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

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 R  3¯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.

Substitution of Nb in LaCoO3 increases the unit cell volume, reduces the symmetry of the unit cell and introduces cation ordering. The chemical substitution leads to suppression of the electronic conductivity and reduces the thermal expansion of the lattice.Figure optionsDownload as PowerPoint slideHighlights
► Cation ordering and crystal structure is reported for LaCoxNb1−xO3.
► Substitution of Nb in LaCoO3 is compensated by reduction of Co3+ to Co2+.
► Thermal expansion of LaCoO3 is reduced by Nb-substitution.
► Electrical conductivity of LaCoO3 is reduced by Nb-substitution.