Mössbauer Spectroscopy: Some Views

Review Paper

Mi ssbauer Spectroscopy: Some Views

Leopold May

Department of Chemistry, The Catholic University of America, Washington, D. C. 20017

(Received 13 January 1969; revision received 24. February 1969)

Some views are presented of the present status of M6ssbauer spectroscopy and its future develop- meuts. The basic phenomenon ~nd equipment are described. Analytical aspects, both qualitative and quantitative, are discussed. Illustrations are given of the applications to structural chemistry, surface chemistry and catalysis, metallurgy, terrestrial and extraterrestrial minerals, and biological systems. INDEX HEADINGS : M6ssbauer spectroscopy; Review; Applications; Instrumentation.

I N T R O D U C T I O N

The year 1968 marks the beginning of the second decade of one of the newest spectroscopic techniques used for the invest igat ion of the s t ructure of materials. Nuclear resonance fluorescence or the MSssbauer effect was first, repor ted in 1958 by Rudolf L. MSss- bauer, for which he received the Nobel Prize in 1961. The invest igations with this technique remained pri- mari ly in the hands of the physicist until about 1960 when one of the isotopes of iron was found to exhibit the ~,I6ssbauer effect. Since tha t t ime the techniques have developed so t ha t it could be used by chemists for the invest igat ion of iron compounds. Since 1967, t in chemis t ry could also benefit f rom investigations with this new method, and other isotopes of interest to chemists have become available. The use in indus- trial and ill research laboratories has been spurred by the avai labi l i ty of commercial ins t rumenta t ion in the last few years. Research in iron (53.5%) and tin (17.7%) has accounted for a m Q o r i t y of the publica- tions in this field}

MSssbauer spectroscopy akin to other forms of spectroscopy is the s tudy of the radiat ion emit ted or absorbed in the t ransi t ion between two or more energy levels. The levels are those of the nucleus, and the energy of the radiat ion is found in the v - ray region. In the observat ion of nuclear resonance absorption, the energy of the t ransi t ion in the source (v-ray emit ter) and the energy absorbed in the absorber mus t be equal (Fig. 1). Since the emit t ing or absorbing

ENERGY LEVELS

gnd I SOURCE ABSORBER

E F G = O EFGdO EFG=O H = O H=O H / O

velocity

(a) (b) (c)

Fro. 1. Nuclear transitions and spectra of M6ssbauer spectroscopy. EFG--Electron field gradient; H--Applied magnetic field.

v - r ay impar ts recoil energy to the nuclei causing a decrease in the energy of the v-ray, it is difficult to observe resonant absorption. MSssbauer ~ found tha t in crystalline latt ices a t t empera tu res significantly below the Debye tempera ture , the nucleus could emit or absorb ~,-radiation M t h litt le or no recoil energy removed from it, and resonance could be observed. I f the nucleus is placed in a different electronic (chemical) env i ronment (as in absorber in Fig. 1), the t ransi t ion energy is changed destroying resonance. To restore resonance, addit ional energy mus t be added to or sub- t rac ted f rom the v-rays of the source (as in Fig. 1) if the source and absorber do not have the same transi- t ion energy, i.e., same electronic environment . The relative mot ion between the absorber and the source adds Doppler mot ion energy to the sys tem permi t t ing the resonance conditions to be reestablished.

I f the nucleus has a quadrupole m o m e n t and the electric field gradient (EFG) abou t the nucleus is asymmetr ic , the nuclear energy levels are split (for iron, Fig. lb) . This gives rise to a doublet in the 2~{6ssbauer spectrum. A Zeeman effect can also be observed in a magnet ic field (external or internal) giving rise to a six-line spec t rum for iron as i l lustrated in Fig. lc. The s imultaneous in teract ion of the E F G and a magnet ic field with the nucleus causes the resulting spec t rum to be asymmetr ic .

