Anal. Chem. 1000, 62, 12513-13913 (C31) DWyW, V. M.; M e t U ~ w ,J. A. D. svf. SCI. 1988, 103, 549-568. (C32) Deft&, W. A.; k d l , L. B.; Seah, M. P.; VAMAS Community Surf. I n t W k 8 Anal. 1988, 13,63-122. (C33) MU’hu, H. J.; Misdrler, S.; V m l , A,; Seller, A.; Rledl, G. Surf. Interface Anal. 1988, 12, 78-82, (C34) Tomich, D.H.; Grazulis, L.; Koenig, M. F.; Grant, J. T. Surf. Interface Anal. 1988, 11, 243-250. (C35) Seah, M. P.; Lim, C. S.; Tong, K. L. J . €&&on Spectrosc. Relat. phenom. 1989. 48. 209-218. (C36) DUdek. H. J.; Bunk, W. Fresenlw’ Z . Anal. Chem. 1987, 320, 422-428. (C37) buesch, P.; Foditsch, W.; Stucki, F. J . Vac. Scl. Techno/. A 1987, 5, 3334-3339. (C38) American Societ~for Testing and Materials Surf. Interface Anal. 1988, 11, 119-124. (C39) Marcus, P.; Olefjord, I. Corr. Scl. 1988, 28, 589-602; Surf. Interfa- Anal. 1988, 11, 569-578. ( a 0 ) sham, J. K. N.; Chakraborty, B. R.; Shlvaprasad. S. M. J . Vac. Scl. T & d . A 1988, 6 , 3120-3124.
(C41) Kuivila, C. S.; Butt, J. B.; Stat,P. C. Appl. Surf. Scl. 1988, 32, 99-121. (C42) Sam, M.; Yamade, R. J . Vac. Scl. Techno/. A 1988, 6 , 24 10-2414. (C43) Mischler, S.; Mathleu, H. J.; Landoit, D. Surf. Interface Anal. 1988, 1 1 , 182-188. (C44) Sobdewski, M. A.; Helms. C. R. J . Vac. Scl. Teohno. A 1988, 6 , 1358-1 362. (C45) Kaufmann, R.; KleweNebenlus, H.; Moers, H.; Pfennig, 0.; Jenett, H.; Ache, H. J. Surf. Interface Anal. 1988, 1 I, 502-509. ((246) Tapping, R. L.; Davidson, R. D.; Jackman, T. E.; Davies, J. A. Swt. Interface Anal. 1988. 11, 441-446. (C47) Hammer, G. E.; Shemenski. R. M. Swf. Interface Anal. 1987, 10, 355-359. ((248) Ness, J. N.; Joyner, D. J.; Chapple, A. P. Zecfltes 1989, 0 , 250-252. (C49) Rogers, J. W.; Kelber, J. A.; Hendarson, M. A.; White, J. M. Appl, Surf. SCI. 1988-89, 35,423-434. (C50) Katrlb. A.; Petit, C.; Legare, P.; Hlialre, L.; Make, G. J . phvs. Chem. 1988, 02, 3527-3531.
Mossbauer Spectroscopy John G. Stevens* Department of Chemistry, University of North Carolina Asheuilk, Asheville, North Carolina 28804-3299
Lawrence H.Bowen Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204
Katherine M. Whatley Department of Physics, University of North Carolina Asheville, Asheville, North Carolina 28804-3299
This is the tenth review in the series of Mossbauer s ectroscopy in which the facilities of the M h b a u e r Effect b t a Center have been used. Durin this period of time we have seen the number of published d w b a u e r papers increase from 900 per year to approximately 1500. This current review has considered over 2700 apers on Mossbauer spectroscopy which have been processedy! the Data Center since the last review (I). As in recent years, we have selected approximately 300 papers to be featured in this review. This compares with the selection of over 771 papers that were included in our first review in this series in 1972 (2).The result is that we have become much more selective over the years and are now c h a i n ? only a proximately 10% of the papers. The rocess of identifying t i e papers to be featured in this review as not been easy and the selection in part reflects some of our own bias based on our particular interest. In spite of this shortcoming, the review does represent a ood summary of what has been happening in the field of dossbauer spectroscopy since the last review 2 years ago. Almost all the apers that are included in thisreview are from the time period)1987-1989. The proceedin s of three international conferences have been published, afl of them as special volumes of the journal Hyperfine Interactions. The proceedings of the International Conference on the A lications of the Mossbauer Effect (ICAME) that was heRin Melbourne, Australia, in August of 1987 have been published in a three-volumeseries. Included in these proceedings are 22 invited papers and 233 short pa ers. Topics that are covered include chemical structure an$ bonding, magnetism, metals and alloys, surface phenomenon, catalysts, mineralogy, geology, archaeology, biological systems, noncrystalline m a t e d , radiation effects, relaxation, phase transitions, lattice dynamics, industrial applications, high-temperature superconductors, and theoretical and experimental developments (3). The proceedings of the Second International Symposium on the Industrial A plications of the Mossbauer Effect (ISIAME),which was feld in Parma, Italy, in September 1988,
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have been published in two volumes. The proceedings include 13 invited talks and 82 papers. The topics of the papers include magnetic materials, amorphous materials, metals and alloys, corrosion studies, surface and film studies, catalysts, mineralogy and new instrumentation (4). The third published roceedin s were that of the Third Seeheim Workshop on d h b a u e r 8pectroscopy that was held in May 1988 at the Lufthansa Training Center in Seeheim, Germany. This volume contains the 25 papers from the invited speakers to the workshop. The volume also includes 117 abstracts of the poster presentations. The topics of this workshop are very similar to those of the aforementioned proceedings, with the addition of sections on small particles and ion implantations (5). In addition to these conference roceedm there has been only one major English-written took on gossbauer spectroscopy published. This is the third volume of Mossbauer Spectroscop Applied to Inorganic Chemistry, edited by G. J. Long a n i F . Grandjean (6). This volume contains 11 chapters on a variety of interestin topics written by varioub experts in the field. These incluje “The Early Days of the Effect” (U. Gonser and C. Schneider), “Mossbauer Effect Studies of Hard Magnetic Materials” (0.A. Pringle and G. J. Long), “Mossbauer Effect Studies of Oxidic Spinels” (R. E. Vandenberghe and E. De Grave), “Evaluation of Distributed Hyperfine Parameters” (S. J. Campbell and F. Aubertin), ‘Radio Frequency Field-Induced Effects in Ferromagnetic roecopic Studies Materials” (M. Kopcewicz),“M-bauer S of the High Oxidation States of Iron” ~ R U S Sand O G. J. Long), “Mossbauer Spectroscopic Studies of Intercalation Compounds” (F.J. Berry), “Mossbauer Spectroscopy of Iron and Iron-Molybdenum Hydrotreatin Catalysts” (A. M. van der Kraan,W. L. T. M. Ramselaar, an%V. H. J. de Beer), ”The Use and Potential of Mossbauer S ectroscopy in Studies of Biological Mineralization” (J. WeEb and T. G. St. Pierre), “Characterization of Semiconductors by Mossbauer Spectroscopy” (G. Langouche), and “Miissbauer Spectroscopy 0 1990 American Chemical Society
