Structure, thermal expansion and electrical conductivity of Nb-substituted LaCoO3

Abstract

The effect of Nb-substitution in LaCoO3 has been examined by evaluating the electrical conductivity, thermal expansion and crystal structure of the solid solutions LaCo1−xNbxO3 (x=0 to 1/3). It was demonstrated that Nb-substitution in LaCoO3 was mainly compensated by reduction of Co3+ to Co2+, and that oxidation of Co2+ could destabilise the solid solution. The ambient crystal structure was shown to transform from rhombohedral R3¯c perovskite (x=0) to orthorhombic Pbnm (x=0.15, 0.20) and finally to B-site ordered perovskite P21/n (x=0.25, 1/3) perovskite with increasing Nb-substitution. The thermal expansion of LaCo1−xNbxO3 was shown to be strongly depressed with increasing Nb-content, and significantly lower thermal expansion was observed for LaCo2/3Nb1/3O3 relative to LaCoO3. The electrical conductivity was reduced with increasing Nb-content, and semi-conducting properties was demonstrated for LaCo1−xNbxO3 in contrast to the metallic behaviour of pure LaCoO3 at elevated temperature. The thermal expansion, electrical conductivity and the stability of the materials were discussed with emphasis on Co/Nb ordering, the oxidation state and spin transitions of Co.