STRUCTURAL, THERMAL AND ELECTRICAL-PROPERTIES OF PR0.5SR0.5CO1-YNIYO3-DELTA PEROVSKITE-TYPE OXIDES

Perovskite oxides in the system Pr0.5Sr0.5Co1-yNiyO3-delta (0<y<1) wer e prepared and characterized by X-ray powder diffraction, dilatometry and electrical conductivity measurements. The Pr0.5S0.5CoO3-delta end member has an orthorhombic GdFeO3 structure, while when Ni is substitu ted for Co. a tetragonal phase is formed, the concentration of which i ncreases linearly in the range 0<y<0.6. Above y = 0.6 the tetragonal i s the only phase present. The lattice parameters of all compositions w ere determined at room temperature. Ni is introduced in the bivalent s tate in the range 0<y<0.4, while Ni3+ is introduced in the range 0.4<y <1. An orthorhombic-to-tetragonal phase transition was detected by dil atometer measurements for compositions with y = 0.2, 0.4 and 0.6. The phase transition temperature decreases with increasing Ni content. The linear thermal expansion coefficient (TEC) is always higher for the h igh temperature modification. The formation of oxygen vacancies V-O(.. )) and the concurrent thermal reduction of Co4+, Co3+ and Ni3+ cations to the lower valence states, were considered responsible for the stee per thermal expansion curves at high temperatures. The TEC generally d ecreases with increasing Ni content, due to the increase of the size o f the unit cell. Pr0.5Sr0.5CoO3-delta and Pr0.5Sr0.5NiO3-delta end mem bers show semi-metallic behavior, while the substitutionally-mixed com positions are semi conductors and exhibit a metal-insulator (M-I) tran sition when the temperature is raised. The conductivity in the semicon ducting region can be described by the small polaron hopping conductiv ity model. The introduction of Ni2+ cations in the Co-rich composition s is preferably electronically compensated by the formation of V-O(..) . (C) 1998 Elsevier Science B.V. All rights reserved.