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

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.