Some observations on the electronic structure of β-UD3

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Some observations on the electronic structure of β -UD3

J. Ward, L. Cox, J. Smith, G. Stewart, J. Wood

To cite this version:

J. Ward, L. Cox, J. Smith, G. Stewart, J. Wood. Some observations on the electronic structure of

β-UD3. Journal de Physique Colloques, 1979, 40 (C4), pp.C4-15-C4-17. �10.1051/jphyscol:1979403�.

�jpa-00218799�

JOURNAL DE PHYSIQUE Colloque C4, suppldment au no 4 , Tome 40, avril 1979, page C4-15

Some observations on the electronic structure of P-UD, ^(*)

J. W. Ward, L. E. Cox, J. L. Smith, G. R. Stewart and J. H. Wood University of California, Los Alamos Scientific Laboratory, Los Alamos, NM 87545, U.S.A.

R6sum6. - Le magnttisme, la rtsistivitt Blectrique, la chaleur sptcifique de basses temptratures et I'ESCA ont Ctt CtudiBs sur des Cchantillons de UD, ayant des densitts proches de la valeur thtorique. La stechiomttrie, la structure et les propriCtts Clectroniques sont discuttes dans un modkle d e mttal pseudo de transition, avec B la fois un comportement localist et itinerant pour les tlectrons f.

Abstract. - Magnetic, electrical resistivity, low-temperature specific heat and ESCA valence band studies have been performed on near-theoretical density UDa samples. The stoichiometry, structure and electronic properties ate discussed in terms of a pseudo-transition metal model, with both localized and itinerant f-electron behaviour.

1. Introduction.

-

Uranium and protactinium hy- dride are unique among metal hydrides, being at the same time stoichiometric compounds, yet possess- ing high electrical conductivity and at the same time a considerable magnetic moment. These compounds crystallize in the A-15 (P-tungsten) structure, and bear no apparent relationship to either the thorium hydrides or the hydrides of neptunium and beyond.

Although a great deal [1] has been published on the properties of UH, and UD,, no attempt has hereto- fore been made to describe these unique materials in terms of electronic structure and possible f-electron effects.

We describe here magnetic, electrical and heat capacity measurements, as well as the first XPS studies on the uranium-hydrogen system. Near- theoretical density solid samples of UD, were made by reacting uranium with deuterium at 207 MPa (2 110 kg/cm2) pressure and 900 "C. The resultant solid hydride pieces were dark grey, brittle and metallic-appearing, with no apparent tendency to either powder or oxidize. Metallographic examina- tion revealed a small void population, and bright inclusions and stringers in the matrix. These were identified by X-ray diffraction as UO, estimated at 6-8 % ; oxygen analysis confirmed this estimate.

2. Experimental results. - Magnetic susceptibili- ty measurements tended to support earlier [2-51 val- ues that were done on powdered samples. Data followed the Curie-Weiss law, and the Curie temper- ature was found to be : Curie-Weiss (166 K), rema- nence (168 K), M2 US. H / M (163 K), and resistivity

-

see below (166K). The paramagnetic moment was 2.26 2 0.11 pB, with an ordered moment (not saturated) at 2.4 K, H = 53.4 kOe, of 0.87 p,.

(*) Work performed under the auspices of the U.S. Depart- ment of Energy.

The electrical resistivity was measured with a conventional 4-terminal AC bridge method, using spring-loaded gold contacts. The data are plotted in figure 1 (left ordinate and lower abscissa). The Curie temperature of 166 K agrees well with the magnetic studies. In general, the resistivity data are typical of a ferromagnetic transition metal like Fe or Co, but with a considerably greater resistance.

Fig. 1. - Electrical resistivity p - ; legends left and lower scales. Heat capacity C

- - -

this work, and

- - -

Abraham et al. ; legends right and upper scales.

A 74 mg sample of UD, was used for the specific heat measurements. The small-sample calorimeter has been fully described elsewhere [6-71. A 27 mg high-purity Ge sample was used as a standard ; an absolute accuracy of better than k 3 % can be expected for the sample size of UD, used. The data are also plotted in figure 1 (right ordinate and upper abscissa), along with earlier values reported by Abraham et al. [8] ; the upswing of their data at low

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979403

C4-16 J. W. WARD, L. E. COX, J. L. SMITH, G. R. STEWART AND J. H. WOOD

temperatures is due to desorption of He gas [9]. Our data fit the equation C = yT

+

^PT3

+

STS, where y = 33.9 mJ/mole-KZ,

P

= 0.201 mJ/mole-K4, and S = 0.000 30 mJ/mole-K6. From P we can derive a Debye temperature 0, = 338 t 10 K. From the large electronic specific heat coefficient ( y ) we calculate the electronic density of states at the Fermi level t o be 7.20 states/eV/atom.

Valence-band and core-level XPS spectra were measured on a Physical Electronics ESCA-SAM instrument. The specimen fracture stage was used, so a fresh surface was exposed at better than 10-'Otorr vacuum, and studied immediately. The calibration of the instrument was checked against gold 4f spectra, before, during and after each run ; UO, and clean uranium spectra were also taken, which compare nicely with the literature [lo, 111.

