CRYSTAL-STRUCTURE, THERMAL AND ELECTRICAL-PROPERTIES OF PR1-XSRXCOO3-DELTA (X = 0, 0.15, 0.3, 0.4, 0.5) PEROVSKITE OXIDES

The crystal structure at room temperature, and the thermal expansion a nd electrical conductivity, from room temperature up to 800 degrees C, of the perovskite-type oxides in the system Pr(1-x)Sr(x)CoO3-delta (x = 0, 0.15, 0.3, 0.4, 0.5) were studied. All compounds have the orthor hombic perovskite GdFeO3-type structure (Pbnm space group). The lattic e parameters were determined by X-ray powder diffraction. The introduc tion of Sr2+ into the lattice is compensated by the oxidation of Co3to 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 a t high temperatures. Above x = 0.4 oxygen vacancies are formed even at room temperature. The thermal expansion is almost linear for x greate r than or equal to 0.15. The thermal expansion coefficient (TEC), dete rmined in the temperature range from room temperature to 500 degrees C , decreases with x, but shows an increase above x = 0.4. The electrica l conductivity increases with x, but when x greater than or equal to 0 .4 it decreases. The conductivity of the undoped sample in the examine d temperature range, and that of x = 0.15 up to 500 degrees C is p-typ e semiconducting, and can be described by the small polaron hopping me chanism. Semi-metallic behavior was observed for x = 0.15 above 500 de grees C and for x greater than or equal to 0.3 in the whole examined t emperature range. The metal-insulator (M-I) transition at room tempera ture occurs at approximately x = 0.25.