0354-204R$06.00/0 0 1982 American Chemical Society

M~SSBAUER sp~cmoscop~

troscopy (CEMS), there have been some other developments worth mentioning. Kashiwase and Minoura (165) have designed a position-sensitive detector that can be used eapeeidy for y-ray diffraction studies. I t has a counting efficiency of 65% and the space resolution is 0.25 mm. They use the detector to separate succegsfully the inelastic background from the 200 Bragg reflection of a lithium fluoride crystal. In another paper, Bara and Bogaa (29) describe a pulse-shaped discriminator which uses electronic blocks. This scheme ef- ficiently reduces the background produced in the proportional counter by high-energy y-rays, and, consequently, results in an increased observed magnitude in the Mossbauer spectra. Numerous improvements in detecting systems for CEMS have been reported during the last 2 years. These improvements allow for expanded applications of this technique.

The temperature range in which CEMS can be done has been greatly extended. Sawicki et al. (258) describe a proportional counter for CEMS at liquid helium temperatures. For high-temperature studies there have been several reports in the literature. Inaha and others (154) describe a conversion electron counter that can he used for temperatures up to 750 K. In 1979, Isozumi et al. (156) reported a CEMS detector capable for making measurements up to 560 K. Recently they report a detector capable of measurements approaching 1200 K (157). At such high temperatures one can directly observe the surface phenomena of various kinds of reactions or study the diffusion of iron into metals and alloys.

There have been several reports of modifications to the basic detector which improve count rates and signal-tonoise ratios. Andersen and Walker (8) describe a helium-filled detector which detects recoilless scattered y-rays by using conversion electrons from an enriched thin stainless steel foil (3000 A). This detector is used to study recoilless Rayleigh scattering from a very soft crystal of barium titanate, an impoasihle task using a conventional Mossbauer setup. Tylisznak et al. (301) built a conversion electron detector which uses electrons with energies below 10 eV. Such a detector greatly improves the capability of making studies of the surface of materials. Of particular interest is the study of surface magnetism. An especially interesting detector is one described by Parellada et al. (223). Their detector, which uses an electrostatic cyl- indrical mirror analyzer, is able, by selectively exciting Mossbauer hyperfine transitions, to produce conversion electrons from states of known spin orientation relative to the internal magnetic field a t the nucleus, and, in addition, can measure simultaneously electrons originating from three different atomic shells.

While almost all of the work that has been reported so far has been with either 37Fe or %n, Shenoy et al. (272) r e p r t successful CEMS systems for the Mosshauer isotopes ' 'Eu and 'a. For a number of the europium-based compounds an enhancement of the resonance effect is reported by using the conversion electron technique.

There are several papers in which the depth selection capability of CEMS is described. Herein lie some of the more unique applications of Mhshauer spectroscopy. With the depth selection Capability, one is able in principle to obtain Mosshauer spectra of particular nuclei that are situated a t a particular depth within the material. Yang et al. (331) report on the depth dependence of the hyperfine interactions in Fe 203 films. In another study Shigematau et al. (274) performed a series of measurements by using a high-resolution electrostatic electron spectrometer to observe sharp changes in the Mosshauer spectra. They obtained spectra from various depths in the materials by using very small energy ranges of the Mossbauer electrons that come from the surface of the sample. Grozdanov et al. (134) report their study of proceases that are related to the passage of low-energy electrons through matter. Their study should aid in the understanding of depth selection Mosshauer spectroscopy. Domke et al: (99) describe a nondispersive electron spectrometer for depth selection conversion electron Mosshauer studies which operates in the lo-'' torr range. Such a spectrometer can he used to study clean surfaces in ultra high vacuum. As with several of the other more recent CEMS systems, a channeltron is used.

There have continued to be improvements in instrumentation for obtaining Mossbauer spectra a t both ends of the temperature range. Herbert et al. (144) describe a very ef- ficient continuous flow cooling unit which can cool independently the source and the absorber in the temperature

ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982 205R

JOm Q. st- Isa p0tesSaofdam)sby at me univaally ot Nom Car- a1 A h + vib. He r&ed Na B.S. (1984) Wee in chsrnisby and Ph.D. (1969) in physical dsrnlmy from Ncdh Carollna stale Univer- ally. He spent three w r n m at Argonne N a t h l Laboratory. a leave of absence at MBxpIBnck-lnslbR Kr Festkixperfaschung. and a year leave of absence in addillon to fw summers st me Universlly of Nil-n. HIS main research interest is MBssbauer S p e a r O s ~ Y and ns BPPlicatlon to the stw

addnbnal interests are with problems of of antimony mmpOUnds and mlnwab. Hb r,;., :j 1 evaluation and dissemination of s~lent i f l~ ' ' data and infarmation. He Is head of WssbBw Effect Dale Center lrom whlch he coedils me h46sbeusr Effect Reference and &la Jamal. He ls on me executive h a r d Of the InlernaWnal Commbsh IM Applica- tbns of lha Mossbauer Effect (ICAME). He is a member 01 the Ametlcan CbmICBi Saclety, A m w i ~ m Physical Saclety. S w a Xi. Sbma Pi Sigma. and me Chemical Society.

GIB n. M. Calls received hb wee In Chemiary (1977) at the Universlly of Utrecht, The Nelhsriands. and his Ph.D.

ty of N l jmpn. The Nehr!anda. His meSb

sbcbon spln behaviw and includes studles on magnetlc phase bansnbns and electron

(1981) in FiQalcai chenavy at the univasi-

deals wlth Mossbaw 51udlBs On dymmic

spin reiaxatb". At Nlj-n he cooperated with several research grou s investigating M o L I w l d mwunda bV "Au &sbauer L

I &~trd;mpy. He spnilast winter at me UnNersity of N m h Carolina at Aeheviile. Since February 1. 1982. he has been a r e search associate st me Dutch State Mines Labxataries st Geleen. The Nelherlands. He is a member ot the Dutch Chemical Socisty (KNCV)

L a w r ~ . H. Bowen Is a profspsa of bemlstry at Nom Carolha state Unhwsily. He receked hls 0,s. horn the Vtginia Milk tary Ins1111110 in 1956 and mS Ph.D. in physi- mi chemisby from the MB-Ch-ns Insti- tute 01 Technoloay In 1981. He has k n on me Iacuny at Nom Carolina state Univ- srsny since September 1981. HIs remrch

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American Phvsical SOCieN. and

INSTRUMENTATION AND EXPERIMENTAL TECHNIQUES

The primary improvements to Mossbauer spectrometers reported have related to the expanded we of microprocessors and integrated circuits. Woodhams reporta further improvements on a microprocessor-controlled M a h a u e r spectrometer ( I & ) , which he had reported in earlier literature. Using an eight-hit microprocessor, he is able to improve the diversification and have a storage capacity of 16 X 1OI6 counts per channel. Bahikova (15) describes a M6sshauer system using integrated circuits in which the nonlinearity is leas than 0.1%. Still another spectrometer using integrated circuits is described hy Pushpalatha and Puttaswamy (240). One of the advantages of using either integrated circuits or microprocessors is the ease with which one can generate saw-tooth, triangular, trapezoidal, or sinusoidal wave forms. Corson (79) describes a method to improve the linearity and stability by a shape correction technique. By this technique, the feedback gain is reduced by a factor of 60 as compared to conventional drives. The simple adjustment that he describes can be made to many spectrometers. A method of correcting Mosshauer spectra for velocity-processing errors in the constant accel- eration mode is detailed by Nassonov and Gergeev (208).

While the primary developments in detectors recently have been for specific use in conversion electron Mossbauer spec-

MOSSBAUER SPECTROSCOPY

range of 4.2-300 K, while maintaining a low helium con- sumption. Low-temperature cryostats using 3He have been described by two groups, both of which have obtained temperatures down to 0.4 K for extended periods of time. Bogner et al. (45) describe a continuously operating 3He cryostat which uses a closed cycle Joule-Thomson refrigerator. The cryostat was especially made so that biological compounds can be loaded without raising their temperature above 100 K. In addition, it is possible to apply a transverse magnetic field of up to 1.5 T. The other low-temperature device is described by Calis et al. (62), in which a zeolite absorption pump is operated in a closed circuit using 3He. In their cryostat the temperature can be controlled to within 10.002 K. Their spectrometer can operate at 0.38 K for 15 h.

On the other end of the temperature scale, two interesting furnaces are described. Asenov and others (14) designed a rotating sample holder which allows heating of the absorber at any temperature in the range from room temperature up to 1500 K. The temperature instability at the high temperatures is less than f0.2 K. Dubois and LeCaer (101) designed a special Mossbauer furnace suitable for fabricating and studying amorphous alloys. The furnace allows for very rapid heat treatment and is especially useful for the study of the thermal stability of various alloys.

