High-temperature modification of Y5Sb3 and its ternary analogue Y5NixSb3-x

The high-temperature modification of Y5Sb3 has been discovered: the Yb5Sb3 structure type, Pnma space group, a=11.867(1), h=9.2247(9) and c=8.0977(8) Angstrom. It has been estimated that the first-order structural transition from the low-temperature form (Mn,Si, type) to the high-temperature form is above 1265 degreesC but below 1400 degreesC. Antimony atoms on one of the two sites can be partially substituted by Ni atoms, which results in a Y5NixSb3-x formula with 0 less than or equal tox<1. The high-temperature structure was refined for a Y5Ni0.38Sb2.62 crystal, obtained by annealing a sample in an induction furnace for 2 h at 1265C. All high-temperature nickel-containing phases Y5NixSb3-x with the ordered Yb5Sb3 structure undergo a decomposition to low-temperature Y5Sb3 (Mn5Si3 type) and a nickel-containing phase, when annealed at 800 degreesC. Extended Huckel tight-binding calculations show that substitution of Sb by Ni is unfavorable with respect to the Y-Sb bonding. The structure of Y5NixSb3-x is stabilized by the configurational entropy, resulting from a statistical mixture of Sb and Ni atoms on one site. Choice of an Sb site to be occupied by Ni is dictated by bonding maximization in the substituted structure. The high-temperature Y5Sb3 and Y5Ni0.38Sb2.62 phases are weak Pauli paramagnets. (C) 2001 Elsevier Science B.V. All rights reserved.