Mossbauer Spectrometry 1. R. DaVoe and J. 1. Spijkerman, Radiochemical Analysis Section, Analytical Chemistry Division, National Bureau o f Standards, Washington, D. C.
A
REVIEW of the applications of Xossbauer spectrometry to chemical analysis, and a bibliography of publications on t'hese applications in 1965 are presented. The principle of the spectrometer as \\-ell as a discussion of errors associated with its use are described. Current theoretical interpretations of some aspects of the hlossbauer spectrum are discussed. Chemical structure analysis is the most prevalent application to date, and other applications, such as metallurgy and quant'itative analysis, appear to be promising. I n 1957 Rudolph llossbauer discovered the recoil-free nuclear resonant absorption of gamma rays which subsequently has had a great impact upon many areas of physics and chemistry (Professor Mossbauer was subsequently awarded the Nobel prize for this work.)
(115).
A description of this new spectrometry is classical in the sense that it may be described in terms of spectrophotometry where a measurement' is made of the nuclear resonant absorption of gamma rays passing from a source of radioact'ivity to an absorber. Prior to Mossbauer's discovery, it was difficult to observe nuclear resonant absorption because the emitted gamma rays imparted sufficient recoil energy to the emitt'ing nucleus to cause a decrease in the energy of the gamma ray, thereby destroying resonant absorption. Mossbauer discovered that in crystalline lattices a t temperatures significantly below the Debye temperature, the nucleus could emit a gamma ray with no recoil energy removed from it; and, therefore, if the absorber nuclei were also in a suitable lattice, resonant absorption could be observed. The above conditions prevail when the recoil energy is insufficient to escite any optical modes of vibration (phonons) in the crystalline lattice above the zero phonon level. Therefore, there is a critical upper limit of about 150 k.e.v. to the energy of the emitted gamma ray. The theoretical line width of the emitted Nossbauer gamma ray can be calculated from the lifetime of the nuclear escited state using the Heisenberg Uncertainty Principle. These line widths are of the order of 10-5 electron volt for most Mossbauer radioisotopes or about 10-l2 of the energy of the gamma ray. This line width is sufficiently narrow to allow measurement of very small changes in the energy
382 R
ANALYTICAL CHEMISTRY
level of the nucleus. For example, it is possible to observe the hyperfine interactions (the effect of the extranuclear electron field upon these lowlying nuclear energy levels). There are several excellent review articles in the literature which describe the theory of the nuclear resonant absorption (50, 107, 114, 158). I n addition there are proceedings of a series of conferences held on the Mossbauer Effect (3,7 , S2,58, 61,84). Prior to 1965 many review articles have appeared in the literature (86, 38, 4O,47,48, 5 7 , 6 6 , 8 9 , 1 5 7 )on the chemical application of the Mossbauer Effect. The first chemical applications were published in 1962 and number several hundred publications to date. A bibliography of all research published to January 1965 was prepared by hIuir et al. ( 1 1 7 ) . The referenced publications in the present review indicate some of the typical work in the field prior to 1965 with emphasis on those contributions that are deemed by the authors to be of above average significance. Every effort has been made to include in this review all pertinent references on the chemical applications published in 1965. THE SPECTROMETRIC TECHNIQUE
As stated above, the line width of the Mossbauer gamma ray is extremely narrow. Because of this the chemical environment of the atom can displace the energy levels, so that resonant absorption cannot occur (unless, of course, its chemical environment is identical in both source and absorber). For Fe57this displacement is approximately 5 x 10-9 e.v. (this assumes displacement of one half width, r). I n order for nuclear resonant absorption to occur we must modify the energy of the source to match the difference in energy levels of the ground and 14.4 k.e.v. excited state in the absorber. For a more detailed discussion see the section on chcmiod shift below. The Doppler effect has been found to be most satisfactory for doing this. The frequency, Y , of the gamma ray a t the absorber for a velocity, v, relative to the source that emits a gamma ray of frequency y o is:
