MÖSSBAUER EFFECT STUDIES OF Te COMPOUNDS

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MÖSSBAUER EFFECT STUDIES OF Te COMPOUNDS

Y. Mahmud, P. Boolchand, S. Hanna, B. Triplett

To cite this version:

Y. Mahmud, P. Boolchand, S. Hanna, B. Triplett. MÖSSBAUER EFFECT STUDIES OF Te COMPOUNDS. Journal de Physique Colloques, 1974, 35 (C6), pp.C6-227-C6-230.

�10.1051/jphyscol:1974628�. �jpa-00215785�

JOURNAL

DE

Colloque C6, supplkment au no 12, Tome 35, De'cembre 1974, page C6-227

MOSSBAUER EFFECT STUDIES OF Te COMPOUNDS

Y. MAHMUD, P. BOOLCHAND (*), S. S. HANNA and B. B. TRIPLETT Department of Physics, Stanford University, Stanford, Ca. 94305, U. S. A. (**)

RBsumB. - On dkcrit Ies dkplacements isomkriques ( I S ) et les kclatements quadrupolaires ( Q S ) de divers comuosks de Te. En reurksentant les dkolacements isomkriaues des comoosks de tellure en fonction d l ceux de compos& analogues d'iohe, on obtient uneielation appEoximativement linkaire pour des paires de composks 2 la fois isoelectroniques et isostructuraux. Des paires qui ne sont pas isoklectroniques, mais qui prksentent simplement une similitude chimique, ne vkrifient pas, cette relation linkaire. On note que pour ces composks similaires les liaisons Te-X sont plus ioniques que des liaisons I-X. Nous essayons de rendre cette observation plus quantitative et nous ktendons une corrklation observke entre l'kclatement quadrupolaire et le dkplacement isomkrique A des com- poses de tellure qui n'ont pas de similitude chimique.

Abstract. - Isomer shifts ( I S ) and quadrupole splittings ( Q S ) are reported for a number of Te compounds. When the isomer shifts for Te compounds are plotted against the isomer shifts for similar I compounds a roughly linear plot results for pairs of compounds which are both isoelectronic and isostructural. Pairs which are not isoelectronic but merely have similar chemical character deviate from the linear relation. For these similar compounds it is noted that the Te-X bonds are more ionic than the I-X bonds. We attempt to make this argument more quantitative and pursue a correlation found to exist between the quadrupole splitting and the isomer shift which is also found to apply to chemically dissimilar Te compounds.

1 . Introduction. - The isomer shifts of nuclear energy levels, which are observable as changes in the y-ray energy in Mossbauer effect (ME) experiments, are a useful source of information on both nuclear and chemical structure. These shifts arise from the electro- static interaction of the nuclear charge distribution with those electrons which spatially overlap the nucleus.

The isomer shift (IS) in a ME resonance spectrum is given by

where Z is the atomic number of the element,

is the difference in the average electronic charge densi- ties over the nuclear volume in the source and absorber, and 6 < R2 >

< RZ >, ^- < ^{' R}

is the diffe- rence in the mean-square nuclear charge radii of the excited state and ground state of the Mossbauer nucleus. Thus a measurement of the isomer shift pro- vides a measure of the product of two quantities, one related to the chemical environment and one to the structure of the Mossbauer nucleus.

One approach which has been used to obtain infor- mation on the ratio of 6 < R2 >/< R2 > for 125Te

(*) Permanent address : Department of Physics, University of

Cincinnati, Cincinnati, Ohio 45221, U. S. A.

(**) Supported in part by the National Science Foundation.

to that for '"I has been to compare isomer shifts for isoelectronic compounds of Te and I. Such a plot of IS(lZ5Te) versus IS(12'I) yields, in principle, a slope which is a measure of the ratio of the two values of 6 < R2 >/< R2 >. Such a comparison has been a subject of controversy [I] since the original work of Jung and Triftshauser [2] and Ruby and Shenoy [3]. At least some of the controversy concerning the original work centered on the use of some [Te : I] pairs which were not isoelectronic but merely chemically similar.

