X-ray absorption spectra and electronic structure of Haucke-phase-type intermetallic compounds

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X-ray absorption spectra and electronic structure of Haucke-phase-type intermetallic compounds

P. Sarode, A. Chetal, C. Mande

To cite this version:

P. Sarode, A. Chetal, C. Mande. X-ray absorption spectra and electronic structure of Haucke- phase-type intermetallic compounds. Journal de Physique Colloques, 1979, 40 (C5), pp.C5-86-C5-87.

�10.1051/jphyscol:1979533�. �jpa-00218950�

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JOURNAL DE PHYSIQUE Colloque C5, supplément au n° 5, Tome 40, Mai 1979, page C5-86

X-ray absorption spectra and electronic structure of Haucke-phase-type intermetallic compounds

P. R. Sarode (*), A. R. Chetal and C. Mande

Physics Department, Nagpur University, Nagpur, India (*) SSCU, Indian Institute of Science, Bangalore, India

Résumé. — Les spectres d'absorption K du cobalt dans le métal pur et dans les composés du type RCo5 (où R est une terre rare ou yttrium) ont été observés et mesurés. On trouve dans le cas des composés que les disconti- nuités principales se déplacent vers les petites énergies par rapport à celle du métal pur ; ceci indique un transfert d'électrons R vers Co. Une courbe tracée entre les déplacements chimiques et les électrons covalents impliqués dans la liaison (calculé en utilisant un formalisme des rayons métalliques d'après Pauling) suggère une corré- lation. Cette courbe est employée pour déterminer des électrons covalents du cobalt dans TmCo5 et YCOs ; des informations utiles concernant la liaison chimique dans ces composés ont été déduites.

Abstract. — The K X-ray absorption discontinuity of cobalt in the compounds of the RCo5 family (R = rare earth or yttrium) has been measured with a bent crystal spectrograph. It is observed that the main discontinuity shifts to the low energy side, with respect to that in pure cobalt indicating a flow of charge from R to Co. The plot between the observed shifts and the number of covalently bonded electrons (calculated using a semi-empirical metallic radii formalism due to Pauling) suggests a definite correlation. This plot is then used to obtain the number of covalently bonded electrons of cobalt in TmCo5 and YCo5 for which the self-consistent radii data is not available. Useful conclusions are obtained about chemical bonding in these two compounds.

1. Introduction. — The effect of chemical combi- nation on the position of the X-ray absorption discontinuities has been the subject of a large number of investigations in recent years. The range of factors affecting the X-ray absorption shifts (e.g. interatomic distances, effective charges, covalencies, coordination numbers, etc.) being so large no one theory seems to be able to account for all the experimental data.

However, we may attempt interpretation of shifts with some degree of confidence in simpler cases where it is reasonable to assume that certain factors are preponderant. We have undertaken this study of X-ray absorption shifts of cobalt in RCo5 (R = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Y, Tm) compounds to observe if any correlation exists between nature of chemical bonding and the X-ray absorption shifts.

2. Experimental procedure. — A bent (mica) crystal spectrograph of 400 mm diameter was used to photo- graph the absorption spectra. The X-ray tube had a tungsten target which was employed as the source of radiation. The wavelengths of the absorption discontinuities were measured on microphotometer traces in each case, as usual at the inflection points.

Further details of the experimental technique are described elsewhere [1].

3. Results. — We observe that in case of RCo5

compounds the Co K-absorption discontinuity shifts towards the low energy side with respect to its position in the pure metal. The chemical shifts observed, though apparently of the same order, are found on careful measurements to decrease in the sequence LaCo⁵, CeCo⁵, PrCo⁵, NdCo⁵, SmCo⁵, GdCo⁵, YCo5, TbCo5, DyCo5, HoCo5, ErCo5, TmCo5.

4. Discussion. — It has been shown [2] that a negative chemical shift in X-ray spectroscopy is characteristic of an electron acceptor (anion). This suggests that in RCo5 compounds cobalt behaves like a kind of anion and that electron transfer takes place from rare earth to cobalt atoms. The electrons that are transferred from R to Co are, perhaps, the outer, loosely bound 5d, 6s valence electrons which go over to the partially filled 3d-band of cobalt.

