Phase transformations of CuCrS₂: Structural and chemical study

Abstract

The structure and composition of the CuCrS₂ powder synthesized by sulfidation of a mixture of oxides Cu₂O:Cr₂O₃ = 1:1 at 850°C and cooled to room temperature at a rate of 60°C/min were studied by X-ray powder diffraction and differentiating solution. A rhombohedral CuCrS₂ phase (space group R3m) was found, which was stoichiometric in composition and had disordering in the copper sublattice because copper was arranged at the tetrahedral and octahedral sites with occupancy 10% at the latter. The structure of CuCrS₂, in which the octahedra were occupied by copper atoms at room temperature, was found for the first time; in known structures, the copper atoms occupied only the tetrahedral sites, while the probability of octahedral occupation appeared around 400°C (order-disorder transition). The partially disordered CuCrS₂ phase is intermediate on the route to complete ordering. The quickly cooled CuCrS₂ powder is unstable; after the second heating to 500°C with prolonged annealing at 390°C→180°C→80°C→25°C, its transition to the stable state was accompanied by liberation of 2–4 wt.% Cu₉S₅. The real composition of ternary sulfide after isolation of the Cu₉S₅ phase is discussed using the data of the structural method, differential dissolution, and magnetic measurements.