Several new computational techniques are described for fittin the Mossbauer data to theoretical spectra. These incluie a noniterative method for fitting Lorentzians (204), new possibilities for least-squares fitting (214), the use of the simplex iterative algorithm (275), and an improved computational procedure for overlapping hyperfine distributions (327). Le Fever (183) describes a complete data acquisition system for operating four Mossbauer experiments, all simultaneously and independently. Low cost microprocessors are used in real time for data collection. These are controlled by a minicomputer, which also performs the data processing. One of the more welcome developments in computer fitting is the work of Muller (205). He has developed a system of Fortran programs for fitting Mossbauer spectra. The system, which has been tested on a number of main-frame and minicomputer systems is relatively easy to learn, has safe and comfortable operating procedures, and allows for simple im- plementation of new theories. He has provided a very complete and comprehensive documentation. (This system of programs call ‘‘MOSFLTN” is available from the Mossbauer Effect Data Center.)

Theoretical work and discussion continue on the possibility of y-ray lasers, sometimes referred to as “grasers”. One of the methods proposed for achieving the laser action requires the pumping of the nuclear level by an extremely high neutron flux. Baldwin and Solem (22) determined the upper bound which exist on the density of neutrons that can be moderated to a specific energy from an intense pulse of neutrons. In another paper (21), however, they show that these maxima are insufficient for the direct excitation necessary to produce a y-ray laser based on the Mossbauer effect. Winterberg (325) suggests another possibility-a strongly magnetized plasma that can assume crystallike behavior due to the stiffness transmitted by the magnetic field. This would allow for observation of the Mossbauer effect at very high temperatures, a condition imperative for the extremely high neutron fluxes that are needed for lasing. Another possibility discussed by Vysotskii et al. (321) uses a modulated beam of randomly distributed relativistic electrons. They claim that the required flux density is possible when realistic parameters of the beam of the modulator and the accelerator are considered. Vysotakii (320) also investigates the possibility of a y laser using polarized nuclei of the lslDy isomer. In a more recent report (23), Baldwin and Solem discuss two-stage pumping of three-level Mossbauer y-ray lasers, but still conclude that the necessary neutron capture rates are unobtainable.

Observing Mossbauer spectra taken at different time intervals following a decay from the Mossbauer state enables one to obtain useful information on the processes that take place in a crystal in the time scale of nanoseconds. Koch and Realo (170) studied the relaxation processes following the electron capture of 61Co in a ZnS sin le crystal. Other materials studied in this way include Cob4.7H20 (37) and Fe- (CN)Z- (169). Besides looking at the time results following electron capture, Vapirev et al. and co-workers (312) report their results of an in-beam, time-dated Mossbauer experiment,

206 R ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982

in which 66Fe atoms are bombarded by 7-MeV deuterons. Spectra in this latter case were also obtained for various time intervals following production of excited 57Fe.

Some of the more unusual experimental arrangements using the Mossbauer effect follow. A y-ray polarimeter was designed by Daniels et al. (86) which consists of a magnetized foil and a single crystal of siderite. In their paper they describe procedures for determining the polarization of the y-ray and the calibration and the use of the resultant polarimeter. Dubovtsev and Filippova (102) studied the role of the surface force in the mechanism of ultrasonic generation in metals by an rf field. The relation between the ultrasonic intensity and the depth of films of different materials spattered onto an foil is described both theoretically and experimentally. In still another experiment Voitovetskii et al. (319) observed NMR Mossbauer double resonance using lalTa. Albanese et al. (3) measured the elastic and inelastic Rayleigh scattering in amorphous poly(methy1 methacrylate) and then studied temperature dependence up to the melting point of this material. In addition to intensity data, they also obtained the relaxation time. In another paper Martin and O’Connor (194) report a diffraction study that shows the release of energy during the annealing of aluminum. The experiment was done at the Bragg diffraction angle, and it was possible to separate the elastically and inelastically scattered components of the diffraction. Van Burck et al. (308) report on the angular and energy dependence of dynamic Bragg diffraction of y by a single crystal of FeBO,. Their measurements represent the first direct observation of the enhancement of nuclear resonance scattering in the case of a thick crystal reflection.

SPECTRAL ANALYSIS Related to the increasing use of minicomputers, some

programs have been published for the fitting of Mossbauer s ectra which require less memory and less computation time tgan conventional programs. Mukoyama and Vegh (204) describe a noniterative method for the extraction of data from spectra with well-resolved lines by transforming the Lorentzian line shape to an expression of linearly related quantities. A more general Fortran IV program is described by Nullens et al. (214); this program uses nonlinear relations between fitting parameters and x2 functions which take into account the uncertainties in the starting parameters.

The difference in chemical environment of Mossbauer nuclei can cause broadened s ectral lines, requiring certain assumptions for the distriiution of hyperfine parameters. A fast method for the analysis of these type of spectra is proposed by Whipple (324), using a Lorentzian-squared function. Levitz et al. (186) use an inverse matrix method on their spectra to obtain a two-parameter distribution map.

Several contributions facilitate the analysis of spectra of polycrystalline absorbers which have both magnetic dipole and electric quadrupole interactions. Pearson and Williams (226) derive an expression for the absorption line shape of such absorbers in large applied magnetic fields. Bowden et al. (51) tabulate useful statistical tensors for this case. The computation time of this type of spectra can be reduced by symmetry considerations, as is done by Hasselbach and Spiering (136).

Henry et al. (140) show an approximation of the area of the Mossbauer line which yields simple expressions for thickness and polarization corrections.

The applicability of any theoretical model to Mossbauer spectra can be tested with the help of some general criteria given by Daniels (85).

Some contributions deal with the analysis of conversion electron Mossbauer spectroscopy (CEMS). Liljequist and others in Stockholm published calculations of the total electron flux in such experiments before. The expression for the total electron flux is corrected in a more recent paper of Liljequist (188). Some modifications are also proposed by Deeney and McCarthy (94). They discuss the time resolution of common detectors with respect to the separation of K- conversion electrons and accompanying Auger electrons. The group in Stockholm also describes (189) an interactive method for the analysis of depth selective CEMS (DCEMS). Another approach for the analysis of DCEMS data has been reported earlier by Proykova (239).

Bara published a series of papers dealing with the analysis and some interesting features of Mossbauer scattering s ectra in several experimental geometries. In one paper 727) a


radiation. In this case sidebands are a result of multiphoton transitions. Analytical expressions for quantum beats observed with a fequency modulated source in a time-gated experiment were obtained by Monahan and Perlow (198). The experiments they describe can be used to measure a very small Doppler shift. Aleksandrov and Afanas'ev (4) give an expression for secondary processes of electrons accompanying the diffraction scattering of Mossbauer radiation, which can be used to analyze very thin surface layers. The theory of double y nuclear magnetic resonance is treated by Dzyublik (106) for the case that the Mossbauer line shape is affected by rf fields causing transitions between the sublevels of the excited state of a Mossbauer nucleus. The effect of electric quadrupole interactions is discussed.

comparison is given of the transmission and the scattering method, considering thickness, line width, and intensity effects. In another paper (28) it is shown that the line width in Mossbauer spectra m,ay be reduced to the natural line width using a triple resonance scattering geometry in which both the source and the primary scatterer are filtered by two other rigidly coupled scatterlers.

Various authors have developed new methods of analysis to derive physical quantities from Mossbauer spectra. Fotev and Ivanchev (119) determine the recoilless fraction from absolute areas of Mossbauer absorption lines. They measure the absolute areas as a function of the absorber thickness and fit the resulting data, thus yielding values for the recoilless fraction of the resonance and atomic absorption cross-sections. Kolk (171) proposes a method to determine the temperature-independent ratio of force constants of impurity-host and host-host interactions for isolated impurity atoms in cubic host lattices. Diffraction methods are appropriate to measure the recoilless fraction of' host materials, whereas this quantity can be determined more accurately by the Mossbauer effect for impurity atoms. Tsankov (300) studied the effect of acoustical modulations applied to source or absorber on the spectrum. In an earlier paper (299) a numerical method is presented to solve the inverse problem of deriving the dis- placement function from the spectrum. A paper on the same subject has been published by Gabrielyan et al. (122).

THEORY In a number of theoretical papers, calculations are given

of special line shapes in the presence of relaxation. The Clauser-Blume model considering stationary relaxation processes has been extended to include nonstationary processes as well. Banerjee (26) expects to observe mixing features in cases where both procemes play a role. In a short note (277) Srivastava points out that in spin-lattice relaxation processes in which the ionic spin fluctuates, AS, = f 2 transitions have to be taken into accoumt. Selyutin (265) discusses the case of a ferromagnet where spin-lattice relaxation causes the spectra to have the Gaussian line shape around the ordering temperature. Expressions for relaxation effects in time-dependent emission spectra, taking place after the decay of the Mossbauer nucleus, arle given by Afanas'ev (1).

