Multiple Caloric Effects Induced by Phase Transitions of Charge-Spin-Lattice Coupled Transition-Metal Oxides

Caloric effects of solids can provide us with highly efficient and environmentally friendly energy systems. The exploration of novel caloric materials is challenging but crucial for the development of future technologies. Some transition-metal oxides containing cations with unusually high valence states show phase 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 [1]. The analogue quadruple perovskite BiCu₃Cr₄O₁₂ containing Cr^3.75+ also shows the large entropy change by the charge disproportionation phase 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 shown to exhibit the multiple caloric effects [2]. The charge transitions, where the charge, spins, and lattice degrees of freedom are strongly correlated, are crucial for the observed novel multiple large caloric properties [3].<br/>[1] Y. Kosugi, <i>et al.</i>, <i>Adv. Func. Mater.</i> <b>31</b>, 2009476 (2021).<br/>[2] Y. Kosugi, <i>et al.</i>, <i>Sci. Rep.</i> <b>11</b>, 12682 (2021).<br/>[3] Y. Shimakawa and Y. Kosugi, <i>J. Mater. Chem. A</i> <b>11</b>, 12695 (2023).