Normal and Inverse Barocaloric Effects in A-Site-Ordered Quadruple Perovskite RCu ₃ Cr ₄ O ₁₂ (R = La and Bi)

Solid barocaloric materials are promising for next-generation cooling applications. To allow flexible heat management, the development of both normal and inverse barocaloric materials, which, respectively, show exothermic and endothermic responses to the application of pressure, is important. Here, we demonstrate a normal barocaloric effect in LaCu 3 Cr 4 O 12 and an inverse barocaloric effect in BiCu 3 Cr 4 O 12, both of which exhibit contrasting phase transitions, intersite charge transfer, and charge disproportionation to relieve the electronic instability of the unusual Cr 3.75+ states. We have found that the Clausius–Clapeyron equation explains the behaviors of the normal/inverse barocaloric effects in first-order phase transitions. Positive thermal expansion-like volume change due to the charge transfer transition in LaCu 3 Cr 4 O 12 leads to the normal barocaloric effects, while negative thermal expansion-like change in BiCu 3 Cr 4 O 12 leads to the inverse barocaloric effect. The sign and magnitude of the extensive variables (e.g., volume) determine the direction and magnitude of the shift in the phase transition temperature, respectively, when an external field is applied. Chemical substitution at the A-site of the quadruple perovskite structure leads to a completely different response of extensive variables, providing heat flow in opposite directions.