I f the source and the absorber are identical, resonance absorpt ion will occur, bu t if t hey are not identical EFig. l (a)~, the resonance absorpt ion will occur a t a veloci ty different f rom zero. This difference is the isomer shift ~ and is re lated to the s-electron densi ty a t the nucleus. I f the spec t rum is split, as in Fig. lb, the difference between the velocities of the two peaks is the quadrupole spli t t ing (AEQ). This spli t t ing is influenced by the configuration of the electronic envi ronment around the nucleus, and its magni tude yields informat ion abou t the bonding of the atom. The extent of the spli t t ing of the lines as in Fig. lc, can be related to the internal magnet ic field in ferromagnet ic and paramagnet ie materials providing a means for measuring the in tensi ty of the magnet ic field in a wide range of materials.

The ~{Sssbauer effect has been observed with abou t 50 isotopes, including several rare earths, t ransuranic

204 Volume 23, Number 3, 1969 APPLIED SPECTROSCOPY

elements, iron, tin, and iodine. I t is with the la t ter three elements tha t most chemical investigations have been made since the sources are most readily available commercially. The preparat ion of the sources of many other elements requires the proximity of a nuclear reactor or particle accelerator since their half-lives are short. The present discussion, therefore, will deal almost exclusively with results obtained with iron and tin.

I. EQUIPMENT

The 2~i6ssbauer spectrometer essentially consists of a device to move the source relative to the absorber (drive), a detection system for the v-radiation, record- ing system, and data reduction system (Fig. 2). The choice of the various components in the system depends upon the isotope being examined, for example, the detector, which may be a scintillation counter or proportional counter, and the method used for record- ing the data, constant-velocity or constant-acceleration. In the former mode, the drive is set at a constant velocity, and the ~,-radiation counted for a convenient time. The velocity is then changed. This is a point-by- point procedure with the results tabulated manually by the observer. The constant-acceleration mode is more satisfactory but requires more expensive equipment to record the data. The output from the detector is stored as a function of the velocity in a device such as a 200-channel analyzer. The spectrum can then be displayed on an oscilloscope, and a photograph of this display made. The ou tpu t from the analyzer can be fed directly to an x - y plotter, typewriter , or teletype equipped with a tape punch to record the data.

The drive used in the commercial equipment is of the electromechanical type. The various types of equipment are shown in Figs. 3-5. They are adaptable for low temperature operations with specially designed cryogenic accessories, which is essential for examining frozen solutions. The equipment shown in Fig. 3 is specifically designed for rapid data acquisition using very active sources, for example 50-100 mC. These packaged units allow one to obtain M6ssbauer spectra with a minimum of difficulty tha t usually accompanies equipment built by the investigator himself.

For bulky samples such as steel plates or surface formations, it is possible to obtain MSssbauer spectra using the scattered radiation. 3,t This allows observations to be made without any sample preparation. The geometric arrangement of the source, sample, and detector are shown in Fig. 6. In this arrangement, the

FIG. 2. Generalized M6ssbauer spectrometer.

E

I DATA REOUGTION

SYSTEM

FIG. 3. M6ssba,mr equipment (Courtesy of the Austin Science Associates).

FiG. 4. M6ssbauer equipment (Courtesy of the Elron Electronics Industries, Ltd.).

FIG. 5. M6ssbauer equipment (Courtesy of the Instrument Division, International Chemical and Nuclear Corporation).

6.3-keV x rays that are scattered from the sample are counted. These x rays result from the internal conversion of the absorbed 14.4-keV ~,-ray by the ~TFe in the sample. The spectrum is inverted from the transmission spectrum since it is an emission spectrum. A conversion electron is also emit ted and with the appropriate detection system could also be used to measure the back-scattering. In addition to determining the nature of the material at the surface, it can be used to measure the thickness of the surface layer. 4

The information extracted from the spectrum included the peak positions, heights, half-widths, and areas. These can be found manually from a plot of the data, but usually the peaks are fit by a least-

APPLIED SPECTROSCOPY 205

FIG. 6. Back-scattering mode for M6ssbauer spectrometer.

squares procedure using compute r techniques. The peaks are fit to a Lorentzian profile whose constants give the desired information. This requires tha t the ou tpu t of the analyzer be placed on paper tape or punched-cards for use with a computer . Goodman 5 has discussed the problems involved in the use of on-line computers and described the appl icat ion to a small computer , the P D P - 8 computer .