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Semiconductors Europium-Containing Compounds" (F. Grand'ean and G. J. Long). We /we mentioned in recent reviews in this series that the most active area of the application of the Mossbauer spectrosco y has been the investigation of amorphous materials (7).&is has continued durin the last 2 years in which there have been over 350 papers, a s&ht decrease from the previous reporting periods. Two topics for which there has been the reatest increase of interest are the stud of thin films and Lh-temperature superconductors. The dbsbauer effect has been very instrumental in providin structure information of these materials at the atomic level. h e r 150 papers have been published on high-tem erature superconductors, most very recently. The areas t i a t have shown markedly increased interest in the use of Miissbauer spectroscopy are surface studies, catalysts, and irradiation experiments. In this latter application, the interest has been primarily in investigation of materials that have been laser irradiated. Other active areas having 100 or more publications include lattice studies, minerals, and the investigation of relaxation phenomena. Other areas in which we have noted increased interest include designs of new spectrometers and the analysis of line shapes. We continue to see developments in these two areas primarily due to the continued advances with computer hardware and software. Almost 10% of the ex erimental papers are investigations using Conversion Erectron Mossbauer Spectroscopy (CEMS). This articular technique has now become widely accepted for stuzying many materials on which it is difficult to obtain information other than by Mossbauer spectroscopy. A part of this particular review will be focused on the various topics mentioned above. While over 100 Mossbauer transitions are known, it has been uite some time since any have been added to the list. G. A. Ilcorobogatov (8)has reported observin the Mossbauer effect in a second transition in the isotope l&e. The energy of the transition is 109 keV, which part of the y cascade that recedes the widely used 35.5-keV transition in 125Te. bopefully, another laboratory will confirm this particular transition because of the questionable nature of the reported data. S7Fecontinues to be by far the most popular isotope in Mbsbauer spedrosco y, while "gSn continues as the second most popular isotope. ur expressed concern in the previous review of the problem in obtainin sources for Mossbauer spectroscopy has not improved. here are only two commercial companies that provide 57C0 sources for iron Mijssbauer spectrosco and one company that is able to rovide sources for ll# n. This particular situation could r3ecome very critical for the entire Mossbauer research community. Despite the fact that it is not possible to obtain commercial sources for any of the other Mossbauer transitions, there continues to be an im ressive number of investigations using isoto es other than t ose of Fe and Sn. Specifically, there haveLen approximately 300 papers on other isotopes since the last review. filEu continues to be the most popular isotope in this group, on which there are 80 published pa rs. Those isotopes about which there have been over 30 pubEations are 121Sband lS7Au.The interest in the latter case has been mostly due to the harmaceutical interest in the many new gold complexes. ?soto es on which there have been over 20 publications include h , lMGd,='Np, and 126Te. The other isotopes on which there have been at least 10 publications in descending order of popularity are 161Dy,171Yb,159Tm,67Zn, and 61Ni. Even though a ma'ority of the research papers are from scientish in non-Endish countries, the number of En lish written papers has shown a continuous increasing trenf ove; the last 20 years 80 that now approximately 90% are published in or translated into English. Another trend noted previously is the large number of reviews in the field of Mossbauer spectroscopy. The last 2 years have not been any different, in which there are over 180 reviews. We have selected approximately one-fourth of these to be mentioned in this general review. Clark et al. (9) have written an excellent review on the applications of Mijssbauer s ectroscop in chemistry. A noteworthy chapter appears in tEe book titred Structural Methods in Inorganic Chemistry, written by Ebsworth, Rankin, and Cradock (10). The other reviews are on the use of Mijssbauer spectroscopy to study various kinds of materials. These include amorphous
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materials (11-13), catalysts (14-16), cement pol mers (I@, semiconductors (19-21), and new magnetic m a t e d (22). Other reviews are on s ial features or processes in materials, 24), spin fluctuations and magnetic such as int defects materia&425,26), magnetism at surfaces and interfaces (27), microcrystals (28), spin transitions in Fe(I1) (29), dynamics of spin conversion (301,diffusion (311,critical phenomena (321, superparamagnetism (33), magnetic phase transitions (341, and corrosion (35, 36). Other reviews are centered around particular Miissbauer isotopes including slNi (37), B7Zn(B),and mNp (39). Other applicationsinclude archaeology (40,41), electrochemistry (42, 43),proteins (44,451, metallurgy (461, and clays and their fired products (47). Several other specialized topics include ion implantation and induced hase transitions (48), developments in synchrotron irra iation (49), magnetic ordering and slow paramagnetic relaxation at very low temperatures (50), magnetism of superconducting systems (511, and the investigation of atomic jump processes in ion beam experiments (52). Two other general topics havin been reviewed are magnetic systems (53)and structure andi bonding in solid com unds (54). We note that the primary journal for the pu%cation of papers on Mossbauer spectroscopy has become Hyperfine Interactions. Duting the period covered by this review, there have been 455 papers, which include 382 published in special volumes as conference proceedin s. We have continued to use the facilities of the Mossbauer Effect Data Center in preparing this review. The Center continues to ublish the Mossbauer Effect Reference and Data Journafhaving just completed the 12th volume in the series in 1989. A complete listing of all the papers that were considered for this review can be found in the 1988 and 1989 volumes and Issue No. I of the 1990 volume of the journal.