Other theoretical papers are concerned with the effect of lattice properties on the recoilless fraction. Clapp (70) predicts a discontinuity in the recoilless fraction near a martensitic phase transition due to localized soft modes. For a Mossbauer nucleus embedded in a metallic host, the contributions to the recoilless fraction of the host lattice of the impurity and the impurity-vacancy pairs are discriminated by Tewari et al. (294). In a later paper ]Roy et al. (252) conclude that a model confined to the harmonic approximation alone does not explain considerable line broadening found in spectra of metals with impurity-vacancy ]pairs. This can be explained, however, by taking into account anharmonic corrections.

Pere udov (228) deticribes the influence of interference effects tetween photoabsorption and nuclear resonance absorption on Mossbauer absorption spectra for E l transitions.

A useful contribution on the optimization of the absorption thickness for Mossbauer experiments is given by Sarma et al. (256), who determined the signal-to-noise ratio as a function of the effective thickness for various values of the effective absorption coefficient.

Some fundamental theories dealing with parameters derived from Mossbauer spectra need to be mentioned here. Antoncik (12) calculated isomer 13hift values as function of the atomic size in several electronic configurations. He applied his theory to calculate the calibration constant, a, for the isomer shift of '19Sn. Bashkirov and Lebedev (33) discuss the case of an Fe3+ ion in a ferroelectiric compound. The weak coupling of the 4s shell and the core cause a polarization of this shell, and the effect on the hyperfine parameters is calculated. Davis et al. (89) consider atomic processes interfering with nuclear transitions in terms of a phase shift of the radiation. This phase shift affects the absorption cross-section, introducing an asymmetric line shape.

Many theoretical descriptions of new or future applications and modifications of the conventional techniques have been published in the last 2 years. Sidebands in Mossbauer spectra can be obtained in various ways. Olariu et al. (216) derive intensities and positions of sidebands due to intense optical

LATTICE DYNAMICS Many investigators have used recoilless fraction data to

elucidate motional and bonding properties in terms of dis- placements, force constants, and bonding energies in the solid state. Studies of the bonding properties of the layer compound FeOCl and related intercalation compounds are reported by Herber and Maeda (142,143). Herber and Davis (141) have given results on intercalation compounds of alkali metals with SnS2. Apart from the characteristic lattice temperatures determined from the recoilless fractions, other hyperfine parameters provide information about the van der Waals type guest-host bond and the effect of intercalation on magnetic ordering. Debye temperatures were obtained by Cereze et al. (66) for ferrous fluorosilicate and by Chandra and Ericsson (68) for FePS8. In tlhe latter case single-crystal and spectral measurements show features of the Goldanskii-Karyagin ef€ect, indicating lattice anisotropy. Forker and Trzcinski (117) derived the Debye temperature for Fe impurities in scandium and found the impurity-host force constant to be equal to the host-host force constant. The ratios of these force constants were also measured and are discussed for '19Sn in Si, Ge, and a-Sn by Petersen et al. (230).

Several studies have been reported on polymers, glasses, arid liquid crystals. Homologous series of Sn(1V) chlorides with organic ligands were prepared and characterized by Barbieri et al. (31). They found that Debye temperatures decrease continuously for each series toward larger organic ligands, in agreement with the theory of vibrating masses presented. Vasquez and Flinn (313) monitored the glass- liquid transition of ferrocene dissolved in o-terphenyl via the recoilless fraction. Measurements on FeSn03-particles in the liquid crystal EBBA by Bhide and Kandpal(42) show anomalous line broadening around the transition to the isotropic liquid phase and line broadening due to Brownian motion in the nematic and liquid phases. LaPrice and Uhrich (178) suggest a rotational diffusion model for diacetylferrocene to explain their recoilless fraction data for two liquid crystalline materials.

A number of experiments on small Fe particles were performed by Hayashi et al. (137, 138). In the first study, the effect of particle motion is averted by embedding them in matrices. Except for the Fe203 surface, no difference with bulk iron data is reported. In the second study gas-evaporated Fe particles of comparable size were measured to verify the effects of particle motion.

Molecular motions in solids were studied in Fe(I1) sandwich compounds by Fitzsimmons and Hume (115). In their model the rotation of a molecule is represented by an electronic field gradient jumping among the x , y, and z directions with relaxation times around s. Quadrupole relaxation was also observed for 1291 in Si by Kemerink et al. (167). They found a large anisotropy of the recoilless fraction and interpreted their results in terms of a static to dynamic Jahn-Teller transition.

Parak et al. (222) studied fluctuations between conforma- tional states of metmyoglobin of the order of 0.2 A. These motions are characteristic for biomolecules in their active state.

Diffusion of defects in Am metal originating after the emission of a-particles was studied with the use of 237Np Mossbauer spectroscopy by Asch et al. (13). At temperatures above 20 K these defects become mobile, as can be seen from recoilless fraction, isomer shifts, and quadrupole interaction data.

A still developing technique to obtain mean square dis- placements of atomic vibrations is the measurement of Ray-

ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982 # 207R


leigh scattering of Mossbauer radiation. Krup anskii et al.

myoglobin molecules in a buffer solution. (177) describe a procedure to determine disp r acements of

PHASE TRANSITIONS Various types of phase transitions have been studied with

the aid of Mossbauer spectroscopy. Most investigations deal with the tem erature dependence of the Mossbauer parameters; some g a l with their pressure and magnetic field dependence. An extensive review on the applications of Mossbauer spectroscopy to study phase transitions is given by Shenoy (269).

Most investigations in this area are concerned with magnetic phase transitions. A new phase of Fel,Co,C1z~2H20 and Fel-,Co,C12 has been characterized by Katsumata and Ta- waraya (166). In this oblique antiferromagnetic phase the easy magnetic axis is at an angle with easy axes of the pure substances. A second-order phase transition in FeCrzS4 is reported by Brossard et al. (54). The transition from the paramagnetic to the ferromagnetic phase leads to orbital ordering and a slow relaxing electric field gradient.

Baines et al. (19) recorded spectra of RbFeC13 in external magnetic fields above TN and found an increase of TN with the magnetic field. Marusak and Mulay (195) report on an antiferromagnetic-to-ferrimagnetic phase transition in FegSlo and explain their results in terms of rearrangement of Fe vacancies. A similar type of phase transition was observed in FeIz by Calis and Trooster (63). Moreover, they found a stepwise increase of the total magnetization with increasing external field.

Some papers deal with the reorientation of easy magnetic axes. Tobler et al. (295) describe the Morin transition in a-Fe203. A Morin-type transition in DyFeo,es75Coo,oo2503 has been revealed by the experiments of Nikolov et al. (213). The Fe3+ spins rotate from the a axis in the direction of the c axis, but jump to the b axis before reaching it, turning the crystal into an antiferromagnet. Geller and Balestrino (125) show two subsequent spin reorientations in samarium iron garnet.

A few authors describe the effects of phonon modes near the magnetic ordering temperature. In two adjacent papers (112,113), Feder-Bukshpan and Nowik measured a series of binary rare-earth iron alloys (RFez) and explain the discon- tinuities in the Mossbauer parameters by changes in vibra- tional modes. Similar investigations have been reported on ternary alloys of the type Sml-,R,Fez by Bauminger and Savage (34). Klimenko et al. (168) investigated the ternary alloy system Fe-Cr-Ni. Around the ordering temperature, spin-spin and spin-lattice relaxation were observed for Fe- enriched regions and Ni-enriched regions, respectively.

Several papers have been published concerning martensitic phase transitions. The recoilless fraction is indicative of softening of modes near this type of transition. We mention two papers describing Mossbauer experiments on a Mn-Cu-Fe alloy (104) and the Fe73Ni27 alloy (172) performed in transmission geometry. Everson et al. (111) discuss scattering experiments on Fe72Ni28.

Other structural transitions have been studied by Wort- mann et al. (328), who used lslTa Mossbauer spectroscopy to study the anomalous temperature dependence of the quadrupole splitting of LiTa03 around the ferroelectric phase transition, and Jex et al. (160), who used Mossbauer y-ray diffraction experiments, among other techniques, to examine the cubic-to-tetragonal phase transition in RbCaF,.

Eliezer et al. (109) identified intermediate phases during the sintering process of /3-Fe5Ge3 from the pure elements. Their study of phase transition kinetics further indicates that mixing of both elements takes place by diffusion of Ge into Fe only.

