moessbauer spectrometry

18
Mossbauer Spectrometry 1. R. DeVoe and 1. 1. Spiikerman, National Bureau of Standards, Washington, D. C. HIS REVIEW supplements our pre- T vious review (ISd), and it is rec- ommended that the reader refer to our previous review since nomenclature and terminology are the same. Emphasis here is given to the applications which have expanded to a considerable extent in certain areas. A11 of the applications to specific compounds have been entered in tabular form similar to the previous review. n7e have modified the format from last time so that a section on tech- niques for analyzing a Mossbauer spectrum could be included. With the exception of a few references in the text only articles published in 1966 and 1967 are included. It is recognized that the review does not contain all of the published work for this time period. It is particularly evident that references which appeared in the literature in late 1967 have been omitted. Certain other omissions have been made; for example, me have not included any references to abstracts of manuscripts or talks-e.g., Bulletin of the American Physical Society. In addition, we have excluded references that were not available at NBS or readily obtainable from the Library of Congress. Without question the most compre- hensive compilation of references and data is that provided by Muir, Ando, and Coogan (32s). The Mossbauer spectrometry group at the National Bureau of Standards has a computer- ized compilation that is similar to that of Muir. An NBS Technical Note is scheduled to be printed annually con- taining general information currently published about papers, but cataloging spectral parameters. The major ad- vantage of this compilation will be its minimum lead time. Mossbauer spectrometry is probably one of the most reviewed subjects. In the last two years several good reviews on applications in specific areas of study have been published (26, S4, 35, 104, 12SJ ISS, 169, 191, 1923, 195, 200, 101, 375, 39.4, 400, .422, 427, 429, 441, 500). It is most fortunate to have these re- views available, as it can be expected that activity in these fields will increase. 204, 223, 224, 257, 292, s14, 518, 524, SPECTROPHOTOMETRIC TECHNIQUE Two years ago the principles of the method were described. This time it is only necessary to amplify those areas that have received the most attention. In addition, certain promising areas of future interest will be outlined. 1. Preparation of Sources. Con- siderable understanding has been ob- tained (mostly through trial and error in the laboratory) about the factors that must be controlled in order to make a good source of “recoilless” radiation. A “good” source can be defined as one which has a long half- life (few hours), few high energy precursor gamma rays, and a high fraction of recoilless emission (and radiation which is unsplit by hyperfine interaction). In order to obtain a single line source it is necessary that the radioisotope be in a highly symmetric lattice site. To obtain a source with a large recoil free fraction, a high effective Debye tempera- ture is required (%‘) (S20, 358) which is related to the Debye temperature by the expression (see previous review 1.34). eo’ = eo ( ~ ~ ~ ~ ~ / ~ ~ ~ ~ ) l / 2 (1) where JI,,,, Mhost represents the atomic mass of the source atom and host (or lattice) atom, respectively. Another factor to be considered for producing good sources is the mode of decay to produce the nuclear energy level that exhibits recoilless emission. Charge states on the atom produced by precur- sor emissions such as p+, p-, or electron capture must be neutralized and the atom must be in a stable lattice position before the Mossbauer transition takes place. This means that conducting metals invariably make a good matrix. Another factor that is not related to the Mossbauer effect is the electronic self-absorption of the radiation by the matrix of the source. Care must be taken to avoid such effects. There are a variety of sources avail- able and the selection of source can be dictated by the type of absorbers-e.g., their chemical shift-which are to be investigated. Suitable sources have been found for only a limited number of radioisotopes. Several new techniques using accelera- tors or reactors have been described (see Table I). The most promising technique is Coulomb excitation (see Table I) (110), where a Van de Graff accelerator is used to generate the Mossbauer radiation by bombarding a thin foil of the isotope of interest with p, n, a or 160 particles. The background radiation is generally high in these experiments, but recent modification in the apparatus reduces this interference (recoil implantation). The nuclei, excited by the Coulomb interaction, recoil from the surface of the target onto a catcher foil. This foil acts as the lattice for the recoiling nucleus which then emits the Moss- bauer gamma ray (4%). Cse of the recoil implantation technique has re- sulted in several new Mossbauer isotopes (see Table I, 4S6). Another method used successfully has utilized the observation of the Moss- bauer effect following a nuclear reaction (78, 166) (see Table I). Radiation damage was extremely small in these experiments, and excellent spectra were obtained. 2. Absorbers. Little new infor- mation is presented in the literature about practical problems of mounting a material in the spectrometer. At- tention to thickness corrections has been appreciable (see below). With one notable exception, uniformity of thick- ness, etc., has not been of great concern (SI 5). 3. Detectors. The quality of a Mossbauer spectrum is often deter- mined by the type of detector used. For gamma rays of energy below 20 keV, the proportional counter gives the best resolution with satisfactory effi- ciency. The solid-state detector can also be used, but it is worthwhile only in special cases where high energy resolu- tion is required. A 90% Kr, 10% meth- ane filling gas for a 2-inch diameter proportional counter gives the best results for mFe Mossbauer spectrometry because the Kr provides good detection efficiency (60%), and the X-ray absorp- tion edge is slightly less than the 14.4- keV 57Fe gamma-ray energy. Sealed counters are relatively short-lived, par- ticularly at high counting rates. The use of P-10 gas (90% Ar-lO% CHh) flow counter eliminates this problem; they have excellent resolution, but their effi- ciency is much lower than sealed counters (15% for a 2-inch diameter counter). Above 20 keV, a thin NaI(T1) scin- tillation crystal is generally used. If pulse height resolution is a problem, the solid-state lithium-drifted silicon detector should be used. A resolution of 1 keV can easily be obtained and the efficiency is excellent in this range. If the Mossbauer energy level is highly converted, it is possible to make a detector that significantly increases the efficiency of the spectrometer. A detector can be fabricated which is 472 R ANALYTICAL CHEMISTRY

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Page 1: Moessbauer spectrometry

Mossbauer Spectrometry 1. R. DeVoe and 1. 1. Spiikerman, National Bureau of Standards, Washington, D. C.

HIS REVIEW supplements our pre- T vious review ( I S d ) , and it is rec- ommended that the reader refer to our previous review since nomenclature and terminology are the same. Emphasis here is given to the applications which have expanded to a considerable extent in certain areas. A11 of the applications to specific compounds have been entered in tabular form similar to the previous review. n7e have modified the format from last time so that a section on tech- niques for analyzing a Mossbauer spectrum could be included.

With the exception of a few references in the text only articles published in 1966 and 1967 are included. It is recognized that the review does not contain all of the published work for this time period. I t is particularly evident that references which appeared in the literature in late 1967 have been omitted. Certain other omissions have been made; for example, me have not included any references to abstracts of manuscripts or talks-e.g., Bulletin of the American Physical Society. In addition, we have excluded references that were not available a t NBS or readily obtainable from the Library of Congress.

Without question the most compre- hensive compilation of references and data is that provided by Muir, Ando, and Coogan (32s). The Mossbauer spectrometry group a t the National Bureau of Standards has a computer- ized compilation that is similar to that of Muir. An NBS Technical Note is scheduled to be printed annually con- taining general information currently published about papers, but cataloging spectral parameters. The major ad- vantage of this compilation will be its minimum lead time.

Mossbauer spectrometry is probably one of the most reviewed subjects. In the last two years several good reviews on applications in specific areas of study have been published (26, S4, 35, 104, 12SJ ISS, 169, 191, 1923, 195, 200, 101,

375, 39.4, 400, .422, 427, 429, 441, 500). It is most fortunate to have these re- views available, as i t can be expected that activity in these fields will increase.