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INSTRUMENTATION The combination of the advances in both computer and electronic devices has brought about several improved developments for Mossbauer spectrometers. Kurinyi (55)describes a device for stabilization of the center position of the Mossbauer source. This particular feature further minimizes instrumental errors due to the stabilization of the Mijssbauer system. Evdokimov et al. (56)describe a device that simplifies the ad'ustment of the Mossbauer spectrometer and makes a multickannel analyzer unnecessary. Most of the advances in Mossbauer spectrometers have been in the improvements of the various special purpose spectrometers. Time-differential Mossbauer emission spectroscopy (TDMES) is important for studying chemical aftereffects and relaxation processes following electron capture of T o incorporated in various materials. Albrecht et al. (57) describe a new s ectrometer for time-differential Mossbauer spectroscopy wkch can handle high count rates and has the capability of being used at various temperatures. The spectrometer has both excellent energy and time resolutions. Kurinyi and Grotov (58) describe a differential Miissbauer spectrometer. The main feature of their spectrometer is increased sensitivity. Reittinger et al. (59) describe a frequency modulation spectrometer which allows for observing directly the phenomenon known as radio frequency (rf) sidebands without interference from the parent transitions. This spectrometer allows for the very careful examination of the rf sidebands for any fine structure. A most recent develo ment in the piezoelectric-type Mossbauer transducer is &scribed by Sakai (60). The spectrometer uses a pair of piezoelectric bimorph actuators. One of the main features of these particular transducers is wide application to other Mossbauer nuclei because of the capabilities of being used at liquid helium temperatures and enerating velocities of over a 100 mm/s. Seberini (61) descriks the development of a constant velocity electromechanicalMijssbauer drive that operates in the range -15 to +15 mm s with the velocity resolution of 0.005 mm/s. This kind o spectrometer is important for Rayleigh scattering of Masbauer resonance (RSMR) and also for selected excitation of hyperfine split nuclear sublevels. For example, this is a way for a source to provide purely polarized monochromatic low-energy photons. With the increased interest in the use of convemon electron Mossbauer spectroscopy (CEMS) there have been continued developments in examining materials at various depths using what has become known as depth-selective conversion electron
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MOSSBAUER SPECTROSCOPY John 0. Stevens Is Professor of Chemistry at the University of North Carolina at Asheville and was named Feldman Professor in 1984. He received his B.S. (1964) degree in Chemistry and Ph.D. (1969) in physical chemistry from North Carolina Stale University. He spent three Summers at Argonne National Laboratory. a leave 01 absence at Max-Planck-lnstitut fur Feskorperforschung. and a year leave of absence in addition to four summers at the University of Nijmegen in The Netherlands. His main research interest is M6SSbaUer spectroscopy and its application to the study Of antimony corn pounds and fired clay materials. His addie tional interests are problems of evalmtion an0 o~sseminmilonor S E I B ~ ~oats C and information. He is the director of the M6ssbaver Effect Data Center and coedits the M8sb8uer Effect Relerence and Data Journsi. He is an the executive boards of the International Commission for Applications of the MOSSbauer Effect (KAME) and National Conferences for Undergraduate Research (NCURI. He is a l a ~member ~ of the Council on Undergraduate R e search and is on the edtorial boards of Hyperfine Interactions, Magnetic Resonsnce Review. and CRC Handbwk of Spscfroscopy. He is a member Of the Amerlcan Chemical Society.American Physical Society. and Sigma Xi.
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tary InstitUte in 1956 and the p h . D h Phwcal chemistry from the MBSsaChUJBns Institute of TechnOlDgy m 1961 He has been ’ on the facuihr at North Carolina State Unwirsity~since6eptember 1961. His research Ip..I interests Currently are the application of M6ssbauer spectroscopy as an analytical techniqw in soil m i n e r a w and the study of magnetic properties 01 SUbStnUted iron oxides related to environmental minerals. He was on sabbatical leave in 1987 at the Laboratorv of Maonstism of the Slate Universtv ~,~ ~=~~ of Gent. Belgium. and in the Summer of 1989 was visning research professor there under the 5ponsorship of the Belgian National Fund tor Scientific Research. He is a member of the American Chemical Society. American Physical Society. the Clay Minerals Society. and Sigma Xi. ~
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Kalherbe M. Whatley is Assmiate Professor of physics at the University of North Carolina at Ashevilie. She recaived her B.S. degree in physics from Wake Forest University in 1977 and the m.D.in nuclear physics from Duke University 01 North Carolina in 1982. She has been on the facuity at the University of North Carolina at Asheville Since August of 1982. Her research interests include M6sSbaUer studies of tellurium compounds and pottery. She is on the Council on Undergraduate Research and is a member of the American Physical Society and Sigma Xi.
Mossbauer spectroscopy (DSCEMS). Aleckseev et al. (62) describe a depth-selective Miissbauer electron spectrometer that contains an energy analyzer of the cylindrical mirror type, a source drive system, and a recording channel. The resolution of the instrument can be set within 1.2-2.5% for a source 10 mm in diameter. Sauer et al. (63) describe a depth-selective version of electron Mosshauer spectrometer which uses an electrostatic cylindrical mirror analyzer. The spectrometer allows depth-selective investigations even of samples with 57Fe in natural abundance. In another development, a low-noise scintilation electron detector has been developed by Kajcsos et al. (64). This detection system is capable of detecting low-intensity and low-energy electrons in high vacuum. Its performance is compared with a channeltron and shows a substantial improvement in counting time, A design of a new retarding-field electron spectrometer for depth selection is described by van Noort et al. (65,66). This spectrometer is suited for samples of diameter up to 10 mm and is suited for energies up to 25 keV. Mosshauer spectroscopy has always been very dependent on the use of computers, in both the collection ofthe data and its analysis. In recent years the impact of computers on Mosshauer spectroscopy has been primarily the use of various
micro rocessors and microcomputers. Cavatorta et al. (67) descrige a fast microprocessor control data acquisition system for Mosshauer spectroscopy which operates in both the pulse height analysis and the multichannel scaling mode. It is able to handle counting rates greater than 20 MHz and provide excellent resolution using up to 8192 channels per spectrum. The channel-advance dead time is less than 50 ns, with the minimum dwell time of 1 ps. Another microprocessor control spectrometer is described by Grogan and Thornhill (68).The useful characteristic of their system is the ability to change to various wave forms and even to design them specifically for each experiment. They are able to control the number of the channels in which the counting is to take place, as well as the sweep rate. Pitzen e t al. (69) describe a four-input multichannel spectrometer which is based on the Motorola 68000 microprocessor. Software has been developed which and on-screen Drovides for a real-time ”eranhic screen disnlav ~, d a t a evaluatlon; Another mirrocumputer Spectrometer in u,hich spectra can he obtained simultaneously with data analysis is desrribed by Rao et al. (701. This system is designed for a microcomputer with 6 kB memory for data acquistion. Cheremisin (71)describes a special algorithm for spectrum processing by the method of least squares which considerably reduces the amount of calculations and consequently permits spectra to be processed on a microcomputer with a Basic interpreter. Another special algorithm, the Newton-Fiaphson, is described by Woodhams (72). This algorithm is implemented on an accelerator based on a network of INMOS transputers, interfaced