Order-disorder transitions in alloys were followed as a function of the temperatures by van Deen and van der Woude (309). They found experimental evidence for an azeotropic maximum in the binary Ni-Fe phase diagram for Nil3FeZ7. The Fe-A1 phase diagram has been investi ated by Oki et al. (220), who found the coexistence of an ordiered and a disor- dered phase for Fe&12&

Amorphous materials have been investigated to monitor amorphous-to-crystalline transformations. Schaafsma et al. (259) studied Fe Bzo and characterized the crystallization products Fe and g 3 B . Using recoilless fraction data, Schurer and Morrish (262) suggest that atomic rearrangement takes

208R ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982

place before the onset of crystallization of amorphous Fe3z- Ni3,Crl4PlZB6. The same group published two papers on amorphous Fee2B12S& (21 7,218). Both the Curie temperature and the crystallization temperature have been determined. The crystallization products are FezB and FeglSig.

Phase segregation into microscopic particles was found to occur in Co-Fe alloys investigated by Eibschutz et al. (107). The two phases are Co-enriched Fe and Cr-enriched Fe. Contrary to earlier findings, Cohen et al. (76) show that in unannealed Co-hardened gold electrodeposits most of the Co is substituted in the Au lattice.

Mossbauer measurement as a function of applied hydrostatic pressure is a still-developing technique. Phase transitions induced by exposing samples to high hydrostatic pressures have been reported by Kapitanov and Yakovlev (164) in SnTe and by Annersten and Nord (10) in Mg3(P0&

ELECTRON TRANSFER, SPIN TRANSITIONS, AND ELECTRON SPIN RELAXATION

Since the Mossbauer effect is extremely sensitive to the electronic environment of the Mossbauer nucleus, changes in the electronic structure are expected to affect the Mossbauer spectrum as well. In this section we describe the effect of the transfer of electrons from one ion to the other, the effect of spin transformation, and the effect of spin flucuations at rates comparable to the Larmor frequency of the Mossbauer nucleus.

Electron transfer, or electron hopping, occurs between relatively stable oxidation states of an element. Well-known examples are the Eu2+ - Eu3+ and Fez+ - Fe3+ transfer mechanisms. Bahgat et al. (18) studied the influence of substitution of iron ions on electron hoppin in ma netite.

first sample contains a very small amount of Fe and Fe3+ ions on octahedral sites, and consequently no electron hopping of the type Fez+ - Fe3+ was found. The less substituted second sample does show this phenomenon. Rosenberg and Franke (249) reviewed the recent work of their group and concluded that electron transfer can have short-range character only. Howe and Dudley (147) measured potassium ferrites containing Ga and Al. The activation energy for electron hopping is found to increase rapidly upon dilution, due to increasing Fe-Fe distances. Ando and Nishihara (9) interpret their spectra of Fel-,Cu,Cr2S4 with the stochastic model of Blume in terms of electron hopping between tetrahedral iron sites in order to obtain valence distributions. Mossbauer studies on bornite ( C U ~ . ~ F ~ ~ . ~ S ~ , ~ ) by Jagadeesh et al. (158) show a temperature dependent line width for the Fe3+ component, while the Fez+ concentration is found to decrease at lower temperatures due to the hindrance of electron hopping. Vaishnava et al. (307) found evidence for electron transfer between Fez+ sites and adjacent Fe3+ or Ti4+ sites in vesuvianite. A more complicated electron transfer mechanism is proposed for two compounds of the type Fe11Fe111z0(CH3- COO),L3 (L = HzO, C5H5N). Dziobkowski et al. (105) assumed relaxation processes between all three iron ions of one molecule to explain their results. Joshi et al. (162) studied Eul,Sr,Fe03 and Ndl-,Sr,Co03 samples, with both 151Eu and 57Fe Mossbauer spectroscopy. They concluded that electron hopping between Fe2+ and Fe3+ ions occurs below TN at fre- quencies less than 107 Hz. The europium ions remain tri- valent.

Some papers deal with electron hopping of the EuZf - Eu3+ type. Rohler and Kaindl (248) determined the activation energy for electron hopping between equivalent Eu sites in Eu3S4. The hopping frequency is found to increase with applied pressure. Coey et al. (73) found two distinct europium absorption lines in EuOl- N, and concluded from their results that electron hopping is $locked because Eu2+ and Eu3+ ions occupy different sites in the lattice.

High spin to low spin transitions in Fe11N6 compounds continue to be the subject of the studies of Konig et al. (173, 174). In these studies, they discuss the apparent hysteresis in two FenLz(NCS)z (L = bidentate organic ligand, coordinated to the Fe2+ ion with two N atoms). Long et al. (190) induced high spin to low spin transitions in a similar compound.

Electronic spin relaxation effects on Mossbauer spectroscopy are a field of increasing interest. Transitions between electronic levels monitored by the Mossbauer nucleus can provide additional information on the electronic structure.

Fe2+ was substituted by Mg2+ and Mn2+ and F$+ bzAl+. 5 The


on A1 0 was investigated. Berry et al. report both l19Sn (38) and lhSk (40) Mossbauer studies of mixed tin-antimony and tin-molybdenum catalysts. Up to ,- 10% Sb has been found to dissolve in the 13n(IV) oxide, as Sb(V). For higher Sb concentrations another oxide phase forms with mixed Sb(II1) and Sb(V). This phase was definitely shown to be Sbz04 by Portefaix et al. (235), who found only about 5 % Sb in the Sn lattice. This group gives X-ray evidence that the Sbz04 i s a surface layer and proposes that it is this mixed-valence layer which is active in propene oxidation. Peev et al. (227) correlate the composition of ammonia synthesis catalysts with their activity, noting the presence of wustite in the Mossbauer spectrum indicates a loss of activity. Xin et al. (330) report Mossbauer studies on alkali metal-promoted ammonia synthesis catalysts. These catalysts have iron present mostly as metallic a-Fe, but some central lines in the spectrum due to small particles and /or paramagnetic species. These lines alppear to be directly related to the catalytically active species.

Several studies have appeared dealing with iron-molybdenum catalysts on SiOz. Mehner et al. (196) report Mossbauer and ESR studies on a multicomponent catalyst. The original catalyst had 40% a-Fe203 and 60% Fez(MoO )3. The form of iron in the used catalyst was mostly P-FeMobl. Carbucicchio and Trifiro (65) studied the oxidation-reduction behavior of iron-molybdenum catalysts on SiOz as affected by the surface area. The original material contained no Fez03, only Fez(Mo04)3 and some Fe(II1) in SOz, the latter dependent on the surface area. Reduction also produces p-Fe- Moo4. The catalytic activity depends on an iron-molybdate surface film. Burriesci et al. (60) used a commercial Fez- (MOO& catalyst for an in situ Mossbauer study of the changes in the catalyst during methanol oxidation. If the catalyst is exposed to the reaction gas and then heated, a-FeMo04 forms, while preheating the catalyst and then introducing the gas converts it to P-FeMoOl. Wive1 et al. (326) performed in situ Mossbauer emission studies on cobalt-molybdenum catalysts used for hydrodesulfurization. Three cobalt phases appear in the Mossbauer spectrum, with a Co-Mo-S surface phase correlating best with the catalytic activity.

The zeolites are important support materials because of their porous structure. Suib et al. (289) used 161Eu to study oxidation states of europium in zeolites under various conditions of treatment. Lee (184) used sodium vapor and N&H4 to reduce Fez+ in Y-zeolite. The latter appears to leach the iron from the zeolite, but Na apparently produces a reduction in the zeolites to small metallic particles.

ION IMPLANTATION AND DEFECT STUDIES Most of the studies of defects involve sourse implantation

b high-energy bombardment. Quite different results can be ol)iStained, depending on the nature of the material bombarded, ai3 well as the relation chemically between source and matrix. Studies of this type appeared before this review period, but a clearer picture is emerging. Mossbauer spectroscopy will likely play an important role in the future of this field.

Concerning metallic defects, the role of Mossbauer spectroscopy in ion implantation studies is reviewed by Recknagel and Wichert (241). The different choices of source probe and matrix to study interstitial or vacancy defects, as well as clustering have been summarized. Pattyn et al. (225) used a double implantation method to study vacancy distribution in tungsten-first 133Xe as radioactive probe for Mossbauer spectroscopy of the 133Cs y. Then stable 132Xe was implanted a t varying energies. The damage profile has a maximum at much less de th than the particle penetration. DeWaard et all. (93) used EgSb and lzgmTe sources implanted in metals to study the effect of helium implantation on the vacancy defects. These "helium-decorated" vacancies are thermally stabilized.