Our interest in pursuing this research is twofold.

Firstly, we are attempting to establish the slope of the linear relationship originally proposed in references [2]

and [3] with a more complete set of isoelectronic pairs [4]. Secondly, we suggest that deviations from the straight line may be correlated with differences in the ionicity between the Te-X and I-X bonds in a given pair of similar compounds.

This correlation between IS and ionicity may be accidental, but it suggests a more direct method for testing its validity. If IS and QS are measured for groups of Te compounds having similar bonds with mixed ionic-covalent character, the ionic character of the bonds should be correlated with a decrease in QS and an increase in IS. We have studied the similar compounds TeCl, and TeI, which exhibit mixed ionic- covalent character and indeed find such a correlation.

Furthermore, we have found that the relation between IS and QS suggested by these two similar compounds also applies to all except two of the materials which we

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1974628

C6-228 Y. MAHMUD, P. BOOLCHAND, S. S. HANNA AND B. B. TRIPLETT have studied in spite of the different bonding configura-

tions involved.

2. Experimental. - The experimental procedure used in the present measurements is described in detail elsewhere [5]. The Mossbauer coldfinger cryostat is shown schematically in figure 1. An important feature

FIG. 1 .

Schematic drawing of the cold-finger cryostat and detector arrangement.

of this apparatus is the provision for calibrating the drive while simultaneously recording spectra of two absorbers maintained at cryogenic temperatures. The results reported here were taken only at liquid helium temperature with source and absorbers at nearly the

same temperature. Comparison of a 1 2 5 S b ( ~ ~ ) (t,,, = 2.7 years) source, which gives an approxi- mately natural linewidth [5, 61, with a 125m~e(Cu) (t,,, ⁼ 58 days) source, showed that the latter h a s a slightly larger linewidth. The former source was there- fore used in the current studies.

Usually only one new absorber was measured in a given run. This procedure allowed us to take a simulta- neous calibration of the velocity of the 125Sb(C~) source (usually with a 125Te standard absorber 4 mg/cm2 in thickness). In this manner we were able to check for systematic differences between the 1 2 5 S b ( ~ ~ ) source velocity and the velocity at the drive (monitored with a source of 57Co(Pt)

and a natural Fe metal absorber).

3. Results and discussion. - Table I summarizes our IS and QS results for the Te compounds studied so far. The results are generally in good agreement with those previously reported in references [2] and [3].

The magnitudes of the reported IS values for Te compounds lie between 1.0 mm/s for TeCl, and

- 1.24 mm/s for H6Te06. These IS values are in agreement with the behavior expected from the che- mical bonding in the compounds if 6 < R2 > is posi- tive. In TeCl,, for example, the Te-C1 bonds are largely ionic, hence the 2 electrons in the 5s shell contribute substantially to the charge density at the nuclear site, and a correspondingly large positive isomer shift is

Absorber

TeCl, TeC1, TeBr, TeI, TeO, K2Te0, K2Te04

NH4HTeO42H2O H6Te0,

ZnTe DyTe ("1

CoTe (") FeTe (") trigonal Te amorphous Te

Teo.02Seo ,,,

T e ~ . ~ 1 4 s ~ . 9 9 6

Absorber thickness (mg/cm2)

-

3.2 2.9 2.5 2.1 5.7 2 3.2 3 4 4 2.8 3.5 4.3 4

Source

This work

-

-

Ref. [9]

Ref. [5]

Ref. [9]

Ref. [9]

(") These materials are known to be magnetically ordered. It is likely that they all show unresolved magnetic

hyperfine splittings. For FeTe the splitting is large enough to produce a pronounced distortion from a single line shape, but the full six line spectrum is unresolved. We have fit the data with a two equal intensity, equal linewidth routine. The resultant splitting is shown in brackets in the QS column.

(b)

The IS and Q S values for amorphous Te are taken from the extrapolation of values for amorphous

Ge,Te,-, to x = 0. See P. Bollchand, B. B. Triplett, S. S. Hanna and J. P. de Neufville [5].