204, 223, 224, 257, 292, s14, 518, 524,

SPECTROPHOTOMETRIC TECHNIQUE

Two years ago the principles of the method were described. This time it is only necessary to amplify those areas that have received the most attention.

In addition, certain promising areas of future interest will be outlined.

1. Preparation of Sources. Con- siderable understanding has been ob- tained (mostly through trial and error in the laboratory) about the factors tha t must be controlled in order to make a good source of “recoilless” radiation. A “good” source can be defined as one which has a long half- life (few hours), few high energy precursor gamma rays, and a high fraction of recoilless emission (and radiation which is unsplit by hyperfine interaction).

In order to obtain a single line source it is necessary that the radioisotope be in a highly symmetric lattice site. To obtain a source with a large recoil free fraction, a high effective Debye tempera- ture is required (%‘) (S20, 358) which is related to the Debye temperature by the expression (see previous review 1.34).

eo’ = eo ( ~ ~ ~ ~ ~ / ~ ~ ~ ~ ) l / 2 (1) where JI,,,, Mhost represents the atomic mass of the source atom and host (or lattice) atom, respectively. Another factor to be considered for producing good sources is the mode of decay to produce the nuclear energy level that exhibits recoilless emission. Charge states on the atom produced by precur- sor emissions such as p+, p-, or electron capture must be neutralized and the atom must be in a stable lattice position before the Mossbauer transition takes place. This means that conducting metals invariably make a good matrix. Another factor that is not related to the Mossbauer effect is the electronic self-absorption of the radiation by the matrix of the source. Care must be taken to avoid such effects.

There are a variety of sources avail- able and the selection of source can be dictated by the type of absorbers-e.g., their chemical shift-which are to be investigated.

Suitable sources have been found for only a limited number of radioisotopes. Several new techniques using accelera- tors or reactors have been described (see Table I).

The most promising technique is Coulomb excitation (see Table I) (110), where a Van de Graff accelerator is used to generate the Mossbauer radiation by bombarding a thin foil of the isotope of interest with p, n, a or 1 6 0 particles. The background radiation is generally high in these experiments, but recent modification in the apparatus reduces

this interference (recoil implantation). The nuclei, excited by the Coulomb interaction, recoil from the surface of the target onto a catcher foil. This foil acts as the lattice for the recoiling nucleus which then emits the Moss- bauer gamma ray (4%). Cse of the recoil implantation technique has re- sulted in several new Mossbauer isotopes (see Table I , 4S6).

Another method used successfully has utilized the observation of the Moss- bauer effect following a nuclear reaction (78, 166) (see Table I) . Radiation damage was extremely small in these experiments, and excellent spectra were obtained.

2. Absorbers. Little new infor- mation is presented in the literature about practical problems of mounting a material in the spectrometer. At- tention to thickness corrections has been appreciable (see below). With one notable exception, uniformity of thick- ness, etc., has not been of great concern (S I 5 ) .

3. Detectors. The quality of a Mossbauer spectrum is often deter- mined by the type of detector used. For gamma rays of energy below 20 keV, the proportional counter gives the best resolution with satisfactory effi- ciency. The solid-state detector can also be used, but it is worthwhile only in special cases where high energy resolu- tion is required. A 90% Kr, 10% meth- ane filling gas for a 2-inch diameter proportional counter gives the best results for mFe Mossbauer spectrometry because the Kr provides good detection efficiency (60%), and the X-ray absorp- tion edge is slightly less than the 14.4- keV 57Fe gamma-ray energy. Sealed counters are relatively short-lived, par- ticularly a t high counting rates. The use of P-10 gas (90% Ar-lO% CHh) flow counter eliminates this problem; they have excellent resolution, but their effi- ciency is much lower than sealed counters (15% for a 2-inch diameter counter).

Above 20 keV, a thin NaI(T1) scin- tillation crystal is generally used. If pulse height resolution is a problem, the solid-state lithium-drifted silicon detector should be used. A resolution of 1 keV can easily be obtained and the efficiency is excellent in this range.

If the Mossbauer energy level is highly converted, it is possible to make a detector that significantly increases the efficiency of the spectrometer. A detector can be fabricated which is

472 R ANALYTICAL CHEMISTRY

Page 2: Moessbauer spectrometry

~ ~~

Table 1. Application of MSssbauer Spectrometry to Chemistry Compilation of Publications for 1966 and 1967

El E2 CY = Internal conversion coefficient K ple1 plP

&le, &I,

[Nuclear data taken from reference number (323)] Abbreviations: EFG = electric field gradient

= Energy of Mossbauer gamma ray 1 and 2, respectively, in keV

= Ka:p X-ray energy in keV = Magnetic moment of excited and ground state producing Mossbauer gamma ray 1,

= Electric quadrupole moment of excited and ground state producing Mossbauer respectively, in nuclear magnetons (nm)

gamma ray 1, respectively, in barns (b = 1 0 - 2 4 cm2).

AE, = electric quadrupole splitting = asymmetry parameter (428) 8s = chemical shift

6 0 = Debye temperature AR/R = fractional nuclear radius change where excited to ground state = ( A R )

H = effective magnetic field a t the nucleus * = indicates that chemical shift of iron compounds was referenced to

€ = magnitude of effect (428)

and radius of ground state = R

sodium nitroprusside

Subject or Mossbauer nuclides material studied

67Fe ( - l / ~ ) Nuclear parameters E = 14.4 keV a = 9

K = 6.5 keV pd = -0.154nm Source preparation

= +0.09024 nm Alloys

&. = +0.285 b Alloys

Carbides

Doped materials

Iron-oxygen

Types Lifetime of excited state FeSiFe. 6H20 K shell fluorescence yield

Observation of effect (Wk)

Fe-Pd

Fe-AI, Fe-Ti

Fe-Cr Fe-Cu

Fe-Ge, Fe-Sn Fe-In Fe-110 Fe-Ah, Fe-Cr-Ti Fe-Ni

Fe-Pd Fe-IZh Fe-Si Fe-Sn Fe-Se Fe-Tm Fe-S' Co-FeSi V-Si-Fe, Mo-Cr-Fe Fe-Co-V Martensite

Cementite Corundum In-Sb, Ga-Sb Fe

N g o Mn-Au

Ni-metal

NiO, &In0 SrTi03

Metals Complex Ca-Fe-0 com-

pounds

Rare earth-Fe208 Rh203-Fe203 M2O3-Fe2O3 FeT08 MnFe03

Remarks 97.7 + 0 . 2 n s Qe = +0.2 b Wk = 0.322 z!z 0.029

136-keS' line Diffusion into Zn, Mo, Sn, A1 Diffusion into Cu, Fe, and Pd Compared with a magnetic suscep-

tibility Two sites

. . . Paramagnetic phase and ferromag-

AE,, CS netic, a: iron precipitate

e

Short range order H us "K Phase transitions Two sites, AE,, CS

(116, 117, 246, (171, 476) 596)

(1 71 , 390)

Disordered cubic' lattice (371 1 H = 0 a t low temperature, CS (474 ) AEq (277) Fine particles (197) H us. yo carbon (182, 166,

602) Quantitative application (206)

Effect of external magnetic field (L86) Two sites ($44, 391)

- e measured ( i 9 )

AEq-crystal field ( 2 1 2 )

0.08 to 1" K polarization of radiation

antiferro- and ferromagnetic prop- ( 7 )

(161) observed

erties Comparison with NMR H us. O K H us. O K. Metal or FeS+ AE,, CS*

H = 554 kOe, 78" K. H >10-2yc = precipitated CoCll <10-2yc Fez+ in lattice sites

Spin interaction measured with

Two sites larized radiation

PO-

(159) (113, 511) (92)