+ y)= -
Y
= &(l
4
Yo
(1
+ ;cos 8)
(1)
where 8 is the angle between t!ic direction of motion and that of the gamma radiation, and c is the velocity of light. For 8 = 0 the energy change is AE = E,v/c, and substituting A E / E , = r/Eo = 3.3 X for Fe5', a required relative velocity of 0.1 mm. per second is calculated. Therefore, it can be seen that only modest velocities which can be easily produced in the laboratory are required. Spectrometer. T h e spectrometer must be capable of producing a relative velocity between the source and absorber and at the same time sensing and sorting the Mossbauer gamma rays according to the instantaneous relative velocity. A Mossbauer spectrum consists of the plot of Mossbauer gammaray intensity as a function of relative velocity between the source and absorber (see Figure 4). I n current practice there are two general classes of device that produce Doppler motion, one generating constant velocity, and the other constant acceleration. The system most used is of the constant acceleration type, since stability problems with the gamma-ray detection system can be averaged over the entire range of velocities in the q-1iectrum. Eccentric cams, hydraulic pistons, (34) and other mechanical devices can be used, but the majority of the systems use an electromechanical transducer. There are many such systems reported in the literature (1, 19, 33, 46, 54, 86,98, 1OS, 110,184, 153). All of the systems involve basic components indicated in Figure 1. X signal generator produces a linear increase in voltage with time. This voltage is transformed into a velocity in the transducer providing constant acceleration. Most systems require feedback which is generated from another velocity sensing transducer on the motion. This feedback provides greater stability of the Doppler drive, and is particularly useful for reducing extraneous motion due to building vibration, for example. It is beneficial to discriminate from photons that are not of the Mossbauer gamma-ray energy; therefore, a single channel analyzer is used to pass only those detector pulses that correspond to the proper energy. These pulses are scaled in the multichannel analyzer. The channel number in the analyzer represents a certain velocity. When a gamma ray is detected, it must be stored in the channel corresponding to the proper velocity. Therefore, some
. i
-
SINGLE CHANNEL ANALYZER
DETECTOR :
ABSORBER
c
ANALYZER
SOURCE
DRIVE COIL SIGNAL GENERATOR
OPERATIONAL AMPLIFIER
POWER AMPLIFIER
-
-L
method of synchronizing the signal generator and the channel address must be devised. There are a number of techniques used, and these are discussed in the references indicated above. rl most convenient method for accomplishing this, as well as to remove equilibrium drift in the motion of the transducer, is found in the NBS spectrometer (137). This system uses the analog output of the address in the multichannel analyzer as a generator for the input signal to the velocity transducer. The address is then advanced with an oscillator of the desired frequency. This system is only as linear as the analog signal from the multichannel analyzer; however, if the dual spectrum measurement approach is used (see below), reproducibility of the motion becomes more important than linearity. I n any case the linearity of the analyzer can be made to exceed the capability of the transducer to follow it. The radiation detectors that are used are proportional counters for energies below 20 k.e.v. and a XaI(T1) detector for high energies. The thickness of the NaI is determined by the energy of the gamma radiation to obtain maximum resolution and efficiency. In order to reduce the time required for collection of a spectrum, it is desirable to be able t o detect as high a count rate as possible. Therefore, a fast preamplifier, single channel analyzer, and multichannel scaler capable of counting a t 10 N H z are very useful. All components except the single channel analyzer are commercially available, and even without this component, the sy>teni allows good spectra to be taken in 2 t o 4 hours for most materials. I n addition to the transmission geometry indicated in Figure 1, it is poisible t o obtain good spectra with the system arranged to detect scattered radiation. This geometry which is particularly suited for the analysis of iron-57 is shown in Figure 2. The
SOURCE yFILTER
RAY
MONITOR COIL
yrRAY
7
~ ~ P ~ ~ ~ I o N A L C O U N T I * OR 8OLID STATE DETESTOR
x-
RAY
U
Figure 2.
