Ligand hole driven metal-insulator transition in a prototypical transition metal double perovskite oxide Ca$_2$FeMnO$_6$

TL;DR

This study reveals that ligand hole localization causes charge disproportionation and Jahn-Teller distortion, leading to a metal-insulator transition in Ca$_2$FeMnO$_6$, with the band-gap controlled by charge-transfer energy rather than Mott-Hubbard interactions.

Contribution

It demonstrates that the metal-insulator transition in CFMO is driven by ligand hole localization and charge-transfer energy, providing new insights into transition mechanisms in perovskites.

Findings

Charge disproportionation causes Jahn-Teller distortion.

Band-gap is controlled by charge-transfer energy.

MIT can be modulated by composition, pressure, or stoichiometry.

Abstract

Ca$_2$FeMnO$_6$ (CFMO) double perovskite was studied using first principles density functional theory and tight-binding (TB) Hamiltonian modeling using extended Hubbard model. We have shown by electronic structure analysis that charge- and magnetic-ordering are driven by charge disproportionation at low temperature caused by partial localization of O-$2p$ ligand holes at alternate Fe sites that creates Jahn-Teller distortion, which leads to metal to insulator transition (MIT) in CFMO. Our results suggests MIT was triggered by negative charge-transfer energy of self-hole doping, responsible for symmetry lowering transitions. Notably, the band-gap was found to fundamentally controlled by the strength of the charge-transfer energy, and not by the Mott-Hubbard interactions, which can be modeled by composition, pressure or stoichiometry modulations. The fundamental insights presented in this work will help understand similar physics and mechanisms in other class of perovskites and correlated metals.