Acta Cryst. (1990). CA6, 1579-1580 Structure of the Heavy-Fermion Superconductor UBe13* BY M. W. MCELFRESH, J. H. HALL, R. R. RYAN,~ J. L. SMITH AND Z. FISK Los Alamos National Laboratory, University of California, Los Alamos, N M 87545, USA (Received 30 November 1988; accepted 27 September 1989) Abstract. Uranium-beryllium, UBe13, Mr = 355 2, cubic, Fm3c, a = 10-268 (2)/~, V = 1082-5 A 3, Z = 8, Dx = 4 359 g cm -3, A(Mo KaO = 0.70926/~, # = 282.8 cm-1, F(000) = 1066.4, room temperature, final R = 0.022 for 74 observed independent reflec- tions [I > 2tr(/)] out of 937 measured reflections. This is the first single-crystal study of the heavy-fermion superconductor UBe13. The U is surrounded by eight dodecahedral cages each containing one Be(I) at the center, encompassed by Be(2). The U--Be(2) dis- tance is 3.013 A while the Be(1)--Be(2) distance is 2-163 A. Introduction. The discovery of superconductivity in UBel3 and a few other heavy-fermion systems stimu- lated an intensive effort to characterize the nature of both the normal and superconducting states of these materials. In the normal state the electronic specific heat coefficient, y, of UBel3 extrapolates to a T = 0 K value of 1-1 J m o l - 1 K - 2 (Ott, Rudigier, Fisk & Smith, 1983). The specific heat jump of about 1 J mol -~ K -l at the superconducting transition temperature suggests that the f electrons responsible for the heavy masses are also responsible for the superconductivity. Although there have been pre- vious structural studies on polycrystalline UBe]3 (Baenziger & Rundle, 1949; Goldman, Shapiro, Cox, Smith & Fisk, 1985), this work represents the first such study on single-crystal UBeI3. 0.027 from merging equivalent reflections in original data set containing 937 measured reflections, result- ing in 74 observed reflections [ I > 2tr(/)] out of 83 measured independent reflections. Least-squares refinement minimized Z w ( A F ) 2, W = [o'2(F) + 0.030F2] - 1, (~2(F) based on counting statistics. Scale factor, isotropic type-II extinction parameter (Zachariasen, 1967; Larson, 1967), positional param- eters, anisotropic thermal parameters for Be(2), R = 0.022, wR = 0-028, S = 1.715, final (A/o')max = 0 037. Final AF Fourier synthesis - 1-96 __ p _< 1.63 e A-3 The single large peak is 0.6 A from the U atom. Scattering factors f, f ' and f from International Tables for X-ray Crystallography (1974). Calcula- tions on a Cray 1 using the Los Alamos Crystal Structure System developed primarily by A. C. Larson (Larson, 1977). Discussion. Final parameters are listed in Table 1., The numbering scheme is shown in Fig. 1. Bond lengths are given in Table 2. The cell constants are consistent with previous X-ray powder-diffraction measurements on samples of a single crystalline :l: A list of structure factors has been deposited with the British Library Document Supply Centre as Supplementary Publication No. SUP 52688 (2 pp.). Copies may be obtained through The Technical Editor, International Union of Crystallography, 5 Abbey Square, Chester CH1 2HU, England. Experimental. Crystals of UBe13 were obtained by slow cooling from an aluminium flux. Crystal dimen- sions were 0.02 x 0-03 x 0-04 mm. CAD-4 diffrac- tometer, 0-20 scans. 0 scan range (0.8 + 0.34 tan0) °. Scan speed 1.0 to 8-2 ° min-i. Background first and last one-sixth of scan range. Graphite-monochro- mated Mo Ka radiation. Unit cell from 25 reflec- tions, 7 _< 0 _< 23 °. Empirical ~ scan and spherical absorption corrections, transmission = 0.39-0.25. (Sin0/A)max = 0 7023 A.-1. Index range 0 _< h ___ 14, 0 _< k _< 14, 0 _< l <_ 14. Standard reflections 2271, 226, max. r.m.s, variation 1.5% with no trends. Rin t --- * This work was performed under the auspices of the US Department of Energy. ? Author to whom correspondence should be addressed. Be2 Fig. 1. The nearest neighbors of U (irregular snub cube) and Be(l) (irregular icosahedron). © 1990 International Union of Crystallography