Effects of electron-'breathing' mode coupling. A theory of the three phase transitions of LaCoO3

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Abstract
Coupling of d-electrons to totally symmetric A1g 'breathing' modes of octahedral metal-ligand complexes in a magnetic oxide is considered in a many-electron formulation. Both dynamical and static strains are taken into account. The strictly performed canonical displacement transformation yields effective interactions. Thermal averaging over vibrational states leads to a temperature-dependent reduction of some intra-atomic interactions and hopping integrals. From the general many-electron Hamiltonian, an effective localised-electron Hamiltonian is derived for LaCoO3 including high-spin and low-spin states of trivalent cobalt together with charge-transfer states. The thermodynamics of this Hamiltonian is discussed in the molecular field approximation, and phase diagrams are obtained from the Landau expansion. The model accounts for the rather unusual temperature behaviour of antidistortive, high-spin-low-spin and charge orderings in LaCoO3:disorder from 0K up to 648K (continuous transition), then order up to 920K (continuous transition) and again disorder up to 1210K (discontinuous transition to high-temperature ordered phase).