I I . A N A L Y S I S

A . Q u a l i t a t i v e

MOssbauer spectroscopy can be used to detect the presence or absence of ~ given element because each element requires a specific source. For m a n y samples, the spec t rum m a y be obtained wi thout al terat ion of the sample. However, for m a n y elements, some compounds, and solutions, low tempera tures are required because the amount of recoil of the nucleus is too large to permit observat ions a t room tempera ture .

The magni tude of the isomer shift ~ is related to the oxidation s ta te of the meta l <r (Table I). I t is easy to distinguish between the various oxidation states of tin if ~ is known. In the case of iron, the various oxidation s ta tes require a knowledge of bo th ~ and ZXEQ, a s well as the change of ZXEQ w i t h tempera ture . For the ionic compounds (high spin), the + 2 and + 3

Table I. Relationship between MSssbauer spectral parameters and oxidation states of iron and tin."

Quadrupole Oxidation st,~te Isomer shift splitling QT b

IRON

0.0 -0.1 0.2 -0.5

~0.0 .~0.0

0.03-0.40

0 ~ 2 . 3 organo iron 0.0 - 0.7 0.0-2.5 2+, high spin 1.45- 1.65 1.7-3.2

low spin 0.00- 0.40 0.0-2.0 3+, high spin 0.60- 1.15 0.0-0.9

low spin 0.00- 0.40 0.0-2.0

Standard : Na2~Fe(CN)sNO]. 2H20 0.00

TIN 0 2.0 - 2.7 ~0.0

organo tin 1.0 - 2.0 0.0-5.0 2+ 3.6 - 4.6 0.0-1.7 4+ --0.3 --t-1.6 0.0-1.8

Standard : BaSnOa 0.0

g o L~

All va lues in r am/see . b Variat ion of quadrupole spl i t t ing wi th t empera tu re , m m / s e c / 1 0 0 K .

states, it is relat ively easy to determine the oxidation state. For the covalent compounds (low spin) an evaluat ion of the var ia t ion of the z~EQ with t empera - ture is required since ~ and Z~EQ fall within the same range of values. The ionic compounds will generally show magnet ic hyperfine spli t t ing a t low tempera tu res or in the presence of an external magnet ic field. The various oxidation states for iodine and its compounds can be distinguished f rom the isomer shift value. 8

Qual i ta t ive identification of components within a complex mixture can be made by comparison of the spec t rum with known MOssbauer spectra. For example, Sprenkel-Segel and H a n n a 9 were able to ident ify tha t the i ron-containing minerals in the Plainview meteori te were olivine and pyroxene (ferrous magnes ium silicates), troilite (ferrous sulfide), and kamaci te (iron-nickel alloy). The rus t on a steel surface was identified using the back-sca t ter ing technique by comparison with known spectra of the different forms of iron oxide to be fl-FeOOH. 4 This conclusion was confirmed by measuring the t rans- mission spect rum at 80K, which was magnet ical ly split with an internal field of 468 kOe. This value of the field aided to confirm the identification of the oxide since the internal magnet ic fields have different values in the various oxides.

B. Q u a n t i t a t i v e

I f the MOssbauer spec t rum is relat ively simple, it is possible to es t imate the relative concentrat ions from the peak areas (Fig. 7). Gibb and Greenwood '° found tha t the values f rom the spec t rum for Fe +2 and Fe +3 concentrat ions compared favorab ly with those deter- mined by chemical analysis in crocidolite. Joye and A x t m a n n '1 measured the ratio of corroded to un- corroded iron in thin iron foils by measuring the areas of four lines, two each f rom each component . A correction was made for the MSssbauer efficiency by measuring the ratio using the results f rom gravimetr ic analysis for each component . Each component had

f

'~ETECTOR ~__~SHIE LD

SsOURGE AMPLE

SURFACE

V E L O C I T Y

F1G. 7. Parameters related to quantitative measurements in MSssbauer spectroscopy. B--Background, H--Height of peak, A--Area of peak.

