Charge/orbital disordered states with smaller volume and higher entropy in transition-metal oxides

Abstract

Some transition-metal oxides such as Ca₂RuO₄, BiNiO₃, and V₂OPO₄ harbor smaller volume and higher entropy states by role sharing of the spin, charge, and orbital degrees of freedom. Effect of lattice distortions on the various charge/orbital patterns can be analyzed by d-p models with full degeneracy of the transition-metal d and oxygen 2p orbitals. Based on the mean-field analyses on the d-p models for Ca₂RuO₄, BiNiO₃ and V₂OPO₄, possible mechanisms of negative thermal expansion with charge and orbital degrees of freedom are discussed. In Ca₂RuO₄ and BiNiO₃, orbital and/or charge states are rearranged across their insulator-metal transitions, and the metallic phases with orbital and/or charge fluctuations can be stabilized at high temperatures relative to the insulating phases without them. In V₂OPO₄, the charge/orbital disordered state can keep relatively smaller volume due to orbital-dependent hybridization in the face-sharing VO₆ octahedron chain.

Keywords

Transition-metal oxides / charge order / orbital order / spin-orbit interaction / charge disproportionation / oxygen hole

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