Novel Functional Oxide Materials

Abstract

Transition-metal oxides show lots of interesting and useful properties. The wide variety of their crystal structures gives rise to various electronic structures, which lead to various chemical and physical properties. The author has been interested in such transition-metal oxides and is seeking new materials with novel functional properties. In this review article new functional properties found in A-site-ordered quadruple perovskite structure oxides with the unusually high-valence cations are highlighted. Negative thermal expansion was found in NdCu₃Fe₄O₁₂ at the intersite-charge-transfer transition temperature near room temperature. The property is useful for developing materials to compensate the normal positive thermal expansion. Significant latent heat was also found to be provided by the intersite-charge-transfer transition in NdCu₃Fe₄O₁₂. The large latent heat is considered to be related with unusual first-order magnetic entropy change induced by the charge transition. The large entropy change can be utilized for thermal control through a caloric effect, which can make effective energy systems for thermal energy storage and refrigeration.