Structural, thermal and electrical properties of Pr0.5Sr0.5Co1-yNiyO3-δ perovskite-type oxides

Perovskite oxides in the system Pr0.5Sr0.5Co1-yNiyO3-delta (0<y<1) were 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 substituted for Co. a tetragonal phase is formed, the concentration of which increases linearly in the range 0<y<0.6. Above y = 0.6 the tetragonal is the only phase present. The lattice parameters of all compositions were determined at room temperature. Ni is introduced in the bivalent state 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 dilatometer 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 high 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 steeper thermal expansion curves at high temperatures. The TEC generally decreases with increasing Ni content, due to the increase of the size of the unit cell. Pr0.5Sr0.5CoO3-delta and Pr0.5Sr0.5NiO3-delta end members show semi-metallic behavior, while the substitutionally-mixed compositions are semi conductors and exhibit a metal-insulator (M-I) transition when the temperature is raised. The conductivity in the semiconducting region can be described by the small polaron hopping conductivity model. The introduction of Ni2+ cations in the Co-rich compositions is preferably electronically compensated by the formation of V-O(..). (C) 1998 Elsevier Science B.V. All rights reserved.