Synthesis, crystal structure, and properties of the rhombohedral modification of the thiospinel CuZr_1.86(1)S₄

The rhombohedral modification of the thiospinel, CuZr_1.86(1)S₄, has been synthesized by the reaction of the constituent elements in an alkali metal halide flux and structurally characterized by single crystal X-ray diffraction techniques. The title compound crystallizes in the rhombohedral space group D_{3 d}⁵ - R over(3, -) m (#166, a=7.3552(2) Å, c=35.832(2) Å, V=1678.76(13) ų, Z=12, and R/wR=0.0239/0.0624). The structure is composed of close packed S layers, with a stacking order of ⋯ABCBCABABCACAB⋯·along the c axis. The Zr and Cu atoms occupy the octahedral and tetrahedral holes between S layers, respectively. Three different kinds of S-M-S layers exist in the structure: layer I has fully occupied Zr and Cu sites, layer II has fully occupied Zr sites but no Cu, and layer III has partially occupied Zr and fully occupied Cu sites. Transport and optical properties indicate that the title compound is a small band gap (1.26 eV) n-type semiconductor.