to a personal computer. It is noted that a very significant speedup factor is obtained with a small number of transputers. For example, the fitting time for a six-line, 512-channel spectrum is 2.6 s per iteration with a four-transputer accelerator compared with about 12 s on a Victor AT computer. In another paper, Nasu and Nakayasu (73) describe a x-square fitting procedure on a microcomputer written in Basic. A common technique to use when detecting Mossbauer radiation in which the intensity of the radiation is too high for conventional nuclear counting is to use the “current integration” method which Cranshaw (74) describes a new detector and a 20 mCi 57C0source. One of the difficulties when using proportional detectors is that beryllium windows contain a small amount of iron and consequently in obtaining a spectrum in which the Mossbauer absorption signal is extremely small there may be an interferring signal caused by the iron in the beryllium. Rimhert and Testard (75) described a new material which may he used instead of beryllium which has good vacuum tightness and high nuclear radiation transmission. The windows that they describe are made of thin high-purity aluminum harriers interleaved in a structural polyimid f h s . The remaining papers on detectors all describe developments and detecting systems that are used in CEMS. Kamzin and Rusakov (76) described a proportional flow counter for CEMS which is operative in the temperature range 1W700 K. They studied various gas mixtures including He + CO, He + CH , and Ar + CH,. A proportional counter offering an even farger temperature range, from 1.5 to 1100 K, is described by Isozumi et al. (77). Their filling gas is pure helium at a pressure of 1atm at room temperature. Another low-temperature sense detector also using pure helium gas which works at temperatures near 4.2 K is described by Katan0 et al. (78). They report that they have been able to operate their detector at temperatures near 4.2 K for more than a couple of weeks at a time. Other improvements of these detector systems include a design for an ultrahigh vacuum spherical electrostatic spectrometer which was developed by Stadnik et al. (79). Specific application of this detector is for that of very thin oxide layers on surfaces of iron foils. Vartanov and Zemskov (80)describe an electroluminescence counter. Depending on the additives to the helium gas, this particular counter can operate in the scintillation or the electroluminescence mode. Evaluation of the performance of electron detectors was reported by two different research groups. Petrikin et al. (81) examined the optimum gas pressures, interelectrode distances, conditions for filtering, and collimation of the y-radiation. The second investigation by Kajcsos et al. (82) examines ways to optimize the use of scintillation counting for CEMS. A furnace has been developed by Heiming et al. (83)which is operative up to 1500 K. They have used the high-tem-
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perature oven to investigate diffusion in ure iron. On the other end of the temperature scale Didenfo et al. (84)have designed a compact cryogen attachment that can be placed on a standard helium Dewar flask which is operative in the temperature range 4.3-300 K. Alonzo et al. (85)have designed a new cryostat that is equipped with a Gifford McMahon cryogenetrator. The vibrations caused by the helium compression/expansion cycle have been limited to approximately 0.05 mm/s. Pasternak and Taylor (86) have developed a technique to obtain very high pressure b means of diamond anvil cells. These articular cells have teen used in applications using the l6lfu, llgSn, and lZsIMossbauer isotopes. Pressures up to 300 kbar have been obtained with these cells. In ano!her high- ressure apparatus, Adlassnig et al. (87)have obtained Mossgauer spectra of the hi h-resolution 93.3-keV resonance in 67Zn. They used B4Canvik and a sandwich asket in which they were able to obtain pressures up to 6 G a. They use a piezoelectric Dop ler drive which is mounted on top of the ressure clamp. "he whole system can be cooled to liquid Kelium temperatures. One of the more recent interesting ex eriments is the use of "imaging" with Mossbauer radiation. Klorton et al. (88,89) proposed and provided an ex rimental demonstration of this concept. While this particuE technique probably does not have any biomedical applications as is the case with NMR imaging, there is the feasibility of applications in the area of material sciences.
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DIFFRACTION AND SCATTERING While the geometry for Mossbauer spectrometers is most often that of absorption resonance, there are a number of interesting experiments that are being performed either in a scattering geometry or in a diffraction geometry. These are areas in which there is room for many developments and improvements in our understanding. Sarma et al. (90)have obtained an expression that describes the effect of thickness of the linewidth of scatter and Mhbauer spectra. In another aper Mei et al. (91) describe the intensities of the y-ray and 8-ray backscattered Mossbauer spectra using 57Fenuclei in different matrix materials. They have been able to explain the previously unusual negative peak intensities that have appeared. Follath and Jex (92) describe the temperaturedependent frequency shifts and resonance scattering of the Mossbauer radiation. One of the results of their theoretical investigation is that in the interference experiments the nuclear scattering is sensitive to the relativistic Doppler shift, while the electronic part is not. The implications of this are that previous interference experiments need to be reinvestigated carefully. Gabrielyan and Kotandjian (93) theoretidy consider resonance scattering, taking into account multiple scattering of the MWbauer radiation transfer win the theory of optical radiation transfer. They also consifer the frequency-modulated y-radiation scattering in a lane-parallel layer. A theoretical model for the analysis of Eackscattered conversion electrons depending on angular and ener distributions has been developed by Lee and Tatarchup(94). The diffraction of 14.4-keV Mossbauer y-radiation from a Fe3B08single crystal was investi ated in the Laue eometry by Tolpekin et al. (95). An anatsis was made of fow spin reorientation phase transition in a crystal affects the interference of resonance nuclear scattering of y-radiation by iron nuclei occupying crystallographically inequivalent 4c and 8d positions. In another case of Laue diffraction, Smirnov et al. (96) study anisotropic scattering. When observing differences between the transmitted and the reflected intensity for two polarization components, asymmetries are caused by strong anisotropy of the nuclear resonance scattering. There are several papers that investi ate various aspects of Rayleigh scatterin of Mossbauer ra8ation. Nienhaus et al. (97) investigated &e angle-dependent Ra leigh scattering from proteins. In particular, the protein $namics can be studied by the determination of the mean s uare displacement using Mossbauer spectroscopy. Specificdy, myoglobin was studied. Van Biirck (98) investigated the nuclear reflection of Mossbauer radiation at an an le near 90°. In particular a single crystal of FeB03 was stuiied. The authors indicate from their investigation that the antiferromagnetic pure nuclear background reflection (33 ll) is a promising candidate for the filtering of Mossbauer lines from white synchrotron l28R
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radiation. Bushuev (99)reports on the investigation of thermal diffuse scattering in the vicinity of the Bragg reflections. Gavrilov et al. (100) re ort on the hi h-frequency ultrasonic excitation that takes pgce in single #i crystals. Zolotoyabko and Gavrilov (101) examine the nonexponential behavior of the Debye Waller factor under coherent ultrasonic excitation conditions. In another paper, Kovalenko et al. (102) have investigated the influence of interference between the nuclear and Rayleigh scattering on the narrowing of resonance lines in Mijssbauer diffraction. In particular, they have examined electron Rayleigh scattering of Mossbauer y-radiation in a Fe3B, single crystal. Siddons et al. (103) report on a new apparatus for the study of nuclear Bragg scattering. Their monochromator in the system provides for an energy resolution of 0.005 eV.
