Complex vs Band Formation in Perovskite Oxides

Abstract

It is argued that there is a critical cation-anion covalent mixing parameter λc such that ligand-field theory is appropriate for λ<λc, but band theory must be used for λ>λc. This provides, therefore, a criterion for distinguishing metallic vs magnetic compounds in those structures, like perovskite, where cation-cation interactions are negligible. It is also argued that λσ>λc can be anticipated where the cations are in a low-spin state. The fact that LaNiO3 contains low-spin NiIII and exhibits no Jahn-Teller distortion suggested that λσ>λc in this compound. Metallic conductivity from −200° to 300°C and Pauli paramagnetism from 4° to 300°K seem to confirm this suggestion. Where λ≈λc, there is the possibility of a phase change in which λ<λc in some directions, λ>λc in others. LaCoO3 seems to illustrate this situation. It undergoes a transition at 1210°K, the cobalt ordering into alternate (111) planes of high-spin Co3+ and planes containing low-spin CoIII. Below 400°K the latter planes contain only CoIII ions. The magnetic Co3+ ions couple antiferromagnetically via Co3+-``diamagnetic CoIIIO6 complex''-Co3+ superexchange to give TN≈90°K.