Structure and Properties of Ba 6 Ni 25 S 27

Ba 6 Ni 25 S 27 , synthesized by solid‐state reaction of BaS, NiS, and Ni at 675°C, is cubic ( Pm 3 m, a = 10.057(1) Å). The structure was refined by using powder X‐ray diffraction and Rietveld methods with R p = 6.28%, R wp = 8.13%, χ 2 = 3.120. The structure, isotypic with Ba 6 Co 25 S 27 , consists of an extended network of Ni 8 S 14 pseudo‐cube clusters, NiS 6 octahedra, and Ba 6 S octahedra. Extended Hückel calculations of this compound indicate the band structure near the Fermi level to be composed mostly of d‐character orbitals of tetrahedral Ni, and reveal a peak in the density of states slightly below the Fermi level. The temperature dependence of the electrical resistivity exhibits a local maximum with thermal hysteresis at around 230 K. Above and below this transition, the slope of the resistivity is positive, with a room‐temperature resistivity (ρ = 0.23 mωcm) in the range for that of a poor metal conductor. Magnetic susceptibility measurements from 4 to 900 K indicate a sharp change in slope at around 225 K, with a steadily increasing susceptibility up to 900 K. The susceptibility below 225 K suggests Pauli paramagnetic behavior. Low‐temperature powder X‐ray diffraction and subsequent structure refinement shows that the lattice parameter, a , undergoes a slight change in slope at temperatures of around 235 K.