MOSSBAUER EFFECT STUDIES OF Te COMPOUNDS C6-229

observed. On the other hand in ZnTe where Te exists nearly in an s2 p6 configuration, the smaller IS results from the shielding of the 5s' electron density by the 5p6 one. For purposes of IS comparison, ZnTe may be taken as a standard. The negative value of the IS found for K2Te0, can then be understood as resulting from the removal of one 5s electron. In this host Te is known to be tetrahedrally coordinated with sp3 bonding. It is understood here that removal of one 5s electron entails a decrease in the electron density at Te which is more than can be compensated by the shielding of three 5p electrons. This view appears to be further supported by the most negative IS reported for Te compounds, that [3] for TeF,. The ideal assumption of a 5s0 p0 configuration implies removal of two 5s and six 5p electrons which gives a net decrease of electron density with respect to ZnTe. These qualitative IS considera- tions already exist on a firm foundation for the case of

171.

In figure 2 the isomer shifts from this work and from references [2] and [3] are plotted against isomer shifts for similar I compounds [2, 41. Points which are

FIG. 2. - Isomer shifts for Te compounds relative to 125Sb(c)

+ I 0 - + I

-

0 0 -

-

-

c" - 0 5 -

plotted against shifts for similar I compounds relativeto Znl29Te [2, 41. Circles : present work, squares : reference 121, triangles

reference [31. Solid symbols indicate pairs of iso- electronicions, and the straight line is a description of these points.

The underlining of H,jTeOs and ZnTe indicates that they were used as sources relative to a standard absorber in an 1291 experi- ment. The configurations indicated on the right ordinate are meant to be guides to the approximate nature of the bonding

in the Te compounds.

T ~ C I ~

46"

Teo2

5 -

re f/q

K2TeCa

&/$ -ibil,S

Z n l e

/. ¹

based on pairs of isoelectronic ions of Te and I are indicated with solid symbols. The slope of the straight line through these points is expected to reflect the ratio of 6 < R~ >/< R~ > for lZ5Te and 1291 where 6 < R2 >/< R2 > is the fractional change in the mean squared nuclear radius between the excited state and ground state of the nucleus. The straight line in figure 2 has the expected positive slope but a magnitude approximately 10 % smaller than the line on a similar plot in reference [3]. The remaining points in figure 2 are based on pairs of compounds which are generally

neither isoelectronic nor isostructural but only similar in valence or coordination.

I t is seen that the points for the TeCl, : 12Cl, and TeO, : KIO, pairs lie above the line. In view of the previous considerations and further data to be pre- sented below, it is tempting to correlate these deviations with the fact that the bonding in the Te compounds is more ionic than in the 12C16 and 10; structures. The validity of the slope of the line in figure 2 is most strongly supported by the H6Te0, : H6Te0, and ZnTe : ZnTe pairs which were measured by use of

~ ~ ~ ~ " T e 0 , and ZnlZ9"Te sources relative to a stan- dard absorber in an 12'1 experiment [4].

Figure 3 shows isomer shifts plotted against quadru- pole splittings for the compounds we have measured as

FIG. 3.

Isomer shifts ( I S ) plotted against quadrupole split- tings (QS) observed for Te compounds and Te dissolved in trigonal Te, amorphous Te, trigonal Se, and orthorhombic S.

Open circles are TeC12 and TeI2 which are expected to differ in the degree of bonding ionicity. Solid circles are for Te in nearly linear covalently bonded chains. The straight line is a least

squares fit to the open and solid circles.

15-

well as previously measured points for trigonal Te metal, amorphous Te, Te in trigonal Se, and Te in orthorhombic S. Except for the unusual compound K2Te03, the points all lie close to a straight line and suggest that an extrapolation to zero quadrupole shift could provide a calibration of the pure 5s2 configura- tion [8]. The data for the dihalides are consistent with the known bond angles. For TeBr, this angle is about 980 [9], not far from the 900 value characteristic of pure p orbital bonding. Since only a small amount of s character participates in the bonding, this compound is expected to have a large isomer shift. The compound TeBr, does indeed have the largest isomer shift of the dihalides measured. Both TeCl, and TeI, have a nearly linear structural unit with X-Te-X bond angles in excess of 1500 [9]. This large angle suggests sp hybridization which gives a larger quadrupole splitting (from the single p orbital participating in the bonding) and a smaller isomer shift (since on 5s electron is now parti- cipating in the bonding). These considerations are only

