Phase diagram description of the CaCu$_3$Fe$_4$O$_{12}$ double perovskite

Abstract

CaCu$_3$Fe$_4$O$_{12}$ exhibits a temperature-induced transition from a ferrimagnetic-insulating phase, in which Fe appears charge disproportionated, as Fe$^{3+}$ and Fe$^{5+}$, to a paramagnetic-metallic phase at temperatures above 210 K, with Fe$^{4+}$ present. To describe it, we propose a microscopic effective model with two interpenetrating sublattices of Fe$^{(4-\delta)+}$ and Fe$^{(4+\delta)+}$, respectively, being $\delta$ the Fe-charge disproportionation. We include all $3d$-Fe orbitals: $t_{2g}$ localized orbitals, with spin 3/2 and magnetically coupled, plus two degenerate itinerant $e_g$ orbitals with local and nearest-neighbor (NN) electron correlations, and hopping between NN $e_g$ orbitals of the same symmetry. Allub and Alascio previously proposed a model to describe the phase transition in LaCu$_3$Fe$_4$O$_{12}$ from a paramagnetic-metal to an antiferromagnetic-insulator, induced by temperature or pressure, involving charge transfer between Fe and Cu ions, in contrast to Fe-charge disproportionation. With the model proposed for CaCu$_3$Fe$_4$O$_{12}$, modified to account for this difference between the two compounds, the density of states of the itinerant Fe orbitals was obtained, using Green's functions methods. The phase diagram for CaCu$_3$Fe$_4$O$_{12}$ was calculated, including phases exhibiting Fe-charge disproportionation, where the two eg orbitals in each site are symmetrically occupied, as well as novel phases exhibiting local orbital selectivity/asymmetric occupation of $e_g$ orbitals. Both kinds of phases may exhibit paramagnetism and ferromagnetism. We determined the model parameters which best describe the phase transition observed in CaCu$_3$Fe$_4$O$_{12}$, and found other phases at different parameter ranges, which might be relevant for other compounds of the ACu$_3$Fe$_4$O$_{12}$ family, which present both types of transitions.