Metal-Insulator Transition and Local-Moment Collapse in Negative Charge Transfer CaFeO3 under Pressure

We compute the electronic structure, spin and charge state of Fe ions, and the structural phase stability of paramagnetic CaFeO3 under pressure using a fully self-consistent in charge density DFT+ dynamical mean-field theory method. We show that at ambient pressure CaFeO3 is a negative charge transfer insulator characterized by strong localization of the Fe 3d electrons. It crystallizes in the monoclinic P21/n crystal structure with a cooperative breathing mode distortion of the lattice. While the Fe 3d Wannier occupations and local moments are consistent with robust charge disproportionation of Fe ions in the insulating P21/n phase, the physical charge density difference around the structurally distinct Fe A and Fe B ions with the "contracted"and "expanded"oxygen octahedra, respectively, is rather weak, of ∼0.04. This implies the importance of the Fe 3d and O 2p negative charge transfer and supports the formation of a bond-disproportionated state characterized by the Fe A 3d5-δL̲2-δ and Fe B 3d5 valence configurations with δ≪1, in agreement with strong hybridization between the Fe 3d and O 2p states. This complex interplay between electronic correlations, strong covalency, and lattice effects, resulting in bond disproportionation, is in many ways reminiscent of the behavior of rare-earth nickelates, RNiO3 (R=rare earth). Upon compression, CaFeO3 undergoes the metal-to-insulator phase transition (MIT) which is accompanied by a structural transformation into the orthorhombic Pbnm phase. The phase transition is accompanied by suppression of the cooperative breathing mode distortion of the lattice and, hence, results in the melting of bond disproportionation of the Fe ions. Our analysis suggests that the MIT transition is associated with orbital-dependent delocalization of the Fe 3d electrons and leads to a remarkable collapse of the local magnetic moments. Our results imply the crucial importance of the interplay of electronic correlations and structural effects to explain the properties of CaFeO3. © 2022 American Physical Society. We acknowledge support by the Russian Foundation for Basic Research (Project No. 20-42-660027). The theoretical analysis of the electronic structure and DFT calculations were supported by the state assignment of Minobrnauki of Russia (theme “Electron” No. AAAA-A18-118020190098-5).