Abstract
CaCu3Fe4O12 exhibits a temperature-induced transition from a ferrimagnetic-insulating phase, in which Fe appears charge disproportionated, as Fe3+ and Fe5+, to a paramagnetic-metallic phase at temperatures above 210 K, with Fe4+ 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: t2g 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.
Highlights
Double perovskites of the ACu3Fe4O12 (A: rare earth) family possess a variety of electronic and magnetic properties of interest for technological applications, e.g. as a high-activity compound for the oxygen evolution reaction that occurs during the oxidation of water, which is very important in the energy conversion reaction for metalair rechargeable batteries[1], or to achieve precise control of thermal expansion coefficients[2,3,4]In 2007, Xiang et al.[5] studied the electronic and magnetic properties of the CaCu3Fe4O12 (CCFO) double perovskite by the use of density functional theory (DFT), and they predicted that this oxide is a ferrimagnetic and half-metallic compound
In Appendix A we discuss results obtained for the model in the case G < U0 where phases exhibiting orbital selectivity appear, i.e. L phases, homogeneous in charge but with orbital disproportionation (ν = 0) or selective occupation of the eg orbitals, which might be of relevance for other compounds of the double perovskite family ACu3Fe4O12 (A: rare earths)
We propose the first microscopic effective model which describes the phase transition experimentally observed in CaCu3Fe4O12, between the charge disproportionated ferromagnetic phase (D-FiM) and the homogeneous paramagnetic phase (H-PM)
Summary
Double perovskites of the ACu3Fe4O12 (A: rare earth) family possess a variety of electronic and magnetic properties of interest for technological applications, e.g. as a high-activity compound for the oxygen evolution reaction that occurs during the oxidation of water, which is very important in the energy conversion reaction for metalair rechargeable batteries[1], or to achieve precise control of thermal expansion coefficients[2,3,4]. In 2007, Xiang et al.[5] studied the electronic and magnetic properties of the CaCu3Fe4O12 (CCFO) double perovskite by the use of density functional theory (DFT), and they predicted that this oxide is a ferrimagnetic and half-metallic compound. At the high-temperature phase they obtained a homogeneous valence (Fe4+) and an orbital degenerate half-metallic behavior. Orbital ordering, charge ordering, or disproportionation on Fe sites occur in the low temperature phase, leading to the insulating character
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