Perez et al. (229) report an extensive study of the use of 57Fe implantation in MgO and LiF for defect characterization by CEMS. The Mtissbauer spectra were obtained down to 4 K, showing superparamagnetic clusters and some Fe2+. Annealing at 700 "C converts most of the iron species to Fe3+. Stanek et al. (279) give a summary of CEMS results at room temperature for 57Fe in a series of alkali halides. Only LiF and KC1 have evidence of Fez+ species. The field gradients of Fe3+ were analyzed in terms of a model involving two vacancies. In KBr the CEMS spectrum disappears after heating at 600 OC.

Van Rossum et al. (310) review the study of defects in group

ANALYTICAL CHEMISTRY, VOL. 54, NO. 5, APRIL 1982 209 R

Many contributions on this subject deal with spin relaxation of Fez+ and Fe3+ ions. Some examples have been reported on ions of other Mossbtiuer isotopes, predominantly in studies of intermetallics.

Hoy and Corson (143) summarize existing theories, present many calculated spectra, and give an a plication for the particular case of KZFeO4 for which they letermine the spin fluctuation rate in the region of the ordering temperature. Electron spin relaxation of the Fe2+ ion has been studied in several host lattices. Nonville et al. published their investigations on dilute Fez+ impurities in ZnS in two adjacent papers (47, 48). In the first ]paper they estimated the differences between the spin-orbit levels of Fez+ by combining absorption and emission measurements. In the second they interpreted the observed reduce spin-orbit splittings in terms of a dynamic Jahn-Teller effect, IRegnard and Durr (244) performed measurements on Fe"+ impurities in CaFz, obtained the electronic level scheme, and concluded that Jahn-Teller effects can be neglected in this case. Nicolini and Reiff (211) studied bis(acetylacetonato)iron(II), in which the Fez+ ions are in approximately octahedral sites surrounded by six oxygen atoms and noted electron spin relaxation effects. Thus far, the occurrence of these effects for similm Fez+ ions has always been explained by assuming a rhombic distortion of the trigonal symmetry of €?eZ+ sites. Srivastava and Singh (278) point out that these efiFects can also be explained taking into account second-order spin-orbit coupling with lowering the trigonal symmetry. Gubbens et al. (135) fitted their spectra of FellClz(pyrazole)z wiith a Gaussian distribution of electron spin relaxation rates. 'The relaxation spectra of Fea+ species are more complex, since more electronic states are usually populated and each induces an effective hyperfine field on the nucleus. Examples are given by Date et al. (88) for Fe3+ ions in LiNbOs and Bhargava and Zeman (41) for Nio.zp- Sno.76Fez04. In the latter study, effects due to ionic spin relaxation and superparamagnetism are discriminated.

Gal et al. (123) explain the 237Np Mossbauer spectra of N P O ~ + ~ in terms of the Orbach-Blume spin-lattice relaxation mechanism. Ira some cases, electronic spin relaxation has been found in intermetallics. Bowden et al. (50) describe 169Tm experiments on paramagnetic TmA13. The spectra consist of two subspectra due to two inequivalent Tm sites, characterized by different relaxation rates. Dunlap et all. (103) used the 151Eu resonance to obtain the temperature dependence of the relaxation rate of Sno,75E&,,z6M06Ss and distinguished between a contribution linear in T, due to coupling between the rare-earth ionic spin and conduction electron spins, and a part independent of T , due to spin-spin relaxation.

CATALYSTS The recent book on IMossbauer spectroscopy (281) contains

a large section of chaphrs on various applications to catalysts. Bussigre (61) reviews the contribution Mossbauer studies can make to heterogeneous catalysis research and gives example applications. Shen et al. (268) describe a high-pressure, high-temperature cell for in situ Mossbauer measurements. The recent literature has examples of in situ studies, studies a t various stages of treatment or reaction, and studies of a number of different types of catalysts.

One important catalytic process is the Fischer-Tropsch synthesis of hydrocarbons from CO and Hz. Niemantsverdriet et al. (212) give a detailed study by Mtissbauer, X-ray, and reaction measurements of the behavior of unsupported metallic iron catalysts. They give particular emphasis to the carbide phases formed, including a new phase with a broad hyperfine field distribut,ion. Schiifer-Stahl(260) reports results on iron in a carbon matrix as catalyst. These small particles have low reaction temperatures and the reactivity correlates strongly with the formlation of ferromagnetic iron carbides. The iron-nickel system on SiOz is the subject of studies by Unmuth et al. (304, 305) using Mossbauer, X-ray, and gra- vimetry. Their work emphasizes fiist the oxidation-reduction of the metallic sDecies and then carburization of the same systems with C6.

Garten and Sinfelt (124) used 67Fe as Mossbauer probe to study the structure of I?t-Ir catalysts on A1 O3 as function of the preparation conditions. Bacaud et al. (16) studied ll9Sn in Pt-Sn on AlzO3, fiiding both Sn(I1) and Sn(IV) ionic species as well as Pt-Sn alloys. A large fraction of the Pt is present unalloyed. The role of Sn ions with poisoning the acidic sites


4 semiconductors by Mossbauer techniques, Implanted source experiments study correlated defects. In trapping experiments the probe is introduced by alloying or thermal annealing, and the defects are introduced separately. The isoelectronic l19Sn has the unique capability of being produced by three distinct sources: Sb, Sn, and Te, all of which may be implanted in silicon, germanium, and diamond. Antoncik (11) interprets the isomer shift of l19Sn interstitially implanted in the group 4 semiconductors as a neutral Sn(0) species highly compressed by lattice pressure. Weyer et al. (322) studied l19Sb implanted in Si and the same group (215) l19”Te. In addition to the usual hyperfine parameters, effective Debye temperatures for the various sites were calculated from the 77 K:300 K area ratios. The two sources give distinct results. For ll9Sb, most sites are substitutional. The interstitial sites increase upon annealing. The llgrnTe have more vacancy-associated defects, and these are more stable with temperature than in the llgSb case. Dam aard et al. (83) used CEMS to study laser an-

compound precipitation between Fe and Si for 1.5 J/cm2 from a ruby laser and for N 10 J/cm2 from a Nd laser. These results indicate liquefaction of the surface layer prior to precipitation. Bergholz (36) gives results of a study of 57C0 diffusion in silicon, as opposed to ion implantation. The y energy profile is used to estimate diffusion depth. Room-temperature Mossbauer spectra show precipitation reactions in the quenched samples. Latshaw et al. (180) used an in-beam technique to implant 67rnFe in group 4 semiconductors, pro- ducing less impurities than in ordinary source implantations. The spectra indicate at least two sites for the iron.

The 111-V semiconductors have a wide choice for Mossbauer studies, but these have as yet been little exploited. Weyer et al. (323) show that tin impurities can be selectively implanted on substitutional sites depending on the source: lI9Sb on V sites and ll9In on I11 sites. Ambe and Ambe (6), in a nonimplantation study, report the effect of proton and a bombardment on the emission Mossbauer spectra in SnSb, SnTe, and Sb Te,. The high-energy recoil is produced bg l%n (p,2n) or a + ?l7Sn. A major effect is seen only in the SnTe (8,211) case where the recoil l19Sn is found on both Sn and Te sites.

In specialized application of ion implantation, van Rossum et al. (311) show that 6 7 C ~ implanted in solid xenon has a number of advantages over the usual matrix isolation techniques. The same group (57) find an unusually high isomer shift for one com onent when 6 7 C ~ is implanted in solid 02.

ENVIRONMENTAL MATERIALS The recent book on Mossbauer spectroscopy (281) has a

major section on environmental applications, including four chapters on various aspects of coal. Among these, particular mention should be made of the review chapter by Montano (199), which gives a detailed summary both of coal characterization and utilization studies by this technique. Problems in particular with quantitative determination of pyrite, FeS,, by Mbsbauer spectroscopy are discussed. Concerning pyrite determination, Jaggi and Rao (159) describe a simple Mossbauer pyritemeter using a 67Co/FeS2 source. The measurement requires only an on/off resonance count without expensive equipment. The proposal could apply to other phase determinations. Only a few of the many papers dealing with coal will be mentioned here. These appear representative of the variety of work being done. Both Mossbauer spectra and X-ray diffractograms aid the study of mineralogical changes occurring in coal during cleanup, pyrolysis, com- bustion, and conversion processes in a paper by Saporoschenko et al. (255). Montano et al. (200) used an in situ cell to measure Mossbauer spectra of the changes in pyrite under conditions of coal liquefaction. Vaishnava et al. (306) also used in situ measurements of l19Sn to study the catalytic role of SnC1, during hydrogenation.