M = variety of metals T = Ho, Er, Mn, Yb, Lu, Y Two sites, A E CS (17)

(Cont&ued )

VOL 40, NO. 5, APRIL 1968 473 R

Page 3: Moessbauer spectrometry

Mossbauer nuclides

Table 1. Application of Mossbauer Spectrometry to Chemistry (Continued) Subject or

material studied

Ferro- and ferricya- nides

Meteorites Minerals

Organic compounds

Types MnFe204 NiFenO4 GaFeOs GeFenO4 Li-Cr ferrite Ni-Zn ferrite Ca-A1 ferrite

CY Fe2O3 Y Fen03 P-7 FeOOH LY FeOOH Oxides and oxyhydrides Transition element com-

Fe-Gd-0

plexes K4Fe(CN)s Sodium nitroprusside

Review Amphiboles Anthophyllite Biotite, jarosite Chalcopyrite Cement Clays Gillespite

Cubanite, sternbergite Cummingronite, grunerite Glauconite Nept unite Orthopyroxenes Perovskites Spinel, FeVzO4 FeCr204, FeVzO4 Triolite, pyrovhotite (FeS)

Wustite Heme protein

Xanthene oxidase Cells of microorganism Purines Phthalocyanine dipyridine

Ferrocenyl carbonium ion Ferrocenetetra cyano-

Porphyrin, pyridine, imida-

Isoquinoline, picoline, pyri-

ethylene

zole

dine Dipyrid ylamine Pyridine-FeClr (Et4N)2FeClr R2FeX4

Bis-N,Ndiethyldithio- carbamato)iron( 11) chloride

Dithiocarbamates Citrate, benzoate, maleate I-2-Dithiolenes Dimethylglyoxime, salicyl-

Diironenneacarbonyl Iron (I1 )-bis (adiamine)

Iron(I1) 1,lO-phenanthro-

Fe(CN)dCNR), Sodium bistetramethyl am-

monium-hexazido-fer- rate (111)

aldoxime

complexes

line

Remarks Ref. Two sites (491 1 Chromium substitution (93)

(454) (158, 251)

H

Two sites, AE,, CS* (44 I (419, 463) H us. O K.

Two sites (9 )

Particles show superparamagnetism (415) AEn. CS. H (321

(137)

A i ; , cs (90,405)

Second-order Doppler shift (214) Standard reference for chemical shift (426) Sign of EFG single crystal (122)

(430) (19, 482) (24) (229) (8 )

Particles show superparamagnetism (369) AE,, CS (472) Planar Fe-0 configuration (100) EFG small due to crystal field can-

celling ionic field

AE,, CS* Two sites, H , CS* CS LE, us. O K. Distortion, Jahn-Teller Effect AE,, CS, single crystal

AE,, H us. O K. Review Spin-s in relaxation Ferricgrome A Morh AE,, CS review AE,, CS

Fe(II1)-EDTA Fe(I1) Fe(II), Fe(III), AE,, CS*

(121) (105)

AE,, CS* (16% 497)

Compare with electronic spectra (77)

(317) (310)

Tetrahedral (185) ~.

X = C1-, Br-, NCS-, NCSe-, R = NMed+NEt4+, N,N’dimethyl-4!- 4’-dipyridyl (+2 ion), a,a-(bistri- phenylphosphonium)-p-xylene

?l4?, 156)

Spin 3/2 ferromagnet (485)

AE,, CS

AE,, cs (41 ) Prussiate, phosphine, arsine, stibene (170)

(Continued)

474 R ANALYTICAL CHEMISTRY

Page 4: Moessbauer spectrometry

Table 1. Application of Mossbauer Spectrometry to Chemistry (Continued)

Mossbauer nuclides Subject or

material studied Types Ferrous poly( 1-pyrazoly1)-

borate chelate (Ir) Acetylacetone chloride Diketone complexes (CH3)4SnaFe4(CO )I6 Alums

Remarks Simultaneous high spin-low spin

AE,, CS

Both F: and Sn measured CS us. K. Catalysis H us. particle size, H, CS* Other Iron metal powder

Iron phosphides FeL CuFeSnS4 FeSO4. 7 H ~ 0 , FeSiF6

Glass (sodium trisilica FezSiO4 RbFeFs FeF3 KFeF3 FeC12.2H20

FeC12. 4Hz0

H = 0, AE,, CS us. Fez+, Sn4+, CS, AE Crystal field contributes to electric

Fez +, Fe3 +, CS, AE,

Antiferromagnet a t Ne61 Point Phase transitions Relaxation effects H = 250 f 10 kG AE = +4.6 i: 0.05 mm./sec.

(liquid He) 7 = 0 . 3 f. 0.04 H = 255 kG a t 77" K. AE, = f 2 . 6 3 mm./sec.

K.

field gradient 6Hz0

,te)

q = 0 . 2 H = 487 =k 15 kG a t 15" K., CS* us.

" K. FeC13

Fe-Mn-0 Borides FeC13-graphite Fe compounds in ice FeS04, Fe~(S0d3 Ion exchange resins FerTiO4

FePOl

&E,, cs Two sites (interlaminar), AE,, CS*

"Jump" diffusion

Comparison with magnetic suscep-

Antiferromagnetic (<26" K.) AE,, CS, review H and CS us. pressure AE, changes sign a t 30 kbar, CS, in-

dependent of paramagnetic phase a t 50 kbar

Large increase, CS, decrease AEq Large increase, CS, increase AE,

tibility

Effect of pressure Heme-protein Doped nickel a-Fe203, FeTi03, FeO

Ferrocene Pyrite Ferro-Ferricyanides Transition metals Fez +, Fe3 + doped, COO,

Fez(C~04)3. 5H~0 Tetrakispyridineiron(I1)

Hot atom effects

COSO4, CoS04.7H20

chloride

Pure free ion computation not correct H , AE,, CS

Thermal decomposi-

Hot atom and radia-

tion

tion effects

Product = FeC~04, Fe304

Codomb excited by 3-MeV oxygen ions

Recoil in Fez03 Electron capture decay in CoCIz.

CoS04, CoF2 and c O Z ( s 0 4 ) 3 . lSHz.O Coulomb excited 66Fel (d,p) 67Fe in

iron coo COO lifetime of Fe+3 COO, NiO no time effects Two forms of COO Fe~(C204)3 nH2O Neutron capture of s6Fe measured in

Fe203, FeS04.7H20. Electron capture recoil in COO, NiO,

Fe(NH4)2(S04)2.6H2Ol CoS04. 7Hz0, CoC1z .4Hz0

CS, AE,

Source produced by Coulomb excita- tion

/ . ~ ~ / z / / A ~ I z 0.559 f 0.012

Radiation effects

(463)

Surface absorption NH3 on Fe(OH)a silica gel On W and Ag On SiOz and AlzOa

"Ni ( Observation of effect

Nuclear parameters E = 67.4 keV ( -6 /2) a = 0.12 K = 7 . 6 keV

Magnetic moment (132, 298)

f ie = f 0 . 3 5 n m M~ = k0.746 nm

E = 137.2 keV (+2) a = 1.2

'&Os (+O) Metal E = 0 .30a t 15" K.

AE,, EFG = 3 . 5 f 0 . 5 X 1017 eD = 375 A 20" K.

V/cm2 (Continued)

VOL. 40, NO. 5, APRIL 1968 475 R

Page 5: Moessbauer spectrometry

Table 1. Application of Mossbauer Spectrometry to Chemistry (Confinued)

Subject or Mossbauer nuclides material studied Remarks

Scattering mode Ref.

