DECOMPOSITION OF SPINEL-TYPE NICKEL-CHROMIUM INDIUM SULFIDES - X-RAY-POWDER STRUCTURE DETERMINATION

The phase diagram of the quaternary system NiCr2S4-NiIn2S4-Cr2S3-In2S3 has been studied at 873 and 1223 K by X-ray powder structure determin ation of quenched samples. At high temperatures, spinel-type Ni-y(Cr2- 2xIn2x)(1-y)S3-2y solid solutions are formed in a large range of compo sition (y: 0-0.5, x: 0.3-1), At temperatures below 1183 K, these solid solutions decompose to Cr-rich and In-rich spinel-type sulfides, wher eby nickel is enriched in the Cr-rich compounds. The decomposition of the spinel-type sulfides is greatly retarded, even at temperatures abo ut 900 K, due to both thermodynamic and kinetic reasons. The cation di stribution established corresponds to a site preference as Cr3+ > Ni2 > In3+ at the octahedral 16d site and square (vacancies) > In3+ > Ni2 + at the tetrahedral 8a site of the spinel structure. The metal-sulfur distances of the solid solutions refined are compared with those calc ulated from characteristic metal-sulfur distances in close-packed stru ctures, taking into account the site occupation established. For the r espective indium-sulfur distances, the following improved values are r ecommended: In-tet-S = 247.1 pm; In-oct-S = 260.6 pm. (C) 1998 Academi c Press.