Micas are iron-containing layer silicates of particular interest to Mossbauer spectroscopists. Bagin et al. (17) report their studies on cation-ordering and hi h-temperature decomposition of biotite. The spectral anafysis assumes six doublets. The magnetism of decomposition products is explained by their model, which includes as end product from high-temperature annealing, a ferrispinel. Bonnin and Muller (46) use single crystals of muscovite to determine the field gradient


nealing of .g Fe implanted in silicon. They found evidence of

The implanted .p Co is oxidized by annealing at 30 K.

angles relative to the crystal. They compare their results with point charge calculations. The angular dependence of the Mossbauer spectrum was also studied in phlogopite by Krishnamurthy et al. (1 76) to compare observed field gradients with their theoretical calculations.

Single crystals of the mineral riebeckite-arfvedsonite were studied by Mossbauer spectroscopy at low temperatures in the work by Bor and Borg (49). The antiferromagnetic ordering is descriEed in terms of an Ising model. Low temperature determinations improve accuracy of Fe2+/Fe3+ ratios. Schwartz et al. (264) report a detailed study of natural iron- titanium garnets, with a new assignment of the doublets in the complex spectra. Heller-Kallai and Rozenson (139) used a variety of physicochemical methods to study the dehy- droxylation of dioctahedral phyllosilicates. The Mossbauer spectra are discussed in terms of the coordination of the two sites.

Silicate glasses of geological interest are examined by Levitz et al. (187). They describe an analysis technique for resolving Fe(II1) from Fe(I1) in the Mossbauer spectrum without making assumptions about the spectral shape of Fe(II1). Regnard et al. (243) used Mossbauer techniques in conjunction with other measurements to study the iron in volcanic glasses. Magnetite in the form of small particles is present in two volcanic ob- sidians. In addition, other ionic Fe species were observed. The relation between iron oxidation states and the conditions of the volcanic production is discussed. Syono et al. (291) studied the changes in ilmenite, FeTiO,, with shock treatment, using Mossbauer and X-ray techniques.

Minerals containing mixed-valence elements continue to be of interest. Burns (58) reviews the intervalance transitions in mixed-valence minerals of iron-titanium. Evans and Am- thauer (110) report pressure and temperature variation of the Mossbauer spectrum of the iron-silicate mineral ilvaite. The complex spectrum contains three doublets and one broad singlet, and many of its features indicate electron delocalization and electron transfer. Long et al. (191) used Mossbauer spectroscopy to study synthetic samples of voltaite, a mixed-valence potassium-iron-aluminum sulfate, with cadmium introduced as a substitute for Fe(I1) in some cases. Spectra at low temperature and in external fields were obtained and interpreted in terms of site occupancy. The mixed-valence iron silicate deerite exhibits electron delocalization, as studied by Amthauer et al. (7) and by Pollak et al. (234). The complex silicate vesuvianite also shows Mossbauer evidence of electron transfer between Fe(II1)-Fe(II), as reported by Tricker et al. (298).

Mossbauer studies of clay minerals are reviewed by Coey (71). Iron oxides in clays and other environmental materials are the subject of a number of recent publications, including a chapter in the book by Stevens and Shenoy (281). Huggins et al. (153) show the potential for using goethite, a-FeOOH, in coal (as described by Mossbauer techniques) to indicate coal oxidation. Ross and Longworth (250) used Mossbauer techniques to study the iron species in a sand irrigated with a heavy metal leachate solution over several years. A ferric hydroxide gel was precipitated by the leachate. Ferrihydrite is a naturally occurring iron oxide of very small particle size. The relation of its Mossbauer spectrum to the other, more crystalline iron oxides is discussed by Murad and Schwert- mann (207). Childs and Johnson (69) report Mossbauer spectra of synthetic protoferrihydrite, both with and without aluminum, and indicate these species may be reponsible for the previously reported P-FeOOH in certain soils. The formation of iron oxides and hydroxides from solution is the subject of a paper by Belozerskii et al. (35). They emphasize the processes in solutions with OH- deficiency. The only crystalline iron precipitate observed is goethite. Two mechanisms are proposed, For OH- Fe3+ > 0.1, high molecular

glomerates in solution for OH-/Fe3+ < 0.1 favor the formation of goethite.

AMORPHOUS MATERIALS AND SURFACE STUDIES

Metallic glasses are reviewed in the next section. Mossbauer spectroscopy continues to be an important tool for studying any amorphous material containing a suitable isotope, because of its sensitivity to short-range order. The Na20-Si02 glass system with different Fe3+/Fe2+ ratios is reported by Ban-

forms tend to produce a gel. T i, e low molecular weight ag-


dyopadhyay et a]. 2 5 . Distinct changes in the internal

on the oxygen network. Brewer et al. (53) provide evidence from lz9Te emission Mossbauer spectroscopy for broken chemical order in melt-quenched glasses of GeSz and GeSez. Large clusters rather than random networks seem to be present. Czjzek et al. (82) discuss the field gradient distributions in amorphous solids as related to atomic coordination.

Although not amorphious, graphite has reduced dimen- sionality, which is emphasized in oriented grafoil. Shechter et al. (267) used IigSn Mossbauer studies to determine the nature of melting in Sn(CH )., adsorbed on grafoil. Two steps are observed, the first step %eing interpreted as a coalescence of two-dimensional "islands". Phillips et al. (232) report 57Fe Mossbauer studies of iron pentacarbonyl decomposition over grafoil. The metallic iron produced has unusual properties due to small particle size. Bukshpan and Kemerink (56) report l19Sn studies of trimethyl- and triphenyltin chloride adsorbed on grafoil. These compounds appear to adsorb in preferential orientation-the trimethyl compound having the molecular axis parallel to the graphite plane while the triphenyltin is perpendicular.

Graphite layered com~uounds with iron and molybdenum or tungsten were studied by Nefed'ev et a]. (209), using both Mossbauer and X-ray techniques. The heavy metal stabilizes the complex of iron with graphite.

Only part of the review by Huffman and Huggins on Mossbauer spectroscopy in the steel industry (150) deals with surface reactions, but surface studies have been of particular importance for that material. The back-scattering CEMS technique is generally used. Principi et al. (237) report a CEMS study of nitrogen implanted high carbon martensite. They consider particularly the influence of interstitial carbon on the formation of surface compounds. Inaba et al. (155) used a CEMS counter a t high temperature to study steel nitriding. The process of boriding on Armco iron was studied by Carbicicchio et al. (649 using many techniques including both transmission and scattering Mossbauer spectroscopy to characerize the multilayer surface. Leidheiser et al. (185) used emission from 57Clo to study the electrodeposited zinc-cobalt alloy on steel. The cobalt, is highly dispersed in metallic form with the zinc. Berry et al. (39) report a Mossbauer study of iron surfaces which have been chemically treated for corrosion resistance by alkali, cyanate, and thiocyanate. Muller et al. (206) describe a proportional counter for CEMS measurement inside a tube and apply it to study corrosion inhibition in brass condenser tubes by ferrous sulfate. Meisel and Gutlich (197) performed extensive Mossbauer studies on corrosion of steel surfaces. The iniluence of HC1 and of various rust trans- formers is discussed. The stability of the various iron oxide phases is related to the thermodynamics of their transformation. Huffman and IPodgurski (151) discuss the use of electron reemission Mossbauer measurements to determine the thickness of oxide layers on corroded iron. Sano and Endo (254) give a review of the application of CEMS to the study of tin corrosion.