K = 64.5 keV pg = f 0 . 5 7 n m pa = +0.64 nm Qe = 1.54 b

E = 100.1 keV (+2) a = 3.97 K = 60.7keV pa = +0.46 nm Qc = 1.87 b

E1 = 46.5 keV Ez = 99.1 keV ( - 6 / 2 ) a1 = 9 . 0 a2 = 4 . 3 K = 60.7 keV po = f 0 . 1 1 7 nm Qi. = 1.61 b

lSZW Nuclear parameters

183W ( - I/*) Nuclear parameters

Observation of effect "'E (t0)111.2 keV (+2) 01 = 2.75 K = 60.7 keV

= 0.52nm Q. = 2 b

E = 122.5 (+2) a = 1.72 K = 60.7 keV Qe = 0.64b

E1 = 129.4 keV ( f 6 / p )

EP = 82.4 keV (+ ' /P ) a1 = 3 K = 66.4 keV lll0 = +0.18 nm &io = + 1 . 3 b

l S 6 W ( + O ) (See Is4W)

lslIr ( + 3 / ~ ) Observation of effect

ARIR measured

Metal, WOa powder single crystal

was, CUWos

In Fe, Co, and Ni

Mixing ratio E2/M1 = 0.005

Lifetime of excited state

By Coulomb excitation, lifetime of excited state and g factor for 2+ states measured, H

(189)

Metal particles (39.2)

Lifetime of 82.4-keV level = 2 . 3 (264, 406) nsec.

Metal

ls3Ir ( + 3 / ~ ) Nuclear parameters E = 73 keV (+1/~)

R-Ir R = Pr, KJrClg, ARIR >

1, Sm, Gd, Tb, Dy, Ho &pa/pg = 3.0 i 0 .1

193) = 1.03 i 0.03 a = 6 K = 66.4 keV pe = 0.56nm po = + 1 . 1 7 n m Qg = 1 . 5 b

Au-Fe

Au-Cu-Ni Au-Mn Metal Au-V, Au-Cu, Au-Mn,

Au-Te

pCs = 0.402 i 0.025 nm H = 1.29 x 103koe

Ez/Mi = 0. I ARIR = + H , ion implantation Backscatter geometry 99-keV transition used, CS, e

AE,, CS

( + 3 / ~ ) Alloys = 77.3 keV (+'/e) = 3.7 = 70.4 keV = 1 0 . 3 7 n m Effect of Dressure = +0.145 nm = +0.56 b

'96Pt ( - 1 / z ) Doped materials Et = 98.9 keV ( - 3 / ~ ) Ez = 129.7 keV ( - 6 / ~ ) 011 = 7 .2 plc = -0.65nm pl0 = +0.61 nm

Fe, Co, Ni Pt, Be, Cu, I r

g9Ru E K

Observation of effect (z6# keV ( + 3 / 2 ) = 19.6 keV Compounds = -0.285 nm

po = -0.625nm Qe = -0.15 b

RuOz, RUOp, ruthenocene CS, H

GROUP IV, V, VI AND VIIA ELEMENTS l19Sn ( + l / d Nuclear parameters Metal and oxide

E = 23.8 keV ( + 3 / ~ ) AR/R = $3.3 X lo-' f 1 by

conversion, electron measurements from 0 shell

Ly =i 5 . 5 Tin halides AR/R > 0 K = 25.8 keV Tin halides ARIR < 0 pa = +0.76nm Halides, oxides, also of Sb AR/R = f 1 . 2 zt 0 . 4 X lo-' po = -1.046 nm Source preparation Barium stannate Compare well with MgzSn Q e = -0.07 b Evaluation of E MgPSn and PdaSn source discussed

(Continued) Pd3Sn Very narrow line width

476R ANALYTICAL CHEMISTRY

Page 6: Moessbauer spectrometry

Table I. Application of Mossbauer Spectrometry to Chemistry (Continued)

Subject or Mossbauer nuclides material studied

Alloy

Mineral

Oxides

Halides

Other Organotin compounds

Hot atom effects Surface absorption

lzlSb ( + 6 / ~ ) Nuclear parameters E = 37.2 keV . . - a = 10.6 K = 26.9 keV pa = +3.359nm Q. Qp = -0 .42b

= -0.75 f 0.09 b

lesTe (+'/-A Compounds E = 35.6 keV

a = 13.3

K = 28.0 keV pa = +0.65nm

pa = - 0.887 nm

Q. = kO.19 b

78Ge ( + O/Z) Observation of effect E = 67.0 keV (+6 /~ ) a = 0.23 K = 10 keV p, = -0.88nm Q, = -0.26 b

Types Co-Sn, Ni-Sn, Pd-Sn

PtcSn, Ir-Sn Pt-Sn, Rh-Sn

cu-Sri Te-Sn Nb-Sn R S n a

Co-Sn Doped Cu, Ag, Au, Mn Arandisite, canfieldite,

cassiterite, cylindrite, francheite, herzenbergite, hulsite, ni erite, nordenskofdine, stannite, teallite

Ba-Ti-Sn-0 SnOz Oxides and fluorides

SnFe garnets SnOz Fe spinel Sn(A1H4)4

Many Halides Sn-CHTX R,SnH,

Triphenyl tins Phenanthroline complexes

with di-n-butyltin dihalide

(n-C4Hg)zSnC12 (n-CIHs)zSnO Phenanthryl tins Phenylene and naphthylene

oxy-tins Barenes (CHa)4Sn?FedCO)la Methyl tins SnOz On silica gel Tetraneophyltin Tr imethy ltin cyanide

Oxides and halides Sbd&

Metal Metal

Metal and oxide

Remarks

CS us. atom % Sn (linear)

E measured CS us. atom % Sn (nonlinear)

E US. ' K. R = La, Ce, Pr, Nd, Sm, and Yb

H compared with NMR

H US. O K., pc,/po = 0.7 f 0.02 H, conduction electron polarization

Tourmaline solid solution AE,, partially resolved Anomalous CS explained by crystal

H Splitting due to inexact composition

field effects

Intermediate of reaction, SnC14 + LiAlH4

Binding in vulcanized butyl rubber Comparison with infrared spectra OD, c Hexacoordinate E US, O K. Quantitative conclusions not

possible Mossbauer parameters correlated

with NMR

Both Sn and Fe measured CS, AE, . . Recoil effect by (n, a) reaction Binding energy, OD Steric hindrance Anisotropic absorption (Karyagin

effect) ARIR = -8.5 * 3 x 10-4 Q. -0.75 + 0.09 b H, ion implantation

Irradiatkn damage causes CS AEp us. K., two sites in NaIOa

AE, is small AE,

H = 148 * 5 kOe pe = 0.74 f 0 . 0 7

Specific heat measured On = 360 "K, Coulomb excitation

nm

e = 0.125

Lifetime of excited state = 2.33 * AR/R = +io-3

0 . 2 X 10-Osec

(Cmtinued)

VOL 40, NO. 5, APRIL 1968 e 477 R

Page 7: Moessbauer spectrometry

Table I. Application of Mossbauer Spectrometry to Chemistry (Continued) Subject or

material studied Ref. Mossbauer nuclides Types HALOGENS

Remarks

1971 (+v2) Nuclear parameters E = 57.6 keV ( +7/2) Compounds a = 3.8 K = 29.2 keV Hot atom effect ps = 1.96 nm pQ = i 2 . 8 0 9 n m Qo = -0.71 b Q, = -0.79b

a 0 4 ARin/\Rize KIClz, IC1 CSys. K. H I in frozen solution Te in Te(OH),

Ionic form Recoil form IOB-s

1201 ( f ' / Z ) Nuclear parameters E = 27.72 keV (+6 / * )