Detector geometry for resonant scattering
cobalt-57 source radiation is passed through a filter (0.002 inch of aluminum) to remove most of the 6.4 k.e.v. x-ray. The 14.4 k.e.v. radiation passes through the detector and excites the iron-57 in the sample. The detector may be either a proportional counter using argon as a counting gas or a solid state counter such as lithium drifted silicon. The excited state of iron-57 then goes to the ground state by two processes, one internal conversion and the other emission of the 14.4 k.e.v. gamma ray. Since the internal conversion is 10 times more probable than emission of the gamma ray, the detection of the 6.4 k.e.v. x-ray generated by the internal conversion results in high sensitivity and low background. Accuracy and Precision. The parameters of interest in a llossbauer spectrum are the absorption peak positions on the velocity scale, the width a t half maximum of a peak, the line shape, and the area of the peak relative to the area under the base line of the peak called the f’ number or fraction of recoilless events due t o both emisqion and absorption. Very little work has been done on the systematic evaluation of the precision and accuracy associated with these parameters. Variables such as position drift, transducer linearity, and vibration can effect the precision
of the velocity scale produced by the Doppler drive. Bras in the transducer affects the accuracy of the velocity scale. The uqe of a standard absorber will assist in evaluating the precision of the spectrometer (see qection on standardization). Another approach is to use a bipolar voltage-to-frequency converter which replaces the detector input t o the multichannel analyzer. This will reproduce the voltage-velocity ramp of the driving signal so that linearity may be te?ted (137). A method of absolute calibration of the velocity scale may be made by measuring the splitting caused by the magnetic dipole interaction for iron metal (130) which ha. also been ineasured by nuclear magnetic reqonance (106). Xore nil1 be said of this below. Another method is to use optical interferometry, or the optical technique of Lloire frhgeq can be used. Fiom perusal of thc above publication., it appears that the preciqion and accuracy which are dependent upon a h o l u t e value of velocity are in the xange of 0.01 t o 0.1% and apl)ro\iniately 0 I % , respectively. The N I E optical interferometric drive ($4) has imprwed upon these values by about a factor of 100. X considerable improvement in the VOL. 30, NO. 5 , APRIL 1966
383 R
precision of the electromechanical drive can be obtained by running two absorption measurements simultaneously on the same Doppler Drive. This involves multiplexing the multichannel analyzer so that subgroups of the address are time shared while using only one analog drive signal. One absorber would be a reference standard, and the other absorber, the material under study (136). If a suitable geometry can be designed, it would be desirable to have only one source for both of the absorbers so that normalization of source differences would not be neceqsary. There are other factors which affect the precision of the absorption peak position. If the source moves an appreciable fraction of the source-detector distance, the inverse square law and the changing geometry for detection will markedly alter the count rate. If constant acceleration is used the displacement of the source will be a parabola with time, and the spectrum is superimposed upon a parabolic base line. If the absorber is moved, the same effect to less degree is observed when a high energy gamma ray precursor can Compton scatter from the absorber into the desired energy range of the detector. I n certain cases of source-detector geometry, a correction may have to be made for the fact that the gamma rays are not all vectored parallel to the Doppler motion. This usually causeq a displacement of the peak to lower velocity (61,147). Considerable line broadening has been observed (108) with increasing absorber thickness, and the shape of the line is modified from a Lorentzian to a Gaussian function. Correction factors are usually evaluated by observing the line width as a function of absorber thickness (50, I @ ) . A inore detailed discussion of the above requirements for Jlossbauer spectrometer instrumentation and of the errors that one can experience in such instrumentation is presented in a review by one of the authors (147). Standard for Calibration. It is very desirable t o calibrate, with a high degree of accuracy, t h e Doppler drive in units of velocity. One way t o do this is through t h e use of a highly uniform and stable reference absorber which produces a spectrum with more than one peak so t h a t the distance between the peaks can be accurately calibrated. Such an absorber has been made available by the Standard Reference Materials Program of the National Bureau of Standards. Thi. absorber is a properly oriented single crystal of sodium nitroprusside which has been carefully calibrated with the NBS optical hlossbauer spectrometer that was mentioned above. 384 R
ANALYTICAL CHEMISTRY
Proper calibration of the velocity scale will enable direct comparison of data on electric quadrupole and magnetic dipole interaction with data obtained on related parameters by other spectroscopic techniques. Source-Absorber Preparation. I n general, the material to be studied is t h e absorber, so the source should be made to produce a line width as close to the theoretical width as possible. T o simplify most requirements for spectral interpretations, sources are usually made so as to exhibit a single unsplit emission line. I n addition, i t is desirable to maximize the signal-to-noise ratio by producing a source which has a large fraction of recoilless emissions (fa, Debye-faller factor). The following expression can be used as a guide for determining this fraction (69,116,158) fe