Mossbauer studies on the steel fibers produced by decomposition of iron pentacarbonyl in a magnetic field are reported by Lashmore et al. (179). These fibers contain a-iron and Fe3C and the magnetic polarimtion is not oriented along the fiber axis. Surface magnetism of thin iron films has been studied by use of multilayer techniques of 57Fe deposited on 56Fe substrates by two groups Somewhat different experimental conditions were used. Hosoito et al. used thick coatings of palladium on the surfacle (146) and thus probed the iron- palladium interface. In the first - 10 A the magnetic hyperfine field at 57Fe first increases over the bulk value, then decreases nearest the interface. This decrease was observed both at 4 K and more drastically at 300 K, where part of the first layer is paramagnetic. 'l'yson et al. (302) grew multilayer films of iron epitaxially on Ag with the 57Fe layer a t various depths and with a coating of Ag to protect the surface. They report also a reduction of 57Fe field at the surface for 300 K, but not as drastic as in the above, and for 4 K the field increases near the surface. This increase was observed also with surface coatings of Au, MnFz, or NaCl but not for a Cu coating.

friction at -50% Fe $+ are ) related to the influence of iron ions

METALLIC GLASSES This specialized class of amorphous materials has been

receiving considerable attention in the literature. The usual

type is prepared in the form of a ribbon from a combination of transition metals with substitutional element such as boron to stabilize the amorphous phase. A common stoichiometry is MmB2@ Mossbauer spectroscopists are interested when M is at least partly Fe. These materials are ferromagnetic with a broad distribution of hyperfine fields. However, interpretation of this distribution differs widely. Gonser et al. (132, 130) report a study of Fe4$Ji&Pl,B6 and Fe80Bzo with linearly polarized y-rays as well as in an external field. A polarization of spins with applied uniaxial stress was observed. The field distribution correlates with the distribution of liquidlike coordination numbers. Franke et al. (120) varied both x and y in the series Fel,B, and (Fel Niy)mB20. The average field dec~eases with x in direct propozion to the change in magnetic moment. WrBnski et al. (329) also studied FeBOBzo in an external field and note the drastic difference in spectra between cooled samples with and without external stress. Vincze et al. (317) used external fields to polarize the sample and combine spectra measured with and without applied field to eliminate textural effects. They note Fe75B25 has a field distribution closely related to that of the crystalline Fe3B as opposed to dense packing. Vincze and van der Woude (318) emphasize this relation in the spectra of (Fe,Ni),B and Zr3Fe, and the distinctly different behavior of the semiconductor As;l'e3, studied with lZ5Te. Oshima and Fujita (221) applied a Fourier analysis to obtain near neighbor configurations in the Fel-,B, series. They conclude that boron has two types of sites-substitutional when in high concentration, thus stabilizing amorphous Fe3B, and interstitial for lower concentration. LeCaer and Dubois (182) discuss the line width asymmetry in metallic glasses in terms of the dipolar field of the 3d electrons.

The chromium glass Fe3zNi36Cr14P12B6 is discussed by Piecuch et al. (233). They explain the two-peak distribution of hyperfine fields as due to magnetic and nonmagnetic iron, the latter surrounded by excess chromium. Schaafsma (322), however, using the method of Vincze (31 7) to combine spectra with and without a magnetic field, found a broad continuous distribution with only one maximum. Prasad et al. (236) report a study of the magnetic fields in Fe74C01$i16 up to 700 K. The deviation from a Brillouin function is analyzed in terms of a fluctuation in exchange interaction. Dey et al. (96) obtained the variation of hyperfine fields and isomer shift with x in the (Fel,Co,)soBzo glass. The iron and cobalt magnetic moments appear to bo constant in the series. Another composition studied by several groups is FemNi3M04Bls. Schurer and Morrish (261) report studies of clamped and unclamped samples after various heat treatments. The ratio of peak areas of the hyperfine spectrum is used to determine the magnetic anisotropy. Kamal et al. (163) report a temperature study of the distribution of fields from 10 K to 673 K. Up to room temperature the distribution is independent of temperature. Irreversible crystallization effects occur at -600 K. Ok and Monish (219) find major differences in the CEMS spectrum compared to the transmission one, indicating a surface crystallization precedes the bulk. Schurer et al. (263) use CEMS and transmission to study the surface crystallization and induced magnetic anisotropy in amorphous Fe78Bl$ilo.

The amorphous alloys Y66(M1-xFex)34 are not magnetically ordered, but the iron quadrupole splittings and isomer shifts werle studied by Tenhover (293) as function of x for a series of transition metals M. There are two iron sites and the nature of these depends on M.

A dilute solution of magnetic ion in a metallic lattice can have distinctive magnetic behavior. The spin glass Pdl,FeyH is the subject of a detailed Mossbauer study by Brand and Georges-Gibert (52). The hyperfine field distribution has a low field cutoff value. Above the ordering temperature an applied external field produces spin relaxation effects. The nonstoichiometric Nil,S is metallic and Roux-Buisson and Coey (251) report Mossbauer studies of oriented single crystals containing iron impurity. The hexagonal metallic lattice causes anisotropic spin glass behavior of the iron.

Amorphous films of Fe,Snl, were studied by 67Fe and lI9Sn Mossbauer techniques. Rodmacq et al. (246) interpret their data on hyperfine fields in terms of the chemical disorder in the system. Structural effects were minimal.

CHEMICAL STUDIES In this section we review papers of particular interest as



chemical applications of Mossbauer spectroscopy, but which do not fit into the s ecialized sections. The emphasis is on isotopes other thanF7Fe or 119Sn. There are certainly many papers omitted in which Mossbauer spectra of new compounds containing these latter isotopes contribute important structural information. However, in those cases the Mossbauer spectroscopy is generally a small part of a variety of measurements and of a straight-forward nature. For the other isotopes the experiments are straight-forward only to a Mossbauer spectroscopist specializing in that isotope, if indeed then. Also, basic new results concerning the interpretation of their hyperfine parameters are still appearing. The book edited by Stevens and Shenoy (281) contains about 10 chapters with emphasis on other isotopes. All these of interest; a few will be mentioned specifically.

Chemical studies with the extremely narrow Mossbauer resonance in 'j7Zn now are becoming feasible with sophisticated drive and detection equipment. Two important papers deserve mention. Forster et al. (118) report absorption experiments on the chalcogenides as well as ZnO and ZnF,. They obtain much better resolution than previously reported and observe a linear correlation of isomer shift with electronegativity, a distinct indicator of chemical interest. The isomer shift calibration is estimated by comparison with '19Sn results. Vetterling (316) reviews the literature on 67Zn, including pressure and temperature effects, quadrupole interactions, and isomer shift systematics.

Moser et al. (202) describe a high-pressure cell for 50 kbar experiments with 237Np at low temperature. The magnetic properties of a number of neptunium intermetallic compounds are reported as function of pressure.

Some I2'Sb experiments are described in other sections of this review. Alamgir et al. (2) report on two types of compounds: base salts of SbBr4- and SbX (OzC6R4)-. The isomer shift relates to the number of short bonds to Sb(II1). The rather surprising result that the SbBr4- compounds studied have zero quadrupole splittin is justified on the basis of p-electron participation in the %onding. Stevens et al. (287) report studies of dialkyl Sb(V) esters. They interpret the quadrupole splittings in terms of an additive model. The same group has studied the series Me,SbCl (288) and again use an additive model to interpret the qua&ipole splittings. The isomer shifts indicate increasing p character in the lone pair with increasing methyl substitution. Krakow et al. (175) summarize the results of their study of the pressure dependence of the isomer shift for metallic Sb (positive) and for lZ7I in KI (no change with pressure). They discuss the systematics of pressure effects with atomic number.

In spite of the difficulties with radioactive absorbers, the Mossbauer effect in lZgI continues to be a popular subject for chemical studies. In double isotope experiments, Jones and Dombsky (161) studied organotin iodides. The Mossbauer data indicate geometrical distortion from tetrahedral symmetry. Dance and Jones (84) report additional studies on the lZ9I Mossbauer spectra of organotellurium iodides. Bondin

spectra of some Pt(I1) and Pd(I1) complexes. Their discussion emphasizes the difference between Mossbauer cis-trans effects and those conventionall studied. Teitelbaum et al. (292) used resonance Raman and 72gI Mossbauer spectroscopy to study the iodine-starch complex. The color is mainly produced b7 I,-. Iodine charge-transfer complexes were studied by l2 I Mossbauer spectroscopy in the work of Sakai et al. (253). These results were obtained in frozen CS2 solution at 16 K. The n-u transfer complexes show two chemical forms of iodine, while PU com lexes show only one. Their results are discussed in terms of Eonding. Noting the advantages of the stable lnI over I?I, Birchall and Myers (43) discuss Mossbauer spectra of lZ7I in IzSbzFll and a number of polyhalide anions. Structural implications are mentioned.