206 Volume 23, Number 3, 1969

two peaks of equal areas, but there was a small amount of overlap between one peak from each component in the spectrum of the corroded foils. The MSssbauer efficiency includes the fraction of the atoms that undergo recoil-free transitions f, and is different for each component. Yoshioka, Gohshi, and Kohno 12 found that the concentration ratio, Fe+2/Fe +3, could be found easily in iron-phosphate glasses if it were assumed that the f ' s for both iron species were equal. In this work, a correction for the background was introduced. Pella et al. ~ studied the effect of several variables on the spectral parameters using synthetic samples of SnO2 in an A12Oa matrix. The variables included sample thickness and concentration, drift in detector response, and source-sample-detector geom- etry. Their results suggest that the ratios of A/B or H / B (Fig. 7) could both be used within the range of concentrations studied (3 to 76 mg of SnO~) and follow an exponential relationship with concentration similar to Beer's law.

The M6ssbauer spectra of complex mixtures may contain as many as twelve absorption peaks, some of which include overlapping lines. Muir TM has described a method of analysis that uses spectrum stripping by comparison with reference spectra using computer techniques. The method involves using the areas of the lines and subtracting the appropriate number of reference spectra. The usefulness of the method was demonstrated by determining the ratio of olivine iron to pyroxene iron for a number of meteorites.

III. APPLICATIONS

A. Structural Chemistry

h~6ssbauer spectroscopy is a useful tool in con- formational and bonding studies of coordination and organometallie compounds. For example, it has been used to establish the identity of cis-trans isomers in coordination compounds of iron and tin2 5 Donaldson, Oteng, and Senior t6 found that an orthorhombic form of stannous fluoride existed in addition to the known monoelinic form from its MSssbauer spectrum. This was later confirmed by x-ray diffraction and vibration spectral studies. Gol'danskii et al2 7 found that the SnF4 structure is octahedral from a study of its M6ssbauer spectrum. The structure of iron dodeca- carbonyl, Fee(CO)z2, was elucidated through the use of its M6ssbauer spectrum and that of the hydride anion Fe~ (CO)lzH- resolving the conflicting x-ray and infrared spectral studies, t8 These few examples serve to illustrate the value of M6ssbauer spectroscopy in the studies of the structure of chemical compounds.

B. Surface Chemistry and Catalysis

MSssbauer spectroscopy, through the use of the back-scattering technique, for example, allows one to examine films of material on surfaces. Since many reactions in catalysis involve reactions directly at the surface, it can be very useful in determining the structure of these materials at the surface and yielding

information concerning the structure of the metal or metal ion with regard to the catalyst. Certainly, it can determine the oxidation state of the atom and information concerning the nature of the bonding. Hobson 19 has studied the effect of ammonia on silica gel impreg- nated with ferric nitrate. His results suggest that a complex is formed between Fe +3 and ammonia, since the M6ssbauer spectrum changes when ammonia is absorbed, and that the process is reversible. Delgass, Boudart, and Parravano 2° have studied supported gold catalysts. They observed changes in the 197Au spectrum of different heat-treated samples, which suggested that the gold species present depended upon the heat-treatment of the alumina. Delgass and Boudart 2t have recently published a review on the application of M6ssbauer spectroscopy to the study of catalysis, which includes the results found with studies of sorption on zeolites and other materials.

Since corrosion occurs at the surface and MSssbauer spectroscopy is non-destructive, the method offers an opportunity to study the corrosion products without interruption for analysis. Although it has been studied by the transmission mode, ~t the back-scattering mode affords an opportunity to study the reaction i n situ. The back-scattering mode has been applied not only to identify the corrosion product 3,4 but can be used to estimate the depth of penetration of the rust into the underlying steel plate. Terrell and Spijkerman 4 have used the amplitude of the back-scattered radiation to evaluate the thickness of the rust layer and estimated that in their sample the rust was about 2 X 10 .9 cm thick.

