AUTA14S2 - CENTERED (AU,TA)13 ICOSAHEDRA ORGANIZED ACCORDING TO THE MOTIF OF A CUBIC CLOSE PACKING

The tantalum-rich phases of composition AuxTa15-xS2 (0.4 < x < 1.1) we re prepared by arc-melting of appropriate compressed mixtures (Au, Ta, Ta1.35S2) and subsequent annealing of the samples in sealed molydenum crucibles. Brittle crystals with silver lustre were grown in tantalum ampoules at 1 700 K within two days using iodine as a chemical transp ort agent. In contrast to Vegard's rule the lattice parameters of the rhombohedral phases which are isostructural with Pd15P2, shrink with i ncreasing gold content. The structures were determined from Rietveld f its of powder X-ray diffraction spectra and confirmed by a crystal str ucture analysis of a merohedral twinned crystal of Au0.7Ta14.3S2: a = 747.7(2) pm, alpha = 59.84(2)-degrees, R3BAR, Z = 1, 760 reflections ( F2 > 2sigma(F2)), 30 variables, R(F) = 0.048. The parts of the volumin a of the domains with distinct orientations are 0.346(4) and 0.654.Top ologically the structure corresponds to a cubic close packed arrangeme nt of Ta12-icosahedra with all non-tetrahedral interstices being fille d. Gold accumulates preferentially in the centres of the icosahedra. T he remaining metal atoms - two per formula and site - populate the ''o ctahedral sites'' which are encased in stretched polyhedra limited by 32 triangulated faces. The ''tetrahedral sites'' are occupied by the s ulfur atoms which themselves have a ninefold, triangulated tetrakaidec ahedral coordination. The complete occupation of interstices together with the specific orientation of the gold stabilized icosahedral (Au,T a)13-clusters ensure a tetrahedral close packing of all atoms. The dis tortions of the packing are quantitatively analysed in terms of dihedr al angles and deviations of the tetrahedral edge lengths from the mean and are compared with those of other tcp structures.