= esp
- [3E,,*/4JlcZk O D ]
(2)
where E, is the photon energy, Jf is the atomic mass, k is the Roltzman constant, and e0 is the Debye characteristic temperature for a monotomic lattice. The source is prepared by introducing the radioisotope into a suitable matrix which has a high nebye temperature. Often this matrix is not of the same element as the Moqsbauer radioisotope, and for a substitutional impurity, a multiplicative correction must be made to the estimate of the characteristic temperature, (Illhast/-l~,mpur,ty)112. Ordinarily the source is then annealed to produce a uniform distribution of sites with minimal distortion of the lattice which could produce line broadening. Information on source preparation appears in the review articles (see Table I). Of particular interest is the discovery of a very suitable Sn119m source which is an intermetallic compound, PdaSn (69, 144). This source has a 24% effect at room temperature conipared to white tin, and a line width of 0.072 cm. per second compared with the theoretical value of 0.069 cm. per second. There is evidence that the energy of the conduction band electrons is an important consideration in the production of good sources. Those matrices which have partially filled conduction bands and high electron mobility such as in the elements palladium, gold, copper, platinum, etc., seem to have the largest f values while those elements which are in the semiconductor or insulator class have lower values. I n addition to the above procedure of simply introducing the long-lived precursor of the hlossbauer levels into a matrix, it is possible to generate the Mossbauer levels by other types of nuclear reactions. One of these is coulomb excitation. Bombardment of a target element with heavy charged
particles, 25 m.e.v. oxygen ions for example (47, 141)) results in the direct excitation of the low-lying Mossbauer levels in the target. Several elements have been done in this way (see Table I). Another method by which these lfossbauer low-lying levels can be pro) by duced is through the ( n , ~reaction observing the de-excitation process of the prompt gamma. The Mossbauer Effect was first observed in potassium by producing an excited state of potassium-40 via prompt gamma excitation (see Table I). The requirements for preparation of the absorber are dictated by the type of material that is being analyzed. For transmission, the thickness must be controlled because of scattering effects as indicated above. For very careful structure analysis, it would be desirable to use single crystals. Powders which are most often available may be compressed between two thin beryllium windows and mounted in the spectrometer. HYPERFINE INTERACTIONS
The preponderance of publications relating to chemical applications has been in the area of structure analysis. As indicated above, it is the hyperfine interaction which alters the energy levels in the nucleus, that is in turn measured by llossbauer spectrometry. Rather large changes in electronic structure alter these nuclear level5 only slightly, but the extreme narrowness of the llossbauer gamma ray makes the technique a very sensitive one. If the source and absorber are in different chemical environments a change in the nuclear levels of the excited and ground state is observed. These changes can occur as the result of three effects which can be described b y considering the classical expression for the multipole moments of the charge distribution between the nucleus and the atomic electrons (43) d = n=O
__ dern n+l
P , (COSe)
(3)
where the potential d is measured a t distance d from a coordinate system containing a unit charge e a t distance T from the origin, with e the angle between r and d . P, is the Legendre polynomial of order n. n = 0 represents the monopole interaction which considers a point charge in an electrostatic field of spherical electron density, p , near the nucleus. For a nucleus of finite radius ( R ) with the same charge, an expression (17 , 168) can be derived for the change in electrostatic energy ( A E ) with nuclear radius. 2T AE = - ZepR2 5
(4)
I
I SOURCE
ABSORBER
AE,
-
AEg =
27r 0
where q is the electric field gradient a t the nucleus, and Q is the electric quadrupole moment of the nucleus, and p is the asymmetry parameter 11=
Zep (Re2- R g 2 ) ( 5 )
If the chemical structure has an influence on p , there will be a change in the energy level differences between the source and absorber to give the chemical shift (6, see further euplanation below). n = 1 represents the electric dipole moment which is nonexistent for the nucleus; however, there is the interaction of the nuclear magnetic dipole moment, p , with a magnetic field, H , which ran result from the spin and/or orbital angular momentum of atomic electrons, where the energy levels are given by the expression :
where mz is the magnetic quantum 1 values of number which has 21 total nuclear angular momentum, Z, Z - 1, . , . ., -1, pn is the nuclear magneton, and g is the nuclear gyromagnetic ratio. The energy levels of both the excited state and the ground state will be split, and the selection rule of m = i l will produce a series of gamma rays with uniformly spaced energies about the degenerate energy level. n = 2 represents the electric quadrupole moment. If an electric field gradient due to atomic electrons exists at the region of the nucleus which has a n electric quadrupole moment, an interaction results where the energy levels are given by the expression: (SO, 158)
+
E -- 41(21 e2qQ - l) ( 1 + +
T,i
T,i
vzz
Ti, are
I
I
Figure 4. Mossbauer Spectrum of NBS Standard for Chemical Shift, sodium nitroprusside, counts/unit time vs. velocity (cm./sec.)
