Synthesis, crystal structures and high-temperature thermoelectric properties of spinel-type cobalt ruthenium oxides

Solid solutions of spinel-type cobalt-ruthenium oxides Co3−xRuxOy (0.50 ≤ x ≤ 0.70, y = 3.8–3.9) were prepared from Co3O4 and RuO2 powders by solid-state reaction at 1173–1273 K in air. The crystal structures of the solid solutions were refined by Rietveld analysis of powder X-ray diffraction (XRD) patterns. The solid solutions crystallized in cubic cells (space group: Fd3¯m) with formal compositions of (Co)tet[Co2−xRux]octOy, where tet refers to the tetrahedral site and oct the octahedral one. The electrical transport property of the solid solutions exhibited semiconducting behavior and positive Seebeck coefficient in the temperature range from 298 to 1073 K. The thermal conductivity increased with increasing temperature. The maximum dimensionless figure of merit of 0.024 was measured for Co2.35Ru0.65O3.86 at 973 K.