Thermodynamics of Concomitant Charge-Transfer, Antiferromagnetic, and Isostructural Phase Transitions in Barocaloric ACu3Fe4O12 Quadruple Perovskites.

We develop a minimal Landau theory of the concomitant intersite charge-transfer, antiferromagnetic, and isostructural phase transitions in the barocaloric quadruple perovskites ACu 3 Fe 4 O 12 (A = La, Pr, Nd, Sm, Eu, Gd, Tb). A non-symmetry-breaking order parameter for the Cu-Fe intersite transfer is taken as primary, with the Fe 3 + staggered magnetization and the volume strain enslaved to the charge transfer through magnetoelastic and charge-strain couplings. The volume and entropy discontinuities, the thermal expansivities and bulk moduli of the two phases, and the slope of the pressure-temperature phase boundary are thereby tied to one another through the model couplings. The model yields a pressure-independent transition entropy, so the latent heat is proportional to the transition temperature, accounting for its near-constancy under A-site substitution. Applied to NdCu 3 Fe 4 O 12 , the theory reproduces the charge, magnetic, and structural responses across the transition from parameters fixed by experimental data, and resolves the transition entropy into comparable charge and magnetic contributions by anchoring the magnetic sector on the intrinsic ordering scale of the charge-transferred state. The barocaloric effect is inverse, with a peak isothermal entropy change at 0.5 GPa of ≃ 73 J K - 1 kg - 1 , comparable to the quasi-direct value of 65 J K - 1 kg - 1 . The theory makes explicit the intrinsic thermal expansion of each phase, a contribution omitted in the entropy construction underlying the quasi-direct determination.