Giant Multicaloric Effects in Charge-Transition Oxides

Some transition-metal oxides containing cations with unusually high valence states show charge transitions to relieve the electronic instabilities. Such compounds often release significant latent heat by the first-order charge transitions. We found that the large latent heat and the corresponding isothermal entropy changes can be utilized through caloric effects by applying pressure (barocaloric effects) and/or magnetic fields (magnetocaloric effects). The A-site-ordered quadruple perovskite NdCu₃Fe₄O₁₂ containing the unusual high valence Fe^3.75+ shows the large entropy change of 84.2 J K⁻¹ kg⁻¹ by the intersite charge transfer transition (NdCu²⁺₃Fe^3.75+₄O₁₂ ↔ NdCu³⁺₃Fe³⁺₄O₁₂) near room temperature. This entropy change can be utilized by applying pressure through the barocaloric effect. The analogue quadruple perovskite BiCu₃Cr₄O₁₂ containing Cr^3.75+ also shows the large entropy change by the charge disproportionation transition (BiCu₃Cr^3.75+₄O₁₂ ↔ BiCu₃Cr³⁺Cr⁴⁺₃O₁₂) at 190 K. Because the charge disproportionation yields a ferrimagnetic phase below the transition temperature, the observed entropy change can be controlled by applying magnetic fields as well as pressure. The compound is thus demonstrated to show the multicaloric effects. The charge transitions, where the charge, spins, and lattice degrees of freedom are strongly correlated, are crucial to the observed novel giant caloric properties.