Phase diagram description of the CaCu3Fe4O12 double perovskite

Abstract

CaCu3Fe4O12 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 - δ) + + and Fe (4 + δ) + +, respectively, δ being a measure of the Fe-charge disproportionation. We include all 3d-Fe orbitals: t 2 g localized orbitals, with spin 3/2 and magnetically coupled, plus two degenerate itinerant eg orbitals with local and nearest-neighbor (NN) electron correlations, and hopping between NN eg orbitals of the same symmetry. Allub and Alascio [J. Phys.: Condens. Matter 24(49), 495601 (2012)] previously proposed a model to describe the phase transition in LaCu3Fe4O12 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 CaCu3Fe4O12, modified to account for this difference between the two compounds, the density of states of the itinerant Fe orbitals was obtained, using Green´s function methods. The phase diagram of CaCu3Fe4O12 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 eg orbitals. Both kinds of phases may exhibit paramagnetism and ferromagnetism. We determined the model parameters which best describe the phase transition observed in CaCu3Fe4O12 and found other phases in different parameter ranges, which might be relevant for other compounds of the ACu3Fe4O12 family, presenting Fe-charge disproportionation and intersite charge transfer Fe-Cu.