- -

-

-

Te IN CHAINS

- -

d

- - -

- -

-

0 5 -

- -

-

C6-230 Y. MAHMUD, P. BOOLCHAND, S. S. HANNA AND B. B. TRIPLETT

meant to be qualitative, but they are consistent with the more extensive study done on Sn compounds by Lees and Flinn [8]. For TeCl, and TeI, where the bonding is similar, the increased electronegativity of the C1- ion when compared with I - is expected to produce a more ionic bond and consequently a smaller quadrupole splitting and larger isomer shift as observed experimentally (Table I).

Also shown in figure 3 are points taken from previous work [5, 101. These points are for Te metal, amorphous Te, Te dissolved in trigonal Se, and Te dissolved in orthorhombic S. In these cases Te occurs in a chain and the X-Te-X bond angle is expected to be about 1050 (except perhaps for Te in S). It is apparent that the corfelation observed for linear dihalides extends also to other linearly bonded Te structures. These structures exist in the form of covalently bonded chains and little ionic character is expected to be present. The variation in quadrupole splitting has been attributed to overlap electronic contributions which increase as the covalent bond length shortens [lo]. It is indeed surprising and possibly fortuitous that two such correlations between quadrupole splitting and isomer shift can be repre- sented by a single linear relationship. In this spirit, we have also plotted our remaining quadrupole split

compounds on figure 3. Only the unusual ionic compound K,TeO, fails to fall near the straight line.

4. Conclusion. - We have presented data here which support the original contentions [2, 31 that it is possible to determine the ratios of 6 < R2 >/< R2 >

for the nuclei of Te and I by comparisons of isomer shifts for strictly isoelectronic compounds. However, the procedure is shown to be susceptible to shifts produced by differences in the electronegativity of the bonds of the Te and I atoms. We have started to study these effects and have attempted to correlate isomer shifts and quadrupole splittings with the degree of ionicity in a bond for similarly bonded compounds (TeCI, and TeI,). We have also noticed that a plot of isomer shift relative to quadrupole splitting yields a single straight line for almost all of the compounds studied which have a significant quadrupole splitting.

This unusually high degree of correlation between a group of compounds having different bonding confi- gurations is not yet understood.

Acknowledgments. - The authors would like to gratefully acknowledge the assistance of N. S. Dixon in these experiments.

References

[I] GREENWOOD, N. N. and GIBB, T. C., in Mossbauer Spectro- scopy (Chapman and Hall Ltd., London), 1971.

[2] JUNG, P. and TRIFTSHAUSER, W., Phys. Rev. 175 (1968) 512.

[3] RUBY, S. L. and SHENOY, G. K., Phys. Rev. 186 (1969) 326.

[4] PASTERNAK, M., Symposia Faraday Soc., 1967, No. 1, 119.

[5] BOOLCHAND, P., TRIPL~TT, B. B., HANNA, S. S. and DENEUF- VILLE, J. P., in Mossbauer Effect Methodology, Vol. 9, edited by Gruverman, I. (Plenum Press, N. Y.-London) 1974.

[6] BOOLCHAND, P., NUCI. Instrum. Meth. 114 (1974) 159.

[7] BUKHSPAN, S., GOLDSTEI~, C. and SONNINO, T., J. Chem.

Phys. 49 (1968) 5477.

[8] LEES, J. K. and FLINN, P. A., J. Chem. Phys. 48 (1968) 882.

[9] H ~ ~ C K E L , W., in Structural Chemistry of Inorganic Com- pounds, Vol. 1 (Elsevier Publishing Company, Inc., N. Y.-Amsterdam-London-Brussels), 1950.

[lo] BOOLCHAND, P., HENEBERGER, T. and OBERSCHMIDT, J., Phys.

Rev. Lett. 30 (1973) 1292.