Real-Space Observation of Ligand Hole State in Cubic Perovskite SrFeO$_3$

TL;DR

This study visualizes the spatial distribution of ligand holes in cubic SrFeO$_3$, revealing their role in its valence state and metallic properties through advanced X-ray diffraction and electron density analysis.

Contribution

It provides the first real-space observation of ligand hole distribution in a transition-metal oxide using high-energy X-ray diffraction and electron density analysis.

Findings

Ligand holes are formed by Fe 3d and O 2p orbital hybridization.

The ligand holes contribute significantly to the anomalous valence state.

The system exhibits metallic behavior without Jahn-Teller distortions.

Abstract

An anomalously high valence state sometimes shows up in transition-metal oxide compounds. In such systems, holes tend to occupy mainly the ligand $p$ orbitals, giving rise to interesting physical properties such as superconductivity in cuprates and rich magnetic phases in ferrates. However, no one has ever observed the distribution of ligand holes in real space. Here, we report a successful observation of the spatial distribution of valence electrons in cubic perovskite SrFeO$_3$ by high-energy X-ray diffraction experiments and precise electron density analysis using a core differential Fourier synthesis method. A real-space picture of ligand holes formed by the orbital hybridization of Fe 3$d$ and O 2$p$ is revealed. The anomalous valence state in Fe is attributed to the considerable contribution of the ligand hole, which is related to the metallic nature and the absence of Jahn-Teller distortions in this system.