Source of Te in Al

Doped iron SnIa Ag120I Very narrow line Tellurium compounds

NOBLE GASES

E = 27.72 i 0.06 keV Mass separated source a = 5.3 rt 0.3 H = 1.1 X 108kOe Both Sn and I measured

01 = 5 . 0 pa = +2.84nm po = f2 .617 nm Q. -0.68b Q, = -0.55b

Metal Compounds

83Kr ( + 9 / ~ ) E = 9.3 keV (+7/2)

Compounds Review

KrF2

(93) (286) Use of *aRb as a source

CS, AE,, Qd = 0.459 f 0.006 b A R / R = +4 f 2 10-4

Qe/QQ = 1.7 rt 0.02 (w a = 11 K = 12.8 keV p, = -0.97nm Qe Q = +0.27 b

= 0.459 rt 0.006 b Solid krypton OD L-37' k. (362) Alkali bromide and bromate Some broadening and distortion due (364) Hot atom effect

to recoil damage ALKALI METALS

Review 4'K (-4) E = 29.4 keV ( -3) LY = 0.35 K = 3.35 keV po = -1.298nm &, = 0.09 b

lsaCs ( f 7 / 2 ) Demonstration of

Compounds

effect Metal, halides

E = 81 keV(fS/2)

K = 32.0 keV a = 1.63 Cesium halides

pe = +3.3nm pg = f2 .579nm QD = -0.003 b

RARE EARTHS Produced by Coulomb ex- General

citation Many CS theory discussed Intermetallics Review

DyNis, DyCo2, DyNi2 DyAL, DyFez, DyCos, H lslDy (+'/z) Alloys

El = 25.6 keV ( -'/-z) E2 = 74:5 keV ( - 8 / ~ ) Mntd - .- - _I_

Metal Metal Doaed gadolinium

pe/pQ = -1.2 i 0 . 1 Qb/QI = 0.85 f 0 . 1

a1 = 2.5 a2 = 0.65 K = 47.0 keV pla = +0.5nm Compounds pZs = f 1 . 6 n m p, = -0.455nm

Q, = +1.8b &is = +1.75b

l'lE~(+'/z) Compounds E = 21.6 keV (+'/z)

K = 42.5 keV p , = +2.5nm pQ = +3.464 nm Qe = f 1 . 2 b QQ = +0.95b

a = 29

1 W h (+s/.,> Comnounds

D+AI garnet DyFeOs Dysprosium-ethyl sulfate

EU203, EuO, EU cs EuSO~, EuBe

EupOs, EuSO4 EuI garnet Metal

Doped CaFs EueOr

Lifetime of excited state 2.14 f 0.2 X 10-%ec e US. ' K., OD = 94.6' K.

\ I , " I _ _ El = 97.4 keV (-s/~) E2 = 103.2 keV (+"/a) ai = 0.41 Hot atom effect

CS us. concentration of Eu a2 = 1.55 K = 42.5 keV pic = +3.2nm p2, = +2.03nm QiQ +2.93b

(Cuntinued)

478 R ANALYTICAL CHEMISTRY

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Table 1. Application of Mossbauer Spectrometry to Chemistry (Continued)

Subject or Mijssbauer nuclides material studied

1"Er Nuclear parameters 1"Er (+O) Compounds

E = 80.6 keV (+2) Q = 7.2 K = 50.2 keV pe = +0.61nm Qe = - 1 . 6 b

166Gd ( -a / , ) Nuclear parameters

El = 60.0 keV ( -6 / z ) E2 = 86.5 keV E3 = 105 keV f f 1 = 7.5 Q = 0.49 K = 44 KeV p,. = -0.564nm f i l l = -0 .27nm Qu = + 1 . 3 b

E = 64 keV I67Gd Observation of effect

Types ErC13 .6Hz0 ErAh Metal

Remarks Ref.

(328) Relaxation effects, H us. K. (490) H measured (382) H = 7.71 f 0.15 X lo3 kOe (381 1 H = 5.6 f 0.15 X lOakOe (489) Spin relaxation effects (347) Relaxation effects (406)

Metal-single crystal ErFeOs

Erbium ethyl sulfate

GdzOa Lifetime of excited state = 0.22

6 . 3 f 0.4nsec. E,, 1 .2 i 0.04

(436)

(316) nsec. (Ea)

nsec. Ea

Very narrow line '68Gd(Y, p)'6?Eu

l@Gd ( + O ) Observation of effect E = 79.5 keV (+2) Q = 5.94 K = 43.96 keV

176Hf Observation of effect

180Hf Observation of effect E = 88.3 keV

E = 93.3 keV HfOz, HfF. Qe( 176)/Q.( 180) measured

1"Ho ( - 7 / ~ ) Observation of effect E = 95 keV ( - 9 / 2 ) Q = 3.12 K = 44.9 keV pl = +4.15nm Qu = + 3 b

ls9Tb ( + 3 / ~ ) Observation of effect E = 58 keV (+6/2)

Q = 10.1 K = 45.5 keV

16gTm(+1/z) Compounds E = 8.41 keV (fa/*) Q = 325 K = 51.9 keV

Metal Metal Crystal field effect

ps = +0.6nm p l = -0.23nm Q6 = - 1 . 2 b

170Y b Compounds E = 84.2 (+2)

Ytterbium iron garnet YbPda, YbNi5, YbNIz

Spin relaxation Spin relaxation

a = 6 . 7 K = 53.6 keV f ie = $0.668 nm

E1 = 66.7 keV ( Compounds E, = 75.9 keV Q = 1 0 pzs p r = + 0 . 5 b

i r a y b

1'lYb ( - 1 / 2 ) Nuclear parameters

= 1.01 f 0.01 nm

Nuclear parameters E = 78.7 keV

f i ~ , = 1.01 i 0.01 nm

YbCla .6H10

174Yb Nuclear parameters E = 76.5 keV

168 -176yb Nuclear parameters Oxide ACTINIDES

Review Metal, AmFa, Am02

UaOa

Coulomb excitation (163, 166)

General 2 4 l b Compounds **U Observation of effect

E = 45 keV Q6 Q = 625

= +11.3 f 0 . 3 b

(44% CS, A E (443) &. = +11.3 rrt 0.3 b Coulomb excitation

(893, 367)

(Continued)

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Table 1. Application of Mossbauer Spectrometry to Chemistry (Continued)

Subject or Mossbauer nuclides material studied Types

23’Np Compounds 237NpOn E = 59.5 keV a = 1.07 K = 103.5 keV f ie = +2nm go = & 5 n m

E = 84.2 keV a = 2 . 8 K = 104.2 keV

231Pa Observation of effect Pa02

Remarks

B = 0.0045

sensitive to either the conversion elec- tron or the emitted X-ray but not the gamma rays in the energy range of the Mossbauer gamma ray (333). The counting rate is much lower in this case, and therefore stronger sources can be used with an overall decrease in data acquisition time.

4. Doppler Drives. I n the decade since hfossbauer’s discovery, the Dop- pler drive has changed considerably from mechanical drives and from elec- tromechanical drives that tinie-mod- ulate the wave forms of motion as input to pulse height analyzers (75, 102, 108, 275, 4G8). Most drives are electro- mechanical and are of two types, con- stant velocity and constant acceleration (GO, 424, 478). The system using con- stant velocity has a scaler-timer which measures the counts from the detector at a given velocity. Often these systems are programmed to cycle at a given velocity until a preset number of counts is collected. The programmer then changes the velocity setting and repeats the process. Such a system can be used with the multiscaler mode of a multi- channel pulse height analyzer (33, 272, 279, 319, 480).