That electron transfer from R to Co is reasonable since cobalt is more electronegative [3]. The work of Cuthill et al. [4] on XPS study of hard magnets PrCo5, SmCo5 and similar compounds of CaCu5 type structure, and of Buschow and Velge [5] on magneto- volume effects in rare earth-transition metal inter- metallics also suggests similar electron transfer.

A measure of correlation if any between X-ray absorption shifts and chemical binding would be

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979533

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X-RAY ABSORPTION SPECTRA AND ELECTRONIC STRUCTURE C5-87

the number of covalently bonded electrons of cobalt, Vi, in the compounds. We calculate this number using Carter's chemical bond model [6] based on the earlier semi-empirical metallic radii formalism due to Pauling [3].

We plot in figure 1 the magnitudes of the chemical shifts

I

AE and AVi, the difference between the pure metal yi (= 6 ) for cobalt and the corresponding

CHANGE IN COVALENT ELECTRONS, AVi

-

Fig. 1. -Plot showing the dependence of the chemical shift

I AE ( on the change in covalent electrons AVi.

in the compound. All the points seem to lie more or less on a smooth curve which passes through the origin. The figure shows that the magnitude of the shift decreases as A Vi decreases (that is, as Vi increases).

We see, thus, that the covalent character of the R-Co bond increases from LaCo, through ErCo, and that there is indeed a correlation between the chemical shift and covalently bonded electrons.

The RCo5 compounds studied being of CaCu,

type structure [7], would have two crystallographi- cally non-equivalent Co sites (viz. trigonal and ico- sahedral) and consequently two values for Vi (or A Vi) in each case. We plot of the mean in each case since the method employed by us would highlight average effects only. Vi values for Co in TmCo, and YCo, estimated from the curve in figure 1 were found to be 5.35 and 5.05 respectively. Carter [6] does not give these values for want of data on d-character of the bonding orbitals and single-bond radii of Tm and Y in these compounds. However, a rough esti- mation of covalent electrons of cobalt in TmCo, made by extrapolating Vi versus the rare earth atomic number curve of Carter gave Vi = 5.45, in fair agreement with our experimentally determined value.

yi value for Co in YCo5 lies between Vi for GdCo, and TbCo,. This is understandable since Pauling metallic radii [3] for the three rare-earths are in the order Gd > Y > Tb.

The Vi values for YCo, and TmCo, suggest that the rare earth-cobalt bonds are more covalent in the latter. The covalent character of Y-Co bond being intermediate between that of Gd-Co and Tb-Co bonds we infer that in RCo, family, covalent character is maximum for Tm-Co bonds.

X-ray absorption spectra determine the difference between the energies of initial core level and the final outer unoccupied electron state. In our case these are Co 1s level and p or any hybrid state with p- contribution respectively. XPS data on the former being unavailable for RCo, compounds, we assume that these shifts are negligible for our group of compounds on the basis of earlier results of Cuthill et al. [4] who observed null shifts for Co 3s core levels in PrCo, and SmCo,. Thus, the chemical shifts of the X-ray absorption discontinuity could be ascribed to the shifts of the outer electron levels hone. The plot in figure 1, thus, represents the dis- tribution of the unoccupied electron states as a function of change of covalence in RCo, compounds.

References

[I] SARODE, P. R. and CHETAL, A. R., J. Phys. C 10 (1977) 153, and [5] BUSCHOW, K. H. J. and VELGE, W. A. J., J. Appl. Phys. 39

J. Phys. Soc. Japan 40 (1976) 1637. (1968) 1717.

[2] DAS GUPTA, K., Technical Report No. 16 (U.S. Atomic Energy [6] CARTER, F. L., NBS Spec. Publ. No. 323 (1971) 385 and Proc.

Commission Contract No. AT (04-3)-221 and Office of 9th Rare Earth Res. Conf. Blacksburgh, Virginia, Il Naval Research, Contract No. Nonr-220(30), June 1963). (1971) 617.

[3] PAULING, L., The Nature of the Chemical Bond, Third Edition [7] LEMAIRB, R., Cobalt 32 (1966) 132.

(Cornell University Press, Ithaca) 1960.

[4] CUTHILL, J. R., MCALISTER, A. J., ERICKSON, N. E. and WATSON, R. E., AZP Conf. Proc. No. 18 (1974) 1039 and Phys.

Rev. 12 (1975) 5335.