Negative charge transfer energy in correlated compounds

TL;DR

This paper reviews the concept of negative charge transfer energy in correlated compounds, highlighting its implications for covalence, self-doping, and material properties, especially in perovskite oxides and battery materials.

Contribution

It provides a comprehensive review of negative charge transfer energy, emphasizing analysis tools like plots and diagrams, and connects this concept to various functional materials.

Findings

Negative charge transfer energy leads to high covalence in compounds.

Self-doping of holes occurs due to negative charge transfer energy.

The review emphasizes analysis tools like diagrams for understanding charge transfer.

Abstract

In correlated compounds containing cations in high formal oxidation states (assigned by assuming that anions attain full valence shells), the energy of ligand to cation charge transfer can become small or even negative. This yields compounds with a high degree of covalence and can lead to a self-doping of holes into the ligand states of the valence band. Such compounds are of particular topical interest, as highly studied perovskite oxides containing trivalent nickel or tetravalent iron are negative charge transfer systems, as are nickel-containing lithium ion battery cathode materials. In this report, we review the topic of negative charge transfer energy, with an emphasis on plots and diagrams as analysis tools, in the spirit of the celebrated Tanabe-Sugano diagrams which are the focus of this Special Topics Issue.