For constant acceleration drive the velocity function can be either a saw- tooth wave form or a triangular wave form (85, 102, 215, 383, 494) with a single or double parabolic displacement (260, 360, 501). The triangular wave form with velocity sweep will result in two mirror imaged spectra on the multi- channel analyzer. The synchronization of the analyzer and the drive in the time-averaging mode is extremely im- portant (107, 319). If a wave-form generator is used, both the analyzer and drive must be synchronized. A simpler approach is to use the analyzer as a wave-form generator as described in the previous articles (134, 135, 425). This idea has subsequently appeared in later reports (5, 69,337,359) .

In the special case where a stationary source and absorber are required, mod- ulation of a single crystal, from which the radiation is diffracted, can be used

(343). This method can also be used to obtain polarized sources when the radiation is reflected at the Bragg angle.

Several new techniques using con- ventional spectrometers have been de- veloped, and are important for specific applications (4, 57) . A derivative Mossbauer spectrometer (70, 71 ) , based upon ideas used in the electron spin resonance modulating technique, is very satisfactory for the determination of small changes in peak position (for example, detection of small changes in chemical shift with temperature or pressure). Another interesting tech- nique is that of simultaneous collection of separate spectra with two different materials on the same Doppler drive by using subgroups of channels‘in a multi- scaling analyzer (399). This procedure increases the precision of measurement of spectral parameters by normalizing transients in the drive-e.g., by using a standard t s . unknown.

5. Cryostats. Many Mossbauer isotopes require low temperature to observe the effect and, in many cases, a variation in temperature is necessary to interpret the hlossbauer spectrum (408, see below). Several cryostats have been described for cooled absorber only, or for both cooled source and absorber (352, 495).

6. Magnetic Fields. As in the case of temperature studies, the appli- cation of external magnetic fields is extremely useful for spectral inter- pretation.

Weak magnetic fields (1 kG) are easy to apply and to align with a permanent magnet. Large fields (50-100 kG) require more elaborate equipment and care must be taken to provide proper shielding and alignment. While several laboratories have installed this equip- ment, no references on this subject were found in the recent literature.

7. Massbauer Nuclear Fluores- cence Scattering. Although trans- mission geometry is generally used, fluorescence scattering has been ob- served and has the great advantage that very thick, bulk samples can be

measured (378, 499). Since the 57Fe Mossbauer level has a large internal conversion coefficient (9.5) it is possible to detect the 6.3-keV X-ray radiation and the 8-keV conversion electrons. Figure 1 shows the nuclear fluorescence scattering spectrum of a cast iron, Standard Reference Material 1174, obtained by detecting the 6.3-keV X-rays. Because the 122-keV precursor gamma ray also produces a 6.3-keV X-ray by electronic fluorescence, the background level is significant, and there is little advantage in detecting the 6.3- keV X-ray in preference to the 14.4- keV energy. Detection of the short range 8-keV conversion electrons would give only the Nossbauer spectrum of atoms near the surface of the material. d very good presentation of the

scattering technique appears in a report by Debrunner and Frauenfelder (126). In addition to this work, calculations on optimum geometry are presented in reference (227). Another paper de- scribes conversion electron coefficient measurements by a scattering technique (1 79) .

8. Precision and Accuracy. It is desirable to have equipment tha t measures with satisfactory repro- ducibility before evaluation of syste- matic errors is made. I n the case of Mossbauer spectrometry we are con- cerned with the reproducibility of peak position and peak area (or height in some instances). Reproducibility of peak position is particularly useful because of the necessity of measuring small changes in chemical shift with temperature or pressure. The variation in peak position can result from extra- neous motion in the Doppler drive, excessive width of the Mossbauer ab- sorption peak, and the random process of radioactive decay (counting statis- tics). I t is possible with a good spec- trometer and known line shape of ab- sorption peak in the spectrum to limit the random error in peak position to that due only to counting statistics (373, 428).

In a transmission spectrum we ob-

480 R * ANALYTICAL CHEMISTRY

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Q + "a++ 0

P P

+ +

+ t s

* + s t t @ +s

s ++ %

@ t B

t

t

+

VOL 40, NO. 5, APRIL 1968 481 R

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serve a count due to the background radiation. This background comes from the Mossbauer gamma ray emitted with some recoil (B) and frcm electronic interaction-e.g., Compton scattering, etc.-between high energy gamma rays from the source and matter near the detector and in the absorber (B’). The height of the peak ( H ) is a satisfactory measure of peak area if it can be as- sumed that the line shape and width are known. Figure 2a indicates these measures on an absorption peak. Fig- ure 2b shows the hypothetical frequency distribution chart for each of the param- eters. The variance of the parameters B, B! and H can be determined by a least-squares analysis of the experi- mental data using known functions for the background and line shape of the absorption peak. These variances arise from counting statistics from spectrom- eter (Doppler drive) inconsistencies and from improper models in the least- squares analysis. It is important to note that quite often the major contri- bution to the variance is that due to counting statistics.

When one attempts to describe quantitative efficiency of a Mossbauer spectrometer, the proper terminology must use the absolute magnitude of H , B, and B’ along with the statistical variance of these parameters.

The notation, fraction effect ( E ) (134, 237, 638, $09) , expressed in terms of the experimental parameters is

(2) H

B + B’ =- B - ( B - H )

B + B’ E =

The variance of E can be calculated from the variances of H , B , and B’.

We can increase E by reducing B’. For example, this means that we try to eliminate gamma rays in the source that are higher in energy than the Mossbauer gamma ray. This can be done by select- ing the proper nuclear reactions. The resolution and, therefore, the selectivity of the detector can be improved. How- ever, care should be taken to assure high efficiency because, if fewer counts for the same period are collected than be- fore, the variance of E will increase. This entire problem can profitably use the concepts of L. A . Currie (118). The problem is one of “detecting” an ab- sorption peak with given variances of the parameters. Qualitatively we can state that the most efficient spectrom- eter is that which gives the largest “measurable” peak per unit time. The term “lowest measurable” or quantita- tive value resulting from an analysis is also defined in reference (118).

B’ can also be reduced by the use of X-ray absorption filters. In the case of S7Fe the 6.3-keV X-ray can be filtered with a 5-mil aluminum foil. For tin a 2-mil palladium foil and for iodine a 4-mil indium foil are recom- mended. A resonant detector (see de-

v)

c 3 0 0

c

I R e l a t i v e Ve I o c i t y

( 0 )

C o u n t s

(b 1 Figure 2a. Parameters related to per cent effect in MGssbauer spectrum Figure 2b. Frequency distribution of measured parameters

scription above) can almost completely eliminate B’.

An additional limitation on the effi- ciency of the spectrometer is the counting rate that can be accepted by the electronic system. Elimination ot B’ allows a greater source strength to be used so that greater precision in the parameters can be obtained per unit time. It is interesting to note that with B’ reduced to a negligible value, very small percentage effects can be measured because the fractional variance of the parameters decreases with increasing total counts collected. (Of course, the time of analysis is increased consider- ably.) The limit can be determined using the nomenclature of reference (118).

