Influence of cobalt substitution on structure and electric conduction of CuCr2Se4

Seleno-spinels with the nominal composition Cu1−xCoxCr2Se4 (for x = 0.1–0.9) and CuCr2−yCoySe4 (for y = 0.1–1.9) were prepared as polycrystalline samples using ceramic method. Change of crystal structure, from cubic to monoclinic symmetry, was observed in the first system with the increase in cobalt amount. In the second system monoclinic phase as well as single-phase CuCr2−yCoySe4 compounds were not obtained. The X-ray analysis was used in order to make phase analysis of the obtained samples to determine their crystal structure and lattice parameters. The scanning electron microscopy test confirmed the chemical composition and shown the variation in local distribution of cations. The porosity of the samples was observed. Electric resistance of the compressed powdered samples was measured at frequency 100 Hz within temperature 100–500 K range. The samples showing the spinel structure, with low Co-substitution, x = 0.1–0.5, exhibit thermally activated conductance with activation energy 90–220 meV above room temperature. The increase in the Co-concentration, x = 0.6–1.0, lowers the activation energy to about 40 meV. These samples show the metallic-type conductivity within intermediate temperature range, 200 ≲ T ≲ 400 K. The small polaron-type conductance, with low value of the activation energy, Ea ≲ 15 meV, was deduced below 300 K for the all Cu1−xCoxCr2Se4 (x = 0.1–0.9) samples. It was found that the electric transport properties correspond both to the chemical composition and to the crystal structure.