Abstract:
Transition-metal oxides show lots of intriguing and useful properties due to wide varieties of their crystal and electronic structures. We are focusing on fundamental solid-state physics and chemistry of such transition-metal oxides and seeking new materials with novel functional properties. In some transition-metal oxides synthesized under extreme conditions, unusually high valence states of cations can be stabilized, and charge, spin, and lattice degrees of freedom are strongly correlated due to strong hybridization of low-lying d orbitals of the transition-metal ions with 2p orbitals of coordinated oxygen ions. Such an oxide often shows a phase transition to relieve its electronic instability, lading to drastic changes in transport, magnetic, and structural properties. A few examples of the interesting phase transition behaviors are presented. LaCu3Fe4O12 shows an intersite-charge-transfer transition of the unusually high valence Fe^(3.75+) ions, accompanying with drastically changes in transport and magnetic properties. Concomitant significant structural change leads to a negative thermal expansion behavior. A similar charge-transfer tansition in NdCu3Fe4O12 is found to give large latent heat near room temperature. Importantly, the large latent heat can be utilized through the caloric effects, which can provide us with highly efficient and environmentally friendly energy systems. Solid-state chemistry of the functional transitionmetal oxids are discussed