The thickness of the absorber affects both the variance of the peak position and B’. Thick absorbers produce line broadening (with deviation from the Lorentzian profile) and excessive beam attenuation. The absorber thickness should be determined from the effective Mossbauer absorber thickness given in Equation 10, reference (134). If this thickness produces gamma-ray attenua- tion of 37% or more, the counting effi- ciency will be low, and nonresonant scattering will be significant. The absorber thickness is then determined by this attenuation process (208).

The accuracy of the spectral param- eters can be evaluated with a spec-

trometer of high precision by designing a series of experiments and observing a shift in peak position and height (or area). Nonlinearity in the Doppler drive can cause significant systematic error in the peak position.

The linearity of a spectrometer can be measured by several techniques. The most direct method utilizes the six-line spectrum of a high-purity iron foil. However, great care must be taken in selecting the proper material to perform this calibration (410). A second method employs high frequency acoustic mod- ulation of a source or absorber super- imposed upon the relative Doppler velocity (114). The high frequency modulation produces side bands for a single line absorber. The side band separation is a function only of the frequency, while the side band absorp- tion depends on the modulation of the acoustic amplitude. This method pro- vides a continuous check on linearity. The most accurate technique employs an optical interferometer (428). Be- cause this method does not involve the Mossbauer effect, it can be used simul- taneously while a spectrum is accumu- lated, and an error signal can be derived to correct any nonlinearity in the Doppler drive.

Geometry of source, absorber, and detector can cause systematic errors by introducing severe distortion into the spectrum via the background. The opti-

482 R ANALYTICAL CHEMISTRY

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mum geometry is entirely determined by the absorber. Theoretically, the best geometry requires the absorber to be half way between the source and detec- tor. This configuration minimizes the scattering (Compton and Rayleigh) pro- duced by the source in the absorber and also minimizes the detection of the scat- tered Nossbauer radiation from the ab- sorber. However, most Mossbauer en- ergy levels have a large internal conver- sion factor which reduces the scattered Mossbauer radiation detected by the counter. In practice the absorber should be closer to the detector than to the source. For a moving source geometry, the counting rate will be a function of the source detector distance, and hence a function of the Doppler velocity. A saw- tooth velocity wave form will give a par- abolic distortion, while the triangular ve- locity wave form will produce a distorted double parabola. A moving absorber geometry eliminates this problem only in the first order, since nonresonant scattering by the absorber also gives parabolic distortion. Large separation of the source and the detector virtually eliminates this distortion but requires intense sources.

Another geometrical arrangement that distorts the line shape and reduces the effective velocity results from the source area and the detector aperture. The Doppler velocity vector ? and the -pray direction make an angle 8, and the effective velocity (V,) is

d

?e = ? cos 8 (3) This velocity error can be several per cent for close geometry (185, 228, 265).

Serious systematic errors can result from using the height as an indicator of peak area unless the line width and shape remain constant (499). Correc- tions for this effect can be measured and applied.

Several other factors can affect the accuracy of the measurements, but these can be eliminated by taking “blank” spectra. Most windows in counters contain impurities which give a spectrum. Beryllium, aluminum, and many plastics contain iron and should be carefully examined.

9. Use of Standards in MSss- bauer Spectrometry. Many of the above-mentioned systematic errors can be eliminated through the use of appropriate standards. The National Bureau of Standards has certified a single crystal platelet or properly oriented sodium nitroprusside. The distance between the two peaks in this spectrum has been measured with high precision and accuracy (426, 428). For velocity calibration, it is useful to have as a standard, a material such as very pure iron foil whose magnetic dipole interaction produces a multipeaked spectrum. The Sational Bureau of

Standards is considering the calibration of such a standard a t the present time.

Quite apart from the problem of systematic errors in the spectral param- eters is the problem of reporting chemical shifts. In order to gain some uniformity in reporting chemical shifts of iron compounds, the use of the NBS sodium nitroprusside is recommended. We have indicated by an asterisk in Table I those who have used the KBS standard. It is very important that those who are not reporting chemical shifts of iron compounds with respect to the NBS standard do so for two reasons. First, conversion of chemical shift data from one compound to an- other is not necessary. Second, it is inadvisable to report chemical shifts with respect to sources or other com- pounds because the chemical shift may not be representative of the “average” chemical structure. The high reproduc- ibility of the NBS standard has been carefully proven so that the worker can be confident that each laboratory which uses the NBS standard as a reference point can intercompare their data.

10. Spectrum Resolution Tech- nique. The fact that the line width is a significant fraction of the band width over which the resonance absorption is observed requires tha t measurements sometimes be made to within 0.001 of a linewidth. This can only be achieved with the aid of machine digital computation. Furthermore, complex hyperfine interactions, or non- equivalent lattice positions of Moss- bauer nuclides, can result in a very complex spectrum with partially over- lapping lines (258). Very few programs are listed in the literature, but it can be assumed that all of the major labora- tories doing Mossbauer spectrometry have them. Several computer programs are now available which can extract the Ptlossbauer parameters from the spectra. These programs can be grouped into three types.

a. Computation of Mossbauer spectra from theoretical considerations. These spectra assist in providing classifica- tion of the types of interactions present, particularly in the case of mixed quad- rupole and magnetic dipole interactions (106, 146, 178, 289).

b. Computer curve fitting, using a linear combination of Lorentzian line profiles, without constraints on line- width, amplitude, and line positions. Since this requires three independent parameters for each absorption line, this procedure is limited to about 12 peaks per spectrum, for most computer storage capacities (95).

c. Computer curve fitting with re- straints. This greatly reduces the number of independent parameters, but requires an a priori knowledge about the sample. This is about the only means to resolve 5’Fe spectra with

magnetic hyperfine interactions and with several nonequivalent lattice posi- tions. Using the known ratio of the magnetic moment of the excited to the ground state, assuming all peaks have equal line width, and using the theoret- ical nuclear transition probabilities causes an iterative least-squares analysis to converge rapidly.

The curve fitting programs are based upon a least-squares procedure which requires a linear function of the inde- pendent variable and hence, a lineariza- tion of the Lorentzian profile [ L ( x ) ] ,

with amplitude A , half width r, and position of the absorption peak, p .

Linearization is performed by re- placing the parameters with an initial value plus a correction term and then by expanding them into a Taylor series.

ANALYSIS OF A MOSSBAUER SPECTRUM

In the previous review the basic phenomena of the hyperfine interaction were described. In addition, some examples were given of the various in- teractions in iron compounds. In this section an attempt is made to provide a systematic procedure for identifying and sorting out the components of a complex Mossbauer spectrum.

The least-squares mathematical tech- niques provide a method for resolving a complex spectrum into a series of peaks giving their position and area. How- ever, the challenging task is the assign- ment of these peaks to groups that re- sult from a specific hyperfine interaction caused by a specific structure. It is not possible to be comprehensive in a few paragraphs, but procedures will be described that outline the method of attack.

It is perhaps obvious that any of the facts deduced from the designed experi- ments reinforce each other by a feed- back mechanism. For example, if some information is available about the electric field gradient tensor, this in- formation can be used to reduce the number of degrees of freedom in the least-squares analysis as described in a previous section.

