Structural study of [formula omitted] and [formula omitted] perovskites

Abstract

The crystal structure at room temperature, and the thermal expansion and electrical conductivity, from room temperature up to 800°C, of the perovskite-type oxides in the system Pr1−xSrxCoO3−δ (x=0, 0.15, 0.3, 0.4, 0.5) were studied. All compounds have the orthorhombic perovskite GdFeO3-type structure (Pbnm space group). The lattice parameters were determined by X-ray powder diffraction. The introduction of Sr2+ into the lattice is compensated by the oxidation of Co3+ to form Co4+ (holes) for low x, and at reduced temperatures on higher x values, or by the formation of oxygen vacancies as x increases and at high temperatures. Above x=0.4 oxygen vacancies are formed even at room temperature. The thermal expansion is almost linear for x≥0.15. The thermal expansion coefficient (TEC), determined in the temperature range from room temperature to 500°C, decreases with x, but shows an increase above x=0.4. The electrical conductivity increases with x, but when x≥0.4 it decreases. The conductivity of the undoped sample in the examined temperature range, and that of x=0.15 up to 500°C is p-type semiconducting, and can be described by the small polaron hopping mechanism. Semi-metallic behavior was observed for x=0.15 above 500°C and for x≥0.3 in the whole examined temperature range. The metal–insulator (M–I) transition at room temperature occurs at approximately x=0.25.