Electronic structure of negative charge transfer CaFeO3 across the metal-insulator transition

TL;DR

This study explores the electronic structure changes in CaFeO3 during its metal-insulator transition, emphasizing ligand hole contributions and strain effects, using spectroscopy and density functional theory.

Contribution

It provides detailed element-specific insights into the electronic structure changes, supporting the bond-disproportionation model and highlighting ligand hole roles in CaFeO3's transition.

Findings

Fe valence remains unchanged across transition

Ligand hole density increases by 5-10% in insulating state

Strain suppresses the metal-insulator transition temperature

Abstract

We investigated the metal-insulator transition for epitaxial thin films of the perovskite CaFeO3, a material with a significant oxygen ligand hole contribution to its electronic structure. We find that biaxial tensile and compressive strain suppress the metal-insulator transition temperature. By combining hard X-ray photoelectron spectroscopy, soft X-ray absorption spectroscopy, and density functional calculations, we resolve the element-specific changes to the electronic structure across the metal-insulator transition. We demonstrate that the Fe electron valence undergoes no observable change between the metallic and insulating states, whereas the O electronic configuration undergoes significant changes. This strongly supports the bond-disproportionation model of the metal-insulator transition for CaFeO3 and highlights the importance of ligand holes in its electronic structure. By sensitively measuring the ligand hole density, however, we find that it increases by ~5-10% in the insulating state, which we ascribe to a further localization of electron charge on the Fe sites. These results provide detailed insight into the metal-insulator transition of negative charge transfer compounds and should prove instructive for understanding metal-insulator transitions in other late transition metal compounds such as the nickelates.