LINE SHAPES Several ways have been developed to provide more precise fitting of the data using algorithms to speed up the amount of time taken to fit a complex spectrum. Mullen et al. (104, 105) describe a Fourier-transform method which is a simple analytical form of the convolution integral for Mossbauer transmission spectra. The anal is is examined for a number of isotopes including lazW a n 8 @ Wwhich includes the interference that has been observed for these particular isotopes. Another deconvolution approach has been described by Nikolov and Kantchev (106, 107). They obtained excellent results for Lorentzian broadened spectra. Rex et al. (108) describe the finite impulse response operator used to obtain improvements in the fitting of Mossbauer spectra. This technique is another deconvolution procedure. Some of the s ecial applicationsof careful line-shape fitt' of spectra inc1ub)e the theoretical invest' ation of time-res013 Mossbauer spectra of magnetic mater& which are subjected to rf magnetization reversal. This investigation is reported by Sadykov and Skvortsov (109). Miglierini (110) examines several different distribution profiles in convolutions of Gaussian and Lorentzian spectra, fitting the profiles from Mossbauer spectra analysis of metallic glasses. Afanas'ev et al. (111) consider cases when there are more than one component according to valences. Another special application is that of diluted ferromagnetic all0 thin films which have been investigated by Urbaniak-Kuc&uczyk and Osetek (112). These spectra are very sensitive to concentration changes of the ma netic atoms. Julian and Daniels (113) provide a thoroug investigation on the collapse of Mossbauer spectra in strong applied rf fields. They conclude the primary effect of the rf field is not to cause fast switching of magnetization but rather to destroy long-ran8e magnetic ordering below the Curie temperature. In two different pa ers, line shape and scattering are considered. Kumar et a f (114) investigates Mossbauer scattering thickness and interference effects. In another paper Morozov (115) provides analytical expressions for the Mossbauer parameters.
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THEORY The theoretical papers covered in this section concentrate on calculation of hyperfine parameters for a variety of systems and by a variety of techniques. Bominaar et al. (116) used all-electron ab initio calculations for FeX diatomic molecules to predict isomer shifts and quadrupole splittings. These are compared with experimental results on matrix-isolated systems. Model calculations were also made to estimate the matrix interaction between FeC and a cluster of rare gas atoms. A tight-bindingband structure calculation was apphed by Lefebvre et al. (117) to a series of antimony tellurides, sulfides, and iodides, with particular attention to calculation of the 5s electron density at Sb and com arison with experimental isomer shifts. Svane and AntonciE (118) used a linear muffin tin orbital (LMTO) method also applied to covalent compounds of antimony as well as tin, tellurium, and zinc. This paper summarizes their previous work and evaluates the isomer shift calibration constant for the isotopes ll9Sn I2%b ' V e , and 67Zn.The variation of the isomer shift with pkessur; for metallic tin is also discussed usin the same theoretical model. Magill and Roberts (119) used relativistic augmented-plane-wave techniques for the Sn versus pressure isomer shift calculations and also report additional experimental data around the phase transition at 92 kbar. Eriksson and Svane
MOSSBAUER SPECTROSCOPY
(120)have applied the LMTO method to calculate isomer shifta and hyperfine fields for some common iron compounds to rovide new calibration constants for 67Fe. h e point charge model for electric field gradient calculation was applied to random networks of corner-sharing octahedra by Greneche et al. (121)with the ob‘ective of simulating amorphous solids such as FeF The distribution resulting from the calculation gives ood agreement with experiment. Distribution of hyperfine belds and the determination of a distribution from the Mossbauer spectrum are discussed by Brand and Le Caer (122). They compare the method of maximum entropy to the usual matrix methods and note that the smoothin parameter can be avoided. Bayreuther (123) surveys calcuktions on ferromagneticsurfaces and thin films and notes the difficulties encountered in explainin theoretically many magnetic phenomena at surfaces. Ihcalov (124)investigated the combined influence of heterophase fluctuation and phonon excitation in ferromagnetic materials. Emphasis is given to the behavior of the recoilless fraction around the transition region. Another area in which theoretical a ers are active is relaxation phenomena. Afanas’ev et a f b25) consider spinlattice relaxation for Fe3+in an axial crystal field. The relation between Mossbauer spectra and magnetic field direction is given in terms of two relaxation parameters. Putzka (126) presents a method for computation of Fe3+relaxation spectra which lends itself to least-squares treatment. Price used a stochastic model to calculate relaxation spectra for FeSiF6. 6H 0 (127)and discusses its application to line broadening at &e 240 K phase transition. Magnetic cluster excitations and their effects on Miissbauer spectra of magnetic materials are discussed by Rancourt (128). The generality of such behavior and interpretation of the spectra in terms of autocorrelation time for fluctuation are emphasized. Adler et al. (129)present a eneral formulation of the Mossbauer relaxation spectra o the Fe(I1) low spin-hi h spin transition in the presence of absorber texture and $nit, thickness. The physics of super aram netism in relation to Mossbauer spectroscopy is d!scussa by Jones (130).
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CHEMICAL AND BIOLOGICAL STUDIES It is difficult with many papers to decide whether they are more ap ropriately mentioned under Materials Studies or in other o the general cate ories of this review. Use of Mwbauer spectra as a smab part of characterization of new compounds continues to be important, but such pa ers are too numerous to cover in this review. This section d!scusses a selection of pa rs which emphasize Mossbauer spectrosco y applied to probgm of a chemical nature, including examppes relevant to biology. Emphasis beyond their fre uenc has been given to papers dealing with isotopes other &an &‘e in compensation for the emphasis on iron studies in most of the other sections. An intereating Miissbauer emission experiment by Devillers et al. (131)involves a 67Ni source in nickel oxalate. The cumulative effects of the double decay to 67Fewere measured b M h b a u e r spectra usin a moving stainless steel absorber. &e 67Nielectron capture 8ecay is ener etic enou h to break the chemical bonds and indeed at 90 If the r e s u b indicate about 60% of the deca s give recoil of ‘j7C0. The perturbed Fe states were annealei at room temperature. Demazeau et al. (132) report two new iron(1V) oxides in the series SrLa&,6Feo,60, with M = Mg or Zn. For both these oxides the Mossbauer spectra show most of the iron is high-spin Fe(1V) in two distinct sites. The 4 K s ectrum of the Mg com ound exhibits antiferroma etic oraering. Engelmann et af (133)have studied LiNb&-Fe20 a system with potential electro-optical a plications, using k t h Mossbauer and ESR s ectroscopy. TRe properties of the iron component depenxed on the quench method, with slow quenching favoring oxidation to Fe3+and cluster formation. One of the classic chemical applications of Mossbauer s ectroscopy was to the carbonyl Fe (CO) In 1988, &andjean et al. (134) reported a detailecf Mossfauer spectra versus T study down to 4 K which shows distinctive variations both in the quadrupole splittings and the relative peak areas. These variations are discussed in terms of vibrational asymmetry and other properties of the molecule. Mi anaga et al. (135)studid the electrical conductivity of Fe&O)s(diene) complexes doped with iodine and used 67Fe Mossbauer