Figure 3. Energy level diagram of Fe57, 14.4 k.e.v. state, showing chemical shift
where Z is the atomic number. The nuclear radius changes on going from the excited to the ground state so that the energy change upon emission of a gamma ray in the source is
I
- VYY
AE,, - AE,,. Therefore, from Equation 5 , the chemical shift, 6 , is 6 = E,
- E,
=
4s AR - ZeR2 - ( p a - p a ) 5 R
v** 6x2'
!sf
61P
'
etc., which
are the three components of the electric field gradient tensor. Since ?nZ is squared, the energy levels with mZdiffering only in sign are degenerate. Here again a splitting of both the e w t e d and ground state is possible with an emission of gamma rays of different energy. The higher order terms of n are not significant. The manifestation of the above interactions allows one to deduce much information about the chemical structure as will be discus>ed below. It is worthwhile to elaborate soniewhat upon the interpretation of the hyperfine interaction for chemical structure analysis. Characteristic of a developing spectrometry are the many problzms of interpretation, and Mossbauer spectrometry is no exception. Many of the qualitative explanations of a year ago are beginning to he reoriented as a result of a more thorough understanding of the spectrometry. The development of such a consiqtent theory is by no means complete, and, in fact, it is just beginning to show promise. Chemical Shift. Since there will be in most cases different chemical environment between t h e source and absorber, the Nossbauer spectrum measures the difference in excited and ground state energy levels between source and absorber which result from both the radius change of the nucleus and the electronic charge density near the nucleus, see Figure 3. The energy of the gamma ray emitted by the source, E., is E , AE,, - AE,,; and the energy required to result in nuclear resonance absorption, E,, is E ,
+
+
where AR is the change in radius on going from the excited to the ground st,ate. -1convention on sign must be adopted here. The positive chemical shift for an absorber with respect to a source is considered to r e d t when resonance is observed while the source and absorber move toward each otherLe., increasing gamma-ray energy. (Suggestions on these convention? mill be made in a XBS publication associated with the Standard Reference Naterial for chemical shift.) I t is important if a t all possible that the sign of AR/R be determined preferably by a means other than 1\10 spectrometry, before relative es of electron densities a t the nucleus can be determined. Although it is generally accepted that p can be replaced wibh the total s electron probability density a t the nucleus, it is apparent that p , d , and f electrons have their influence (I5S). The value AR/R has a minus sign for iron-57 so we expect that as the s electron density decreases, the chemical shift increases in the posit,ive direction. Nossbauer spectroscopy has provided a correlation between the chemical shift and 4s electron density. Such an allproach has been used by Kalker, Wertheim, and Jacquerino (156') to show a linear relationship between per cent 4s electron density and chemical shift for various d configurations of the coordination compounds of iron. This is a n over-simplification, but the use of this approach is pertinent, and the correlation of chemical shift with suitable molecular orbital calculations of these compounds using data from x-ray diffraction and infrared spectrometry, VOL. 38, NO. 5 , APRIL 1966
385 R
etc. should allow the extension of this method t'o a wide range of iron coordination compounds. 1 similar correlation for Sn1I9 has been obtained by Lees and Flinn (100) using 5 p electron density. Electric Quadrupole Splitting. The electric field gradient a t the nucleus which is responsible for electric quadrupole splitting cannot result from the norniallj- spherical electron distribution of the filled electronic core, but, it can occur as a result of the perturbation of the core by t h e outer electronic configuration. For Fe57 the nuclear angular inonientuin of the ground state is 1/2; so froin Equat'ion 7 the degeneracy of t'his st,ate is not lifted. For the excited state, I is 312 and the 14.4 k.e.v. level is split int,o two states. This produces two peaks in the spectruiii, such as shown for the S U S Standard, sodium nitroprusside, in Figure 4. Inberaction of partly filled d-orbitals with the filled shell:: of the iron core can produce a gradient a t the nucleus. It is suspected that' a preponderance of unpaired electron spins in the d level can influence the gradient through spin-spin coupling. relationship between the magnetic susceptibility and electric quadru,)ole splitting for iron compounds has been found (38, 129). Even with a symmetrical distribution, such as the d5 state of iron, an electric field gradient can be exhibked, and this could be attributed to the spin-spin interaction of the d elect,rons with the ligand field. For iron coordination compounds one would expect to find the maximum quadrupole qditting in a high spin dfi configuration of iron with the low spin d5 next. The high spin dj, and low spin d6 produce very small electric quadrupole splitting. Magnetic Dipole Splitting. I n a similar manner a magnetic field can be generated in the region of t h e nucleus through interaction of the unpaired spins of outer electrons with the completely filled inner core of s electrons producing a Fermi contact interaction with the nucleus. Here again a correlation with magnet'ic susceptibility is possible (38). There are other mechanisms for producing magnetic fields a t the nucleus. ri field can be produced by a net orbital angular monientuin in the out'er electrons. Quite often no magnet'ic hyperfine splitting is observed unless the material is magnetically ordered, but there are notable except'ions. Of particular interest is the possibility of deriving st,ructural information from the spin-interaction process which can occur as a result of the discrete energy levels in the outer electrons. I n certain iron compounds (258) where spinspin interaction is small, electrons in each m, (spin magnetic quantum num386 R