Sham et al. (266) present a new calibration for the isomer shifts of lWAu. They use the relativistic Hartree-Fock method to calculate charge distributions. The electronic properties of Au(1) and Au(II1) compounds are discussed. Parish et al. (224) report lg7Au results on new three- and four-coordinate Au(1) complexes. They relate the Mossbauer parameters to the coordination number and discuss the additive model for the quadrupole splittings,

A number of Mossbauer studies of hydrogen-storage intermetallic compounds have appeared, some dealing with 57Fe

information is presented. Dickinson et al. (97) report l2 $: I

and other with more unusual Mossbauer isotopes. Shenoy et al. (273) review the literature, including CEMS studies of the surface, mechanism of hydrogen absorption, and de ra-

Mossbauer results on Eu2RuH6, as well as 99Ru results on related hydrides. The Ru(I1) is diamagnetic, while Eu(I1) becomes magnetically ordered at 28 K. Cohen et al. (77) used the 161Dy Mossbauer transition to study hydrogen absorption in dysprosium metal as well as intermetallic compounds DyM,. The addition of hydrogen reduces the electron density at Dy. Low temperature magnetic ordering was observed. Both 57Fe and 161Dy Mossbauer results on DyFe, hydrides are reported by Niarchos et al. (210). The magnetic properties of the two isotopes change upon hydriding in a way associated with the lattice expansion. Cohen et al. (78) used lSIEu as Mossbauer probe to study the degradation of LaNi, with thermal cycling of hydrogen adsorption-desorption. They discuss the mechanism of the intrinsic degradation. Both source and absorber studies are reported on Fe-Co probes in P-PdH, by Probst et al. (238). The temperature variation of the recoilless fraction, isomer shift, and line width are related to the effects of hydrogen absorption. One of the best hydrogen storage compounds is TiFe. Two independent reports in the 1980 literature concern 57Fe CEMS studies of TiFe with essentially the same result. Both Shenoy et al. (271) and Blasius and Gonser (44) find ferromagnetically ordered iron on the surface after hydrogen-cycling of TiFe, which is proposed to be the catalyst for dissociation of H2. The latter group report that TiFe, which has not first been thermally activated, does not contain the surface magnetic iron and does not absorb hydrogen.

MISCELLANEOUS APPLICATIONS Besides the many applications of Mossbauer spectroscopy

reviewed in the previous sections, there are a number of miscellaneous applications that, added to the above, testify to the versatility of this technique. Ali et al. (5) demonstrate the use of Mossbauer spectroscopy to qualitatively and quantitatively characterize the iron that is contained in various medicaments. In another study Rodmacq et al. (247) used Mossbauer experiments to investigate Nafion ion exchange membranes. Besides the usual isomer shift and quadrupole coupling data they found the Mossbauer fraction to be quite important in studying these materials. Cusack et al. (80,81) report their investigations on the treatment of wool by inorganic tin and monoorgano tin to make the wool flame-re- sistant and mothproof. Recently there have been a number of papers in which studies on the effect of laser annealing are discussed (90, 91, 231). It is possible to study a variety of materials to locate very precisely iron, tin, tellurium, iodine, and other atomic sites before and after the annealing process.

Certainly one of the more productive and interesting applications of Mossbauer spectroscopy has been its contribution to the understanding and the characterization of ancient pottery found in various parts of the world. Most of the work during these last 2 years has been the study of fiing conditions on a variety of pottery and ceramic materials. Coey et al. (72) studied sherds from northeast Iran which date back to approximately 3000 B.C. They were able to determine the firing conditions within 50 "C for some of the samples studied. Eissa et al. (108) report their study of Islamic pottery from different Arabian regions. They were able to make comparisons with the results of their studies. In another investigation Riederer et al. (245) studied very carefully the colored layers which represent the different degrees of oxidation on a number of ancient Egyptian ceramics. They were able to obtain information on the various firin temperatures. The authenticity and the age of pottery an% ceramics at Glozal, France, has created a great deal of discussion. Bakas et al. (20) studied a tablet from these finds and, as in other studies noted above, it was possible for them to characterize the firing temperature of this tablet. Renaissance Sgraffito pottery of Venetian Paduan was studied by Lazzarini et al. (181) in order to characterize the production techniques.

The last study to be mentioned in this review uses CEMS, a technique discussed in more detail in earlier sections. Longworth and Atkinson (193) studied the surface glazes on various archeological materials. While their spectra are of relativley poor quality, they do note differences in a number of materials that were studied. There is need for further work

dation by cycling. Fernandez et al. (114) report %Ru and 6 Eu



to improve the quality of the spectra and to explore more extensively the use of CISMS for studying ancient pottery and ceramics.

ACKNOWLEDGMENT We have been assisted in the preparation of this review by

Virginia E. Stevens, who extensively proofread this review. Joyce Weatherspoon, Rhonda Ramsey, and Mary Jane Win- frey assisted with the typing. We also wish to note our ap- preciation to Richard White and Chris Matayabas, who aided in retrieving and organizin the literature. The support of the National Science Founiation to L.H.B. is gratefully ac- knowledged.

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Magnetometry: Instrumentation and Analytical Applications Including Catalysis, Bioscience, Geoscience, and Amorphous Materials

L. N. Mulay” and Indumatl L. Mulay

Department of Materials Science and Engineering, 136 Materials Research Laboratoty, The Pennsylvania State University, University Park, Pennsylvania 16802

I. PROLOGUE A. Eulogies to Professor J. H. Van Vleck and Dr. R.

M. Bozorth. In our 1978 review we paid a tribute to Professor J. H. Van Vleck of Harvard University for receiving the 1977 Nobel Prize along with two other physicists. His pioneering research on the “Electric and Magnetic Susceptibilities’! (which appeared in the form of an advanced textbook in 1931 under the above title) brought forth the beauty and truth concerning modern magnetochemistry and magnetophysics. Now, with a saddened heart and a burdened pen we must offer our sincere eulogy not only to Professor J. H. Van Vleck, who passed away in October 1980, but also to Dr. R. M. Bozorth, who passed away in January 1981. Dr. Bozorth contributed immensely to our understanding of modern ferromagnetism and wrote the first English comprehensive treatise on the subject in 1951. These two giant geniuses have left this world, as we all must do some day, but their memories will linger on through their contributions to magnetism. Life seems to have a magical means of propagating itself, especially in the case of geniuses, for they live eternally through their “karma”, which, in Hinduism means the “deeds or duties”, which one must perform without expecting a reward, although the re- wards are there to be bestowed by the Universal Creative Intelligence. L.N.M. had the privilege of listening to the masterly lectures of Professor Van Vleck, and we must conclude this eulogy by quoting John Keats who wrote the following lines in his “Ode to a Grecian Urn”: When old age shall this generation waste,

Than ours, a friend to man, to whom thou say’st, Thou shalt remain, in midst of other woe.

Beauty is truth, truth beauty-That is all

B. Scope of This Review. In this eleventh review, we survey important trends in instrumentation and applications, especially in the realm of analytical chemistry including catalysis, bio- and geoscience, and amorphous magnetic materials. The last two categories are becoming technologically very important in recent years. It has been well established that magnetometry has proved to be extremely useful in the characterization of these systems a t the micro- and macro- scopic levels, dealing with their electronic and bulk structures, analysis of various components present, and so on. The first ten reviews on “Magnetic Susceptibility” appeared during 1962-1980 (91,102-110). This 1981 version covers literature mostly from about January 1980 to December 1981 and some

Ye know on earth, and all ye need to know.

216 R 0003-2700/82/0354-2 16R$06.00/0

earlier work. In response to an editorial plea, we have made this review more concise than the previous ones. In doing so it seemed imperative that we depict the exceptionally novel trends in the applications of magnetometry. Readers will note that we have now adopted the term “magnetometry”, because it encompasses not only the principles of magnetism but also the measurement and applications of magnetic susceptibility (x) as well as of magnetization (M) which are usually studied as a function of temperature (T) and the applied field (H). The susceptibility (x) is simply equal to M / H x can be ex- pressed in units of per unit mass or volume. A table of conversion from the “cgs” to the ”S.I.” units for magnetic parameters is given in Table I.

11. GENERAL LITERATURE A. Conference Proceedings, Books, and Chapters.

“Current Trends in Magnetism” edited by Satya Murthy and Madhav Rao (140) represents the proceedings of the third topical seminar (Dec 1980) of the Indian Physics Association and covers invited lectures on fascinating topics such as (a) transition metal and rare earth complexes, (b) muon spin rotation, (c) spin waves, (d) solitons and magnetic chains, (e) mixed valence, and (f) amorphous systems-spin glass and many more. Mulay and co-workers have two papers on “The Magnetic Characterization of Ni/Al,O3’’ and on the “Characterization of Fe/Zeolite Catalysts” in this book (140). Reference to the last two papers will be made under the section on catalysis.

The proceedings of the Magnetism and Magnetic Materials (3M) Conferences, which are held annually in the U.S. continue to appear in a special issue of the Journal of Applied Physics (usually in March following the conference) and in Special Publications of the American Institute of Physics. The proceedings of the International Conferences on Mag- netism (Intermag) which are held in various countries, including the U.S. in alternate years, are published usually in special issues in journals such as the Institute of Electrical and Electronic Engineers (IEEE)-Magnetic Transactions and related publications. The invited lectures in all cases appear in about 8 page long articles, which are indeed very informative and entertaining, and range all the way from “Advances in Magnetic Theory”, “Technical Applications”, “Magnetism Applied to Archaelogy”, and “Magnetism in Living Systems”. Magneticists are urged to look up the several hundred entries each year in such proceedings, because they

0 1982 American Chemical Society

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Moessbauer spectroscopy