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spectrosco y to determine the nature of the iron s ecies involved in t\e conduction: mainly a low-spin Fe(I8 species. Kretchmar et al. (136) studied electrophoretically purified human transferrin by Miwsbauer s ectroscopy. This rotein has two metal sites and thus can e! monoferric or $ferric. From spectra of the monoferric species at 4 K in an applied field, the spin Hamiltonian could be determined. From the s ctra of labeled %Fe5?Fediferric transferrin, it was shown tEt one site occupancy had little effect on the other. A six-coordinate, high-spin Fe(II1) protoporphyrin has been studied by Mossbauer spectroscopy while encapsulated in a detergent micelle. According to the paper by Medhi et al. (137),it is the micelle that apparently stabilizes the complex in a form similar to the haemproteins. The aqueous solution was frozen to 78 K to obtain the Mossbauer spectrum, which was an asymmetric doublet with typical spin relaxation behavior and with similar parameters to aquo met myoglobin. Another biologically related paper is the l19Sn Mossbauer study by Barbieri et al. (138)of organometallic SnR? bonded to rat hemoglobin. Spectra at 4 K in an external field were taken in order to investigate the bonding. Results on model systems are also discussed. A quite different application of ll9Sn Mossbauer spectrosco y is that of Andreasen et al. (139)to bonding of tin fluor& and oxide molecules isolated in a noble metal matrix. The use of ion beam anal is in conjunction with Mossbauer spectra to characterize t g s e species is emphasized in this review. The l2ISb Mossbauer spectra of cyclopentadienyl antimony halides are discussed by Berlitz et al. (140), in relation to other Sb(II1) compounds. Isomer shift trends have been related to electronegativity differences for the ligands and *-donation ca ability. Parish (141) has reviewed the recent chemical apphations of lnAu Miwsbauer s ctroscopy, emphasizing the effects of structure and bondin or the three oxidation states. Vibrational motions of golf cluster compounds have been analyzed by Smit et al. (142)from ID7Au Mossbauer results in con’unction with specific heat measurements. The difficult Bgku Mossbauer spectra of a number of complexes relat’ to “ruthenium red”-Ru302(NH3) have been obtained a 3 a r e discussed by Wagner et a\. (143) Charge delocalization of the odd electron in the oxidized 7+ complexes is proposed to explain the Mossbauer s ectra. Jov6 et al. (144) discuss isomer shift trends for v37Npin simple inorganic compounds. A ligand bond distanceisomer shift correlation is proposed for hexavalent compounds. The effect of coordination number for Np(V) and Np(V1) is surveyed. It is shown that overlaps occur in the isomer shifts of the higher oxidation states. Mossbauer s ectroscopy of actinides in general has been reviewed by K a h u s (145). He gives examples in particular from 237Np.Intermetallic compounds of neptunium have been studied by Mossbauer spectroscopy at high prwure by Potzel et al. (146). This paper emphasizes the m netic properties and the range of delocalization behavioryound for the 5f electrons. Finally in this section mention should be given of a review by Krop (147) of Mhsbauer s ectroscopy on the “R-T” intermetallics with rototype Nd2fe B. Although 67FeMossbauer results have Eeen important Kr these materials, five of the rare-earth isotope constituents have been also studied by Mossbauer spectroscopy with varying success. Emphasis in this review is given to magnetic properties.
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AMORPHOUS MATERIALS In the last 2 years over 350 apers have been published involving Mossbauer effect stuies of amorphous materials. Many of these pa ers reported findings of Mossbauer effect research on m e d c magnetic materials. Miglierini and Sitek (148)approximated the Miissbauer spectra of Fe-rich metallic glasses with six broadened lines having line position relations similar to those of cy-Fe. Dubiel et al. (149)investigated the influence of an external magnetic field on the reentrant spin-glass transition temperature of a Cr76Fe26 alloy. They observed a linear relationship between the external field and the transition tem erature and interpreted their results in terms of a mean-fiepd theory model. Van Cauteren et al. (150) compared the results of Mossbauer spectroscopy and perturbed an lar correlation s ectroscopy in the study of the magnetic ghavior of the a&y Aul-*Fe,. Ryan et al. (151) observed transverse spin freezing in amorphous Feg2Zr, combining high-field magnetization measurements with ANALYTICAL CHEMISTRY, VOL. 62, NO. 12, JUNE 15, 1990
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Mossbauer spectroscopy with and without an external magnetic field. A review of recent 57Fe and 61Ni M6ssbauer spectroscopic studies of Fe and Ni magnetic moments in crystalline and amorphous alloys was presented by Stadnik and Stroink (152).Fries et al. (153)found strong evidence for chemical short-range order in Fe-poor Fe-Zr and F e H f alloys. The h dime arameters of the Fe-rich alloys exhibit complex a p p E fielzand temperature dependencies. Matteazi et al. (154)examined the alloy Metglas 2826 after three types of heat treatments: batch annealing, continuous laser annealing, and m netic annealing. They found that uce structural relaxation and imlaser annealing can provement of the sot! magnetization properties of Metglas 2826 and that annealing in a magnetic field can produce a controlled change in the easy magnetization axis direction. M6ssbauer spectroscopy was shown to be effective in characterizing the changes induced by the heat treatments. In another stud of the effect of heat treatments on amorphous materials, Civaguera-Mora et al. (155)found that changes were induced in the long-range order parameters but not in the short-range order parameters of Fe,Ni&,-, glassy ribbons. Two distinct relaxation mechanisms were seen to be induced by the heat treatment, one that forces the local magnetization to be more aligned with the ribbon plane than in the asquenched glass and the other that randomizes it. The separate effecta can be generated by selective heat treatments. emEik et al. (156') produced magnetic anisotropy in FeyCr&igSb and Fe&&m amorphous alloys by stress annea ing a ove the Curie temperature. Mossbauer spectroscopy proved to be sensitive to the effects of the induced anisotropy and indicated the preferred formation of longitudinally magnetized domains by creep annealing of the amorphous ribbons. Pollard and Foley (157)measured the Mossbauer s ectra (x = 10, 50) and F$" at of amor hous Fe,Ni 4.2-300 and in &f ap lied magnetic fields. hey reported temperature-indueexchanges in the fluctuations and correlations of the hyperfine parameters. The standard deviation of the magnetic h rf'ine field was also found to vary with tem rature, d e p e n G on the Fe concentration. ZemEik and Hav icek (158)made isothermal measurements of crystallization kinetics at temperatures between 260 and 350 "C for FemColJ3 and F C021B15 amorphous alloys. Addition of Co to F e - B % d m% magnetic amorphous alloys improves their saturation magnetization. Sanchez et al. (159)observed extremely short range order, involving only a central atom and its Fe nearest neighbors in the amorphous phase of rapidly quenched Fe B14ribbons after partial crystallization at 420 "C. Lindero% et al. (160)examined ultrafine particles of amorphous FeszBw The amorphous structure of the particles was found to be similar to that of amorphous films and ribbons. Le Caer and Bauer-Grosse (161)combined Mbsbauer spectroscopy and electron diffraction to investigate the structures of new carbides formed by crystallization of amorphous Fel- C, alloys (0.29 < x < 0.32). The structures were found to change continuously with x. The composition dependence of the Mijssbauer spectra and thermal expansion in rapidly quenched amorphous Fe-Y and (Fd3o)Yalloys is described in Ishio et aL (162).The Fe atom in the alloys was found to have a magnetic moment when it had more than six Fe neighbors. The magnetic moment increased with increasing average number of Fe neighbors. Wiesin er and Hilscher (163)presented recent progress in basic a n b applied research on Nd-Fe-B-type permanent magnets. Nd-Fe-B-type materials have attracted much attention in recent ears and show eat technological promise. Nagayama and (164)used fkjssbauer spectroscopy in conjunction with differential scanning calorimetry, X-ray diffraction, and thermomagnetic measurements to examine the c stallization processes for a wide range of amorphous Fel %d1J3$+ alloys (0.1 5 x I0.6,O5 y I1). Ryan et al. (1657 also considered Nd-Fe-B-type materials. They examined a range of all0 s of the form (Fe -xNd,)lmB (0 Ix I 25,5 S y I20) andlcompared the eflects of metai and metalloid alloying. Pillay et al. (166 167) found that results of bulk ma etic response and 5tFe Mossbauer study of a new reentrant spin glass system, and the series Fe rRul ( P & R :l3Fe refute the interpretation of data on the average hyperdne field at the Fe site in terms of well-defined phase transitions, as in the Gabay-Toulouse model. The reentrant behavior was thought to arise from small clusters constraining
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the dynamics of bigger quasiferromagnetic clusters and appeared to be a continuous process. Hayashi et al. (168)used conversion electron Mossbauer spectroscopy (CEMS) to study ion-beam-induced reactions of multilayered Fe/Si thin films. A significant mixing was found to occur in the interface region between the overlayered films and the substrate even in the case of coating filmsthicker than the irradiating ion ran e. This mixing led to a nonuniform magnetic hyperfime fiefd in the mixed amorphous layer. Antimova et al. (169)produced feqomagnetic, m y n e e p t i c glasses by introducing magnetic microparticlea, in this case Fe, into the pores of a high-silica microporous glass and then thermally consolidating the material into a quartzoid glass. Results are reported on the formation of magnetic particles at various stages of the heat treatment from 290 to 1370 K. Kaganovskii et al. (170)considered ferrosilica gels of different compositions and found that the Mhbauer spectra enerally corresponded to amorphous particles of Fe(OH)1+2, l-x (0 5 x 5 1) of size 3-5 nm. The data suggested an appreciable interaction among the ferrosilica compounds. Myaen (171)used 5'Fe Mlissbauer spectroscopy to determine the abundance of ferric and ferrous Fe in silicate melts. Experimental data from binary, ternary, and quarternary systems can be combined to derive numerical expressions for melts with at least eight components that relate redox equilibria of Fe oxide to temperature, 0 fugacity, and bulk chemical composition of the melt. Borosilicate glasses, important because they are suitable host matrices for the immobilization of high-level radioactive waste, were the subject of studies by Music (172)and Hunter et al. (173). Music found the valence state of Fe and its coordination in borosilicate glasses to depend stron ly on the chemical composition, Fez03content, and the tecfinique of the preparation of the lass. Hunter et al. compared results of wet chemical and dossbauer spectral analyses in determining the Fe2+/Fe3+ratios of simulated nuclear waste glass samples. Mossbauer spectrosco y was not recommended for use on highly radioactive samp es since the sample's radioactivity can influence the Mossbauer spectrum. Tanaka and Soga (174)were also interested in the chemical state of Fe atoms, but in the systems Fe203-P205,-TeO -SrO, -BaO, -PbO, and -BiZO3 All the Fe atoms were foun8 to exist as Fe3+ions in all glasses except FeZO3-P 05,in which Fez+ions also existed. The effect of covalency otFe-0 bonds on the magnetic properties of the glass was also discussed qualitatively. Li et al. (175)pre ared BaFelzOl fine particles with the glass-ceramic methofand investigad the effect of technology factors on the Mossbauer spectra and H,. Nishida et al. (176) made 57FeMossbauer measurements of semiconductingand superionic conducting silver vanadate glasses containing 40 mol % AgI and 1mol % 57Fe203and found that these glasses are composed of V04 tetrahedra. The introduction of A into the V205matrix results in a drastic change in the gPass O matrix. Many other amorphous systems were examined by a variety of researchers. Sawicki et al. (177)studied the Au-Si interface in implanted and deposited thin films. The Au-Si interface is of interest because of its significant role in hi h-speed VLSI electronic devices. Enzweiler et al. (178)deckced the local chemistry of Te impurit atoms in amorphous and crystalline SiSez from 12Teand 9I Mossbauer spectroscopy. Characterization of the Te sites PO ulated yielded information on the medium-range order in ! h e p McNeil et al. (179)combined Mossbauer spectroscopy wth Raman scattering to examine the structure of Ge Sn,Sez glasses in the range 0 I x 5 0.7. Results indicated: phase separation of GeSe and SnSez in the amorphous state. Chambouleyron et al. 9180) also considered Ge-Sn amorphous alloys. They reported on the structure of defects around Sn atoms in the amorphous alloys and on the influence of atomic hydrogen on the structure of such defects and concluded that a new Sn octahedrally coordinated bonding configuration was present. The new bonding confi uration may be the origin of the degradation of the optoekctronic properties of Ge-Sn alloys found experimentally. Hellstern et al. (181)determined the glass-forming ranges in mechanically alloyed transition-metal-Zr alloys. Glass formation by mechanical alloyin was found to be possible in the central composition range. 6unlap et al. (182)reported
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the first experimental indication that a transition metal may occupy the icosahedral site in a quasicrystal. The value of the quadrupole splittin from measured "Fe Mossbauer spectra of melt-spun TiMk Fe Sils indicated a highly symmetric Fe site in the i c o s E d r d structure of the alloy. Few Mossbauer studies have been performed on natural Saragovi-Badler and Labenski (183) applied f k g a u e r spectroscopyto the study of volcanic glasses and closely associated clay minerals. The volcanic glass samples were shown to have a hi h ercentage of Fe2+in octahedral coordination and some !'eg in tetrahedral and octahedral coordination.