Valence band spectra for UD, are shown in figure 2.

A broad concentration of electronic charge occurs between about 8 eV and the Fermi level, overlapping the f-level(s), and a well-defined 5f peak is seen at 2 e V (as compared to 1 eV for UO,). The whole spectrum looks very much like that of a transition metal with d < 5, if one imagines the U 5f electrons playing the part of transition metal d's.

ELECTRON VOLTS Fig. 2.

-

Valence-band XPS spectra for 0-UD,.

Examination of the U 4f doublet shows the deute- ride to appear at 379.1-389.8, as compared to 380.3- 391.1 for UO, and 377.0-387.7 for clean uranium.

The peaks are clearly defined, along with the adja- cent satellite structure. Note that the shifts of all peaks (4f, 6p, Sf) are about 2 eV, relative to the metal ; i.e., the Fermi level has been raised by this amount.

3. Discussion. - A combined overview of the data presented show UD, to be a metallic substance with

a large density of states at the Fermi level. Magnetic properties indicate possible localized f-character, but the moment is not large enough for either U'4 (3.62 p,) or U', (3.68 p,) ; we have not seriously considered transition-metal-like band magnetism, because of the sharpness of the 5f peak in the ESCA spectrum. The valence-band structure of the hy- dride, for an electropositive metal like uranium, can be considered in terms defined by Switendick [12-141 as the formation of hydrogen-derived levels below the Fermi level, to which a certain amount of metal d-f charge is donated ; i.e., the hydrogens are so- mewhat hydridic but also obviously metallic. The number of metal and hydrogen electrons that must be accommodated in bands of metallic character makes the observed rise in the Fermi level quite reasonable. Switendick [I51 has recently performed some calculations on UH,, and finds several broad hydrogen-derived levels actually spanning the f- levelts), with strong evidence of both metal-metal and metal hydrogen f-overlap. This is in excellent agreement with the ESCA picture, and implies that there are both localized and itinerant f-electrons present. One can speculate that exchange interac- tions considered by Chan [l6] might be partially involved, the non-bonding t,, orbitals accounting for most of the magnetic properties.

Relativistic Wigner-Seitz band calculations for the f wave functions show that considerable f- broadening should be expected for the short U-U (3.2

A)

bond in the structure. The unusual A-15 structure can be understood in terms of these strong bonds that link together the face atoms through infinite chains in the x, y and z directions ; the formation of the hydride can be viewed as the simple expansion of the a-uranium lattice in the z- direction, the '001 planes remaining virtually un- changed. There are two kinds of U-atoms present, as required by the A-15 structure. One can postulate that the metallic character resides mostly in the type I atoms that show the strong bonding, whereas the type I1 atoms can account for most of the magnetic moment. In addition, the type I1 atoms would act as scattering centres, thus raising the resistivity.

In conclusion, we view UD, (or UH,) as a pseudo transition-metal-like intermetallic compound of defi- nite composition, possessing both itinerant (bonding) and localized (magnetic) f-electron beha- viour

.

Acknowledgments.

-

Many thanks are due V. 0.

Struebing for magnetic measurements, Dr. R. B.

Roof for X-ray results, Dr. E. G. Zukas for metallo- graphy, and Dr. D. T. Cromer for illuminating dis- cussions on bonding and structure.

SOME OBSERVATIONS ON THE ELECTRONIC STRUCTURE OF P U D 3

References [I] Metal Hydrides, W. M . Mueller, J. P. Blackledge & G. G.

Libowitz eds. (Academic Press, New York, N.Y.) 1%8 ; Chapter 11.

[2] GRUEN, D. M., J. Chem. Phys. 23 (1955) 1708.

[3] LIN, S. T. & KAUFMAN, A. R., Phys. Rev. 102 (1956) 640.

[4] HENRY, W. E., Phys. Rev. 109 (1958) 1976.

[5] KARHEVSKII, A. I. & BURYAK, E. M., Zh. Eksp. Teor. Fiz. 42 (1962) 375.

[6] STEWART, G. R., Cryogenics 18 (1978) 120.

[7] STEWART, G. R. & GIORGI, A. L., Phys. Rev. B 17 (1978) 3534.

181 ABRAHAM, B. M., OSBORNE, D. W., FLOTOW, H. E. & MAR- CUS, R. B., JACS 82 (1960) 1064.

[9] FLOTOW, H. E. & OSBORNE, D. W., Phys. Rev. 164 (1967) 755.

[lo] VEAL, B. W. & LAM, D. J., Phys. Rev. B 10 (1974) 4902.

Ell] NAEGELE, J. R., MANES, L. & CHAUVET, G., (1977) to be published.

[12] SWITENDICK, A. C., Solid State Commun: 8 (1970) 1463.

[I31 SWITENDICK, A. C., Int. J. Quantum Chem. 5 (1971) 459.

[I41 SWTENDICK, A. C., J. Less-Common. 'Met. 49 (1976) 283.

[IS] SWITENDICK, A. C., private communication.

[16] CHAN, S., R o c . 2nd Int. Conf. Electronic Structure of Actinides, J. Mulak, W. Suski & R. Troc eds.

(Wroclaw, Poland, Ossolineum) 1976, p. 327.