Phase transformations of CuCrS2: Structural and chemical study

The structure and composition of the CuCrS2 powder synthesized by sulfidation of a mixture of oxides Cu2O:Cr2O3 = 1:1 at 850A degrees C and cooled to room temperature at a rate of 60A degrees 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 400A degrees 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 500A degrees C with prolonged annealing at 390A degrees C -> 180A degrees C -> 80A degrees C -> 25A degrees 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.