Novel functional properties of charge-transition oxides synthesized under high pressure

Oxides containing unusually high-valence transition-metal ions often exhibit charge transitions to relieve the electronic instabilities. A-site-ordered quadruple perovskites LnCu3Fe4O12 with the unusually high-valence Fe3.75+, which are synthesized under high-pressure conditions, show intermetallic-charge-transfer transitions. In this review article, novel thermo-related functional properties induced by the charge transitions in LnCu3Fe4O12 are highlighted. A large negative-thermal-expansion behavior was observed at the intermetallic-charge-transfer transition temperature. The negative-thermal-expansion property is primarily caused by the size effect of constituent ions by the charge changes. 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 intermetallic-charge-transfer transition in LnCu3Fe4O12. 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.