STRUCTURE AND PROPERTIES OF BA6NI25S27

Ba6Ni25S27, synthesized by solid-state reaction of BaS, NiS, and Ni at 675 degrees C, is cubic (Pm (3) over bar m, a = 10.057(1) Angstrom). The structure was refined by using powder X-ray diffraction and Rietve ld methods with R-p = 6.28%, R-wp = 8.13%, chi (2) = 3.120. The struct ure, isotypic with Ba6Co25S (27), consists of an extended network of N i8S14 pseudo-cube clusters, NiS, octahedra, and Ba6S octahedra. Extend ed Huckel calculations of this compound indicate the band structure ne ar the Fermi level to be composed mostly of d-character orbitals of te trahedral Ni, and reveal a peak in the density of states slightly belo w the Fermi level. The temperature dependence of the electrical resist ivity exhibits a local maximum with thermal hysteresis at around 230 K . Above and below this transition, the slope of the resistivity is pos itive, with a room-temperature resistivity (rho = 0.23 m Omega cm) in the range-Tor that of a poor metal conductor. Magnetic susceptibility measurements from 4 to 900 K indicate a sharp change in slope at aroun d 225 K, with a steadily increasing susceptibility up to 900 K. The su sceptibility below 225 K suggests Pauli paramagnetic behavior. Low-tem perature powder X-ray diffraction and subsequent structure refinement shows that the lattice parameter, a, undergoes a slight change in slop e at temperatures of around 235 K.