e
ANALYTICAL CHEMISTRY
ber) level can produce their own hyperfine int'eraction, and set of six peaks in t,he Mossbauer spectrum. Temperature Dependence. I t is imperative that Mossbauer spectra of a suhstance be taken as a function of temperature. Observation of the electric quadrupole splitting and chemical shift as a function of temperature can provide valuable information about chemical structure. For example, an anomaly in the temperature dependence of chemical shift for iron in palladium has been observed (12). The sixth electron in excess of the half filled shell in the de configuration for iron causes a sharp temperature dependency of the electric quadrupole splitting. The molecular orbitals arising from the d atomic orbital are split by the crystal field and are populated in accordance with the 13oltzniann factor exp(-TP/kT) where TT' is the energy separation of the levels. As the temperature is lowered so that TY >> k T , the lower level is populated preferentially, and the electric quadrupole splitting is a t it,s highest value. -kt higher temperatures all levels are populated equally and the electric quadrupole splitting approaches zero. The d5 iron level has a temperature dependency that is related only to the tal field parameters, and it is a much smaller effect . Transitions in magnetic ordering with temperature (Le.) through the Curie point) can be observed. I n the case of the internal spin interaction of some iron compounds variation of temperat'ure will alter the relative population of the m, levels and, therefore, the relative intensities of the three sets of six lined spectra. Several devices for doing Mossbauer spectrometry with cooled source and/or absorber have been described in t'he literature (6, 27, 154). Pressure Dependence. The application of pressure upon a crystal of inoderate compressibility d l have a net result of simply changing the bond lengths. If the crystal is perfectly symmetric one would expect to decrease all bond lengths equally (assuming t'hat t h e pressure can be applied evenly), and a subsequent change in l$ol* and, therefore, the chemical shift would result. If there were asynimet'ry in the crystal which could cause an electric quadrupole splitt,ing, the applicat'ion of pressure will alter the splitting. Shear forces could be used to indicate the orientation of this asymmetry with the cryst'al axis. The method is exceptionally sensitive and can detect asymmetries which are not nieasurable by x-ray diffraction (29). The theory and instrumentation of such pressure experiments appear in references (51) and (82). Measurements in metallic
+
tin have been made up to 100 kbar (124), and pressure measurements also have been made of Fe57 in titanium, vanadium, and copper (39). APPLICATIONS
Standardization. I n order to use any of the parameters t h a t can be obtained from a Mossbauer spect'runi, it is well to ascertain the level of confidence that can be placed upon these measured parameters. For example, there must be a consistent way of reporting the chemical shift of cornpounds. I n the past the shift has been reported with respect to a particular source. I t can be verified that the cheniical shift of even supposedly identical sources varies considerably from source t'o source (66, 117). Another approwh is to relate the chemical shift to an absorber or" a part,icular material. Many absorbers hare been ujed such as iron foil, but recent measurements have suggested that varying amounts of superimposed electric quadrupole splitting on the magnetic dipole splitting in these foils make it a questionable standard. The National I3ureau of Standards has produced a standard (248) which is a suitably oriented single crystal of sodium nitroprusside which has a symmetric-doublet Mossbauer spectrum as a result of electric quadrupole splitting, Figure 4. The procedure is to place the standard in the spectrometer, and to measure the midpoint in units of velocity between the two peaks of the resultant spectrum, and to call this midpoint t'he zero chemical shift (6 std.). The spectrum of the compound of interest is measured, and the peak positions, or the centroid of the electric quadrupole splitting or magnetic dipole splitting are determined with respect to the midpoint of t'he standard. It is proposed that this be called the differential chemical shift, 6, = 6, - 6std. By using such a standard, errors do not arise as the result of using different sources. I n addition, the use of the single crystal precludes the broadening that can occur in polycrystalline materials, which makes t'he interpretation of the velocity spectrum more difficult (127). This standard also serves the purpose of calibrating the velocity scale as indicated in the section on instrumentation. Structural Analysis. Table I lists all of t h e 1965 publications t h a t relate to t h e chemical structure analysis of compounds by Mossbauer spectrometry. .I short description appears in the table for those papers that have been reviewed by the present authors. I n the first column the nuclide and the energy level, E , which exhibits the Nossbauer Effect are listed. The nuclear angular momentum ( I ) is indicated in parentheses after the E value
~
Table 1.