MATERIALS STUDIES In this section we review example applications of Mbbauer spectroscopy to the study of phase transitions, defect structures, surface effects, and pro erties of microcrystals. Also mentioned are Miissbauer stuiies at high temperature, high pressure, and in external fields. Changes in Mijssbauer parameters around a phase transition can often lead to increased understanding of the atomic and electronic rearrangement. De Grave and Vochten (184) measured the temperature dendence of the Miissbauer spectra for iron uranyl phosphate ydrate. The drastic changes in the ferrous hyperfine parameters at 240-260 K are related to a change in the nature of the water layer from ordered to quasi-liquid. The quadrupole splitting distribution exhibited two distinct peaks around the transition region. High-temperature electron delocalization in the mixed-valence iron silicate ilvaite has been well studied in the past. The low-tem rature magnetic transitions are the subject of a paper by I& Xuemin et al. (185). This mineral has two distinct N6el temperatures, one at 116 K for both Fe2+and Fe3+on the A sites and a lower one at 36 K for Fe2+on B. The large difference between these transitions is ascribed to spin frustration at the B sites. Pasternak and Taylor (186)used both '%n and 1qMijrssbauer spectra to study the high-pressure phase transition of SnIl from covalent molecular crystal to amorphous metal. A diamond anvil cell was used for measurements up to 27 GPa. These authors discuss a model for the high- ressure phase which accounts for its hi h conductivity. T l e ferroelectric hase transition in LiNb& doped with 67Fewas studied by %mov et al. (187). single crystals were measured in a furnace under a reducing atmosphere up to 1500 K. The recoilless fraction exhibits strong anisotropy. At about 1000 K, well below the transition temperature, a broad singlet ap ars in the Mossbauer spectrum which is attributed to Epping diffusion and quadrupole relaxation. Another high-temperature study deals with pure iron metal in the range 1200-1500 "C. Using a s cial furnace, Heiming et al. (188)investigated the line broagning due to diffusion in the fcc y-phase up to 1390 OC, the * h-temperature BCC &phase,and the transition region. The md agreement between the calculated diffusion coefficient ror the y-phase and that determined by tracer studies, in contrast to some previous alloy diffusion studies. There is considerable interest in defect structures in semiconductors. MBssSauer studies have been used in particular to stud the nature of the deep-level DX center in G d s and Gal-pyb"9 Gibart et al. (189)report "gSn Mijrssbauer spectra of tin- oped Gal- A1,As for the complete range of x . The spectra were fitted to three singlets and to a doublet plus a singlet, corresponding to shallow donors and clustered species. However, the x = 0.3-0.4 samples had spectra indicating an additional site, which was associated with the DX center. These spectra are also discussed by Van Vechten (190), who offers an alternate interpretation of the DX center as an antisite defect. Langouche (191) surveys l2Ve and lZeI Mossbauer experiments in G d s and Gal,A1,As. Although low do ant concentrations lead to substitutional site occupang,&her concentrations give a defect c o d ation, which exhlbita uadrupole splitting m the 1qspectragm implanted -'e. h e latter is tentatively attributed to the DX center. Emission Mossbauer s ectroscopy was used by Okada et al. (192) to study llQSbadlsorbed on the surface of the ferrimagnetic CoFe204. The broad emission spectrum from the llsmSndaughter was analyzed in terms of a hyperfine field distribution with two distinct components at room temperature and three at low temperature. A tentative model for adsor tion which accounts for sites with different supertranskrred hyperfine fields is presented.
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Externally applied magnetic fields can assist in interpreting low-temperature Mijrssbauer data but sometimes produce new effects such as magnetic phase transitions. Dai et al. (193) report a field-induced phase transition in TQ,$eo S3at 4 K and an applied field of about 1.5 T. This materid is a quasi-one-dimensional conductor with three distinguishable chains, two of which exhibit charge density waves. The Mossbauer spectra indicate that these chains become antiferromagneticall ordered at high fields. Saifi et al. (194) 0 down report a detailedrhlossbauer study of ZnCr,Ga to 1.4 K, with and without an externally applied%el$. The low-temperature spectra varied depending on the substitution x ( x was varied from 0 to 1with all samples having 1% 57Fe doping). For hi h n > 0.97 a first-order antiferroma netic transition was o%served. This ordering is perturbed y increasing Ga to produce a spin glass state with observable relaxation effects in the Mossbauer spectra. A complex magnetic phase diagram is presented with at least seven regions marked out on the T versus n plot. Smit et al. (195) studied the magnetic chain compounds FeC204.2H20and RbFeC13.2H20 at low temperature and in an external field. They present a model for the nonlinear excitations that produce relaxation effects in the Mossbauer spectra. In their model two distinct subspectra have similar static Mbssbauer parameters but different relaxation mechanisms and rates. Another quasi-onedimensional chain compound, KJ?eFS.H 0, has been studied at very low temperature, down to 0.3 d i n a helium-3 cryostat, by Taklcs and Reiff (196). A three-dimensional ordering was observed below 1K, and thisordering was shown to be antiferromagnetic by external field measurements. The effects of Al substitution on the magnetic field distributions for hematite, a-Fe203, were studied for substitutions up to 28% by De Grave et al. (197). Several fitting procedures are compared and a model-independent method is judged to be best in fitting results over a wide range of A1 content and temperatures. Among Mossbauer studies of lattice dynamics, the work of Bauminger et al. (198) on iron-depleted ferritin deserves mention. Although the iron storage protein has been much studied in the past, this paper shows the development of the iron core with only 4-40 irons er protein molecule. The spectra show the presence of F$+ and dimeric Fe3+as well as Fe3+ clusters. High-pressure Miissbauer studies of single-crystal Sn are reported by Opalenko and Kornilova (199). From measurements in two different orientations, the determined the variation of anisotropy with pressure of the eQSn Mossbauer probability and relate this to a theoretical model for the lattice dynamics. Schaer et al. (200) used the 67Zn Mossbauer resonance to study the temperature de endence of the Mijssbauer parameters in single-crystal ZnO. fery little anisotropy in the recoilless fraction was found, and the data were fitted to a Debye model. The isomer shift exhibits a e due to phonon-induced c temperaturedependent c transfer. The unusual Wr oasbauer effect was used to stu y
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structure are reported. The recoilless fraction was fitted by a Debye model. Melting of the bubbles did not occur even up to 230 K. Mijrssbauer spectxoscopyof thin iron filmshas been an active research field, with papers dealing with the structure of the film itself and with reactions occurring with the substrate. CEMS (conversion electron Mijssbauer spectroscopy) has been common1 used for these studies. Macedo and Keune (202) report CJMS studies of l e 1 7 monola er 57Fegrown on Cu(100). These iron films have the BC8 y-structure and the CEMS results show they are paramagnetic at room temperature and order antiferromagnetically at 65 K. Iron films on Ag have the more common BCC struture of a-Fe but exhibit different properties depending on the face of the Ag crystal used as substrate. Using CEMS on films grown on Ag(100), Volkening et al. (203)show that the orientation of the magnetic moment depends on film thickness, being perpendicular to the plane for