In the description that follows, general examples of common electronic states in iron and tin compounds will be used. No intermetallic compounds or alloys will be described because of the added complexity due to their structure.

pure electric quadrupole interaction (EQI) produces two peaks for iron and tin, three for nickel, five for iodine, and seven for tellurium. Several papers have described the pertinent theory

VOL 40, NO. 5, APRIL 1968 483 R

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(28,106,199,224,230). A pure internal magnetic dipole interaction (MDI) produces six peaks for iron (239, 240, 448, 464, 479, 488, 488). Combinations of electric quadrupole and magnetic dipole interactions are often observed. Several theoretical discussions have appeared (130, 174, 273, 282, 477). Because of the superposition of the many peaks and other effects, one ob- serves considerable difference in the peak intensities. In addition, peak asymmetry that is produced by mixed quadrupole and magnetic dipole inter- action can be due to relaxation effects (68, 123, 124, 177, 211, 274, 344, 346, 353, 361, 465). These effects are the result of the interaction of the fluctuat- ing magnetic field produced by atomic electrons with the nuclear llossbauer levels (59, 350). The field fluctuation rate is controlled by spin-spin and/or spin-lattice interactions, and the peak asymmetry is a function of the relaxa- tion rate. If the relaxation rate is fast enough the apparent observed field is zero. For those compounds that pro- duce a single peak, magnetic relaxation can produce peak broadening. In addition, diffusion in a nonideal crystal causes broadening of peak (286, 287). To complicate matters somewhat fur- ther, asymmetric peak intensities in a polycrystalline material can be caused by a crystal orientation dependence of the Debye-Waller factor. For a single crystal, the peak asymmetry is a func- tion of the crystal orientation with respect to the direction of the Moss- bauer radiation. The chemical shift is another effect that can be observed in a Mossbauer spectrum (40, 284). Quite often it is necessary to evaluate those peaks produced by electric quadrupole and magnetic dipole interaction before a centroid or position of the degenerate level can be used to express the chem- ical shift. Considerable information in certain spectra can be derived from measuring the fraction of effect or the Debye-Waller factor (257,854).

It can be readily understood that direct interpretation of a single Moss- bauer spectrum without further experi- mentation is usually impossible. In fact, it is always true that data from other spectroscopies such as microwave spectroscopies, infrared] magnetic sus- ceptibility, etc., are required to provide assistance with the proper interpretation of the spectrum. There are a number of experiments that can be designed to assist in separating the various interac- tions described above.

Temperature Change. Measuring the Mossbauer spectrum as a function of temperature provides much information and should be performed in almost every instance for both magnetic and quadrupole interactions.

An example of the utility of tempera- ture variation for magnetic spectra is

the study of the behavior of the internal magnetic field near the Curie (Ned) temperature of a ferromagnetic (anti- ferromagnetic) compound or alloy.

Variation of a quadrupolar spectrum with temperature often reflects a change in Boltamann population of atomic energy levels. For instance, significant temperature dependence is usually ob- served for high spin de iron because the additional electron above the half-full d shell is free to Boltamann populate the d levels. In contrast high spin d5 iron exhibits little if any temperature de- pendence since the d shell is half full.

The two relaxation effects (spin-spin] and spin-lattice) cannot be distin- guished by temperature change. The asymmetry increases with decreasing temperature for spin-spin relaxation, but the relationship is unpredictable for spin-lattice relaxation.

Temperature dependence of chemical shift is not particularly significant from the standpoint of determining interac- tions. The value of the chemical shift can be very helpful in determining the structure under study. Many publica- tions (see Table I) contain catalogs of chemical shifts and the clustering of specific electronic configurations around definitive regions of chemical shift provides useful information.

The anisotropy in the Debye-Waller factor approaches zero as the tempera- ture is decreased. However, it is diffi- cult to distinguish this temperature effect from certain of the spin-lattice relaxation effects. This can be ac- complished by molecular dilution. The structure is then isolated from the lattice and the spin-lattice interaction is removed. This dilution procedure is often not a simple task. Sometimes dilute substitution in a lattice of similar type is possible. Some work has been done using solvation of the molecule (without interaction) and then freezing the solution to take the spectrum.

Occasionally one can observe a pre- ferred orientation of a polycrystalline sample caused by simple preferential packing (due to crystal shape) in the absorber mount. This will cause an asymmetric EQI. In fact, measure- ment of such asymmetry as a function of angle of incidence to the gamma ray provides data about the sign of the electric field gradient in the compound.

Finally, the use of an external weak magnetic field (1 kG) to polarize the source results in population of only certain levels in the absorber (196,417). This results in decreasing the complexity of the spectrum particularly in those cases where the MDI occurs with two separate structures in the lattice. It is also possible to apply a large magnetic field (50-100 kG) to a compound with predominant EQI that will allow mea- surement of the electric field gradient tensor.

APPLICATIONS

All of the applications are itemized in Table I. There is a noticeable increase in the number of applications on iron and tin compounds, and a lack of ap- plications with other elements. Several new isotopes have been found, primarily with the assistance of the Coulomb scattering technique. An interesting acceleration in the study of pressure effects is noted (126, 127, 178, 502,803). These experiments require very great care in their design to be assured that the expected compression of the lattice is being achieved.

Some interesting theoretical papers have appeared on the problem of re- sidual charge states produced by nuclear reactions prior to the decay of the Mossbauer level (166, 271, 340, 342).

Some effort has been made to correlate the Mossbauer parameters of tin com- pounds. Although considerable diffi- culty has occurred particularly in the assignment of AR/R (see Table I) some interesting progress has recently been demonstrated (111, 188, 190, 222, 504).

Another interesting study is the effect of small particle size on the Mossbauer spectral parameters (6, 65, 285, 295, 532, 339, 404, 462).

Several new applications that were not described in the previous review are the study of surface absorption (82, 109,110,143,470) and the measurement of intermediate compounds produced during chemical reactions (Table I.)

It would be improper not to mention the possibility of measuring quantita- tively the presence of known structures in a material. The present authors have recently reviewed work that demonstrates the feasibility of mea- suring relative concentration of oxi- dation states. It still remains to be demonstrated if the problems outlined in the last review article can be resolved satisfactorily to allow direct quantita- tive measurement of a specific structure.

SUMMARY

It may be of interest to attempt to generalize about the place Mossbauer spectrometry appears to be taking in the total complex of spectrometries which allows deductions about chemical struc- ture to be made. Mossbauer spectrom- etry is, of course, closest to the micro- wave spectrometries. In particular, i t often competes most closely with KMR. In the case where well defined hyper- fine levels exist-e.g., with most iron compounds-the much higher precision of S M R is superior to the Mossbauer method (455). However, when there is considerable spread in the levels, Moss- bauer spectrometry is very valuable in assigning appropriate frequency regions for NMR study. Of course, for mea- suring electric quadrupole interaction,

484 R ANALYTICAL CHEMISTRY

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Mossbauer spectrometry becomes unique if the spin state of the ground level is and that of the Mossbauer level is greater than '/z. There is little question but that those working in fields of Mossbauer spectrometry have much to learn from the microwave spectroscopists about the pertinent hyperfine interactions, and increasing efforts have been made to understand the relationships between these closely related fields.

I t is notable that only a limited number of papers have been published on a wide variety of applications to many areas in science, from surface absorption to the measurement of nu- clear parameters. Development of these ideas probably is slowed by a lack of elements to which this spectrometry can be conveniently applied. It is un- doubtedly the task of both the radio- chemist and physicist to remedy this situation. Recent developments in radioisotopic sources and the recoil implantation technique may provide the stimulation for expanding the method to other elements.

Fortunately, the importance of iron chemistry particularly in metallurgy provides much needed impetus for ex- tension of the applications. Improve- ment in instrumentation, particularly in radiation detection systems will also provide increasing interest in this unique spectrometry.

ACKNOWLEDGMENT

We could not have produced this com- pilation of references within the time schedule imposed upon ourselves with- out the much appreciated assistance of Robert Boreni, who operates the com- puterized information retrieval system, and Miriam Oland who carefully typed the entire manuscript.

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VOL. 40, NO. 5, APRIL 1968 489 R