Application of Mossbauer Spectrometry to Chemistry-Compilation
of Publications for 1 965
El, E2
= Energy of Mossbauer gamma ray 1 and 2, respectively, in k.e.v.
a
= = =
Internal conversion coefficient Kap x-ray energy in k.e.v. f i l e , p1, Magnetic moment of excited and ground state producing hIossbauer gamma ray 1, respectively, in nuclear magnetons (nm.). &la, &I, = Electric quadrupole moment of excited and ground state producing Mossbauer gamma ray 1, respectively, in barns (b. = 10-24cm.2). [data taken from reference number (117)] Abbreviations: EFG = Electric field gradient EQS = electric quadrupole splitting CS = chemical shift f = Debye-Waller factor 00 = Debye temperature = charge radius of nucleus R K
Mossbauer nuclides Fe57 ( - 1/2) E = 14.4 k.e.v. (-3/2) ff = 9 K = 6 . 5 k.e.v. p$ = -0.154nm p, = f0.09024 nm. Q. = f 0 . 2 8 5 b.
Remarks Subject or material studied TRANSITION ELEMENTS Coulomb excitation, 3 m.e.v. alpha New method of source preparation FeS6(d,p)Fe5' Measurement of nuclear parameters
Studies of coordination compounds
Phase transitions in solvent Surface adsorption Relaxation effects Hot atom and radiation effect
Ni61( -3/2) E = 67.4 k.e.v. (-5/2) a = 0.12 K = 7 . 6 k.e.v. p8 = hO.35 nm. p, = 5Z0.746 nm. osy SO) E = 137.2 k.e.v. (4-2) a = 1.2 K = 64.5 k.e.v. pg = f O . 57 nm. Os'y +O)
E = 155 k.e.v. ($2) a = 0.84 K = 64.5 k.e.v. po = +0.49nm.
~183(-1/2) El = 46.5 k.e.v. ( -3/2) E2 = 99.1 k.e.v. (-5/2) LY1 = 7 . 4 012 = 3.6 K = 60.7 k.e.v. pg = i ~ 0 . 1 1 7 n m .
Debye-Waller factor; mean square displacements Source produced by Coulomb excitation
Observation of effect, scattering studies
Gyromagnetic moment, 1st 2( +) state Observation of effect, scattering studies
Gyromagnetic moment, 1st 2(+) state Observation of effect
01 = 5 . 9 i 6, 9 . 2 i 0.5, and 9 . 0 rt 0 . 4
Magnetic moment = f0.0924 5Z 0.00007 nm. cm.2 Cross section = 1.91 i 0.14 X Various carbonyl ferrates Heme compounds Ferrocyanides Spinel oxides Crocidotite and Amosite Ferric hydroxide Ferrous formate Tetrachloro-ferrate(I1) ion FeC12with molecular orbital calculations Sign of EFG in ferrocene by applied magnetic field Xagnetixation of ferric ammonium sulphate Iron in glasses Iron in BaTiO, Metal ferricyanides Graham's salts Large number of iron-organic complexes Salts in ice General theory On tungsten Dithiocarbamates Hematite Auger effect Radiolysis of oxalate In zinc Stainless steel 25 1l.e.v. oxygen ions
References
(93) (85, 180)
iissi
Source, Re metal Absorber, Os metal Os186 effect = 9.770 en-l86 = 312 f 7 OK. Temperature =