C. Metallurgy

The application to the study of metals and alloys includes identification and quantitative analysis of the various phases and the measurement of specific properties of the phases. The M6ssbaucr spectrum can be used to measure the magnetic properties. For example, the measurement of the six-line spectrum of ferromagnetic iron gives the value of the internal magnetic field. The various phases of the iron-carbon have different values for the internal magnetic fields with slightly different spectra. One phase, austenite, is not ferromagnetic, and its spectrum has one line. Thus, it is possible to analyze for this phase in iron as well as the other phases in steels. 22 M6ssbauer spectroscopy is useful in the investigations of magnetically dilute alloys because one observes the distribution of the alignments of the atomic spins whereas the average alignment is observed with measurements of magnetic susceptibility. 23 This permits discrimination of short range from long range effects in alloy systems. The observations of both chemical and magnetic order within an alloy can also be made in complex alloy phases in systems, such as Pt~_xFez+x? 4 The results of a study with dilute Fe-Cr alloys have been correlated with stress-microstrain measurements to study the mechanism of solid-solution strengtheningi 25 It is also possible to use M6ssbauer spectroscopy in the elucida-


tion of phase diagrams of alloy systems containing iron, tin, and other metals, such as the rare earth elements.

D. Minerals, Terrestrial and Extraterrestrial

A number of studies have been made to determine the oxidation state of iron, the nature and the amount of the components in both terrestrial and extraterrestrial minerals. A number of studies have been reported with silicates as well as other iron containing minerals2,~°.26 Considerable progress has been shown in analyzing meteorites to identify the iron containing components2 ,'4,~7 Although the spectra of meteorites a r e complex, information concerning the identity and the amount of the various iron sites present has been obtained.

I t has been possible to distinguish between limenite (FeTiO3) and magnetite (Fe304) because hyperfine magnetic splitting is observed only for the Fe30478 The ore can be speedily classified as titanium (ilmenite) o r iron (magnetite) ore since the spectrum of magnetite has six lines whereas the spectrum of the other ore is a doublet. Scatter techniques have been used in this application.

Zuckerman has examined 11 museum-identified tin minerals with the result that two samples were renamed. 29 A tin detector weighing about 3.5 kg has been developed in Russia for the assay of tin in ores and minerals. This device operates in both transmission and scattering modes2 ° This is perhaps the first field application of MSssbauer spectroscopy.

E. Biological Systems

Metallobiochemicals are excellent candidates f o r study with MSssbauer spectral techniques because some contain iron. Many proteins have been examined to provide information concerning the structures and to assist in the elucidation of the mechanism of action of these proteins2 ~ Among the hemoproteins are the oxygen carriers, hemoglobin and myoglobin, and the enzymes, cytochrome c and eatalase. Gonser, Grant, and Kregzde 32 found that the M6ssbauer spectrum of human red cells at 4K had four lines. Two of these disappeared from the spectrum when the cells were treated with N~, deoxygenating the hemoglobin. The positions of these two lines are identical with two lines found in the spectrum of Q-hemoglobin. The two lines that remained in the spectrum of the blood cells were also found in the spectrum of CO2-treated red blood cells. These results suggest that the CO2 is not directly bound as a ligand to the iron confirming other evidence that it is bound to some other part of the molecule. M6ssbauer spectroscopy has been used to study other complexes of hemoglobin as well as the iron-organo moiety, heine, of the hemoproteins. A number of studies have been made with non-heme proteins such as ferridoxin to help elucidate its structureY 3 A study of the nitrogen- fixing bacteria Azotobacter vinelandii has suggested that iron is involved in the nitrogen fixation process24

IV. SUMMARY AND OUTLOOK

In this discussion, an at tempt has been made to indicate the present status of this new form of spectroscopy with regard to its usefulness in several selected areas. Although presently it is principally applied by chemists and others to the study of iron and tin compounds, with the greater availability of devices such as reactors, particle accelerators, etc., for producing sources of different nuclides and an increase m the speed of delivery from these devices to the M6ssbauer laboratory, the range of nuclides for use in chemical systems will increase very rapidly in the near future. Nuclides such as 127I, 33Ba, and '21Sb may soon become readily available to the chemist. The future developments in Coulomb-excitation of sources may lead to this method becoming a laboratory tool. Many of these will require low-temperature cryostats (liquid helium), but machines are becoming available for the production of these temperatures25 More rapid acquisition of the data will increase the acceptance of the MSssbauer spectrometer as a routine tool in the laboratory. Presently, data reduction is accomplished by fitting the data through a computer system. The widespread use of computer-sharing systems with a terminal located near the spectrometer reduces the time needed for determining the M6ssbauer parameters. With the advent, of automated, computer con- trolled instrumentation, the time for computer fitting should be reduced to the extent that the parameters could be evaluated within minutes after the spectrum has been measured.

The above illustrations of M6ssbauer spectroscopy are indicative of the extent to which it can be used as a nondestructive analytical method. Its use as a field-tin detector 3° can be extended to other nuclides and other minerals. I t has even been proposed to be used for the analysis of extraterrestrial surfaces such as the moon and Mars. a6 It can provide a unique elementary analytical system since the M6ssbauer effect is specific for the one nuclide being investigated. Although the investigation of the quantitative analytical aspects has begun only recently, it will be rapidly expanded so that the full potentialities of this technique will probably be known within the next five years.

The back-scattering technique points the way to an expanded use of M6ssbauer spectroscopy. The samples need not be ground into particles or even remow~d from place. Surface reactions and compounds can easily be studied as to composition, thickness, and rates of reactions without altering the reaction system. Application to steel corrosion a,4 illustrates the power of this technique, and we can expect its use in this area to expand rapidly in the future. As we have seen, the study of catalysts need not be limited to iron containing materials.

The use of this technique has suggested that it can be applied to the automatic control of composition of a complex mixture since it is nondestructive. Initial studies indicate that this may indeed be possible. This

208 Volume 23, Number 3, 1969

was app l i ed to the con t ro l of t he t i n c o n t e n t in ores and e n r i c h m e n t p roduc ts27 T h e expans ion of th i s a p p r o a c h to t he con t inuous ana lys i s of o t h e r m e t a l - con ta in i , lg ores shou ld be seen w i th in t h e nea r f u t u r e as M S s s b a u e r s p e c t r o m e t e r s for th is a p p l i c a t i o n become aw~ilable.

T h e r e are o t h e r i n d u s t r i a l m a t e r i a l s and processes in which the m e a s u r e m e n t of t he i ron phases a re impor ta~ l t in a d d i t i o n to t he me ta l s a n d al loys. E x t e n s i o n of t he a p p r o a c h e s used p r ev ious ly , for example , could l ead to an u n d e r s t a n d i n g of t he role of i r o n - c o n t a i n i n g pa r t i c l e s in t he h a r d e n i n g process of cements . I t has been used to measu re t he r a t e of h y d r a t i o n in c e m e n t 2 s

M S s s b a u e r s p e c t r o s c o p y p r o v i d e s a un ique tool for m a g n e t i c m e a s u r e m e n t s , in a d d i t i o n to p r o v i d i n g i n f o r m a t i o n a b o u t t he i n t i m a t e s t r u c t u r e a r o u n d the a t o m being o b s e r v e d b y 5![Sssbauer spec t ro scopy . A p p l i c a t i o n s to such c o m p l i c a t e d s y s t e m s as p r o t e i n s and me teo r i t e s i l l u s t r a t e t he va lue of th i s new tool a n d p o i n t to t he f u t u r e in which i t s h o r t l y will t a k e i ts p lace a long wi th t h e more t r a d i t i o n a l fo rms of spec- t r o s c o p y d e v o t e d to t he s t u d y of t he s t r u c t u r e of ma te r i a l s .

A n o t h e r use to which i \16ssbauer s p e c t r o s c o p y can be m a d e is t he m e a s u r e m e n t of m o v e m e n t . Th i s is i l l u s t r a t e d b y s tud ies on the ear 39 in which the source is m o u n t e d on the ea r s t r u c t u r e whose m o t i o n is to be measu red . T h e m o v e m e n t of t he ea r s t ruc tu re , such as the ea r d r u m , can t h e n be m e a s u r e d b y us ing a s t a t i o n a r y absorber . I t is poss ib le t h a t th i s m a y be useful in o t h e r s i t ua t i ons inaecessb i le to t he usua l m e t h o d s for m e a s u r i n g v ib r a t i ons . Thus , we see t h a t M 6 s s b a u e r s p e c t r o s c o p y of the fu tu r e will be used b y chemis ts , me ta l lu rg i s t s , spec t ro scop i s t s , etc. , for t he s t u d y of t he compos i t ion , s t ruc tu re , a n d p rope r t i e s of m a t e r i a l s ; and b y engineers for v i b r a t i o n a l ana lys i s .

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Mössbauer Spectroscopy: Some Views