Crystal field splitting in correlated systems with the negative charge-transfer gap

TL;DR

This paper investigates the unique crystal field splitting behavior in transition metal compounds with negative charge-transfer gaps, combining theoretical analysis with ab-initio calculations on Cs$_2$Au$_2$Cl$_6$ to reveal covalency effects dominate Coulomb interactions.

Contribution

It provides a theoretical framework for understanding crystal field splitting in systems with negative charge-transfer gaps and confirms predictions through ab-initio calculations on a specific compound.

Findings

In negative charge-transfer gap systems, covalency influences level ordering more than Coulomb interactions.

Ab-initio calculations show $e_g$ levels are below $t_{2g}$ levels in Cs$_2$Au$_2$Cl$_6$.

Conduction bands are mainly ligand $p$-states with minor $d$-state admixture.

Abstract

Special features of the crystal field splitting of $d-$levels in the transition metal compounds with the small or negative charge-transfer gap $\Delta_{CT}$ are considered. We show that in this case the Coulomb term and the covalent contribution to the $t_{2g} - e_g$ splitting have different signs. In order to check the theoretical predictions we carried out the ab-initio band structure calculations for Cs$_2$Au$_2$Cl$_6$, in which the charge-transfer gap is negative, so that the $d-$electrons predominantly occupy low-lying bonding states. For these states the $e_g$-levels lie below $t_{2g}$ ones, which demonstrates that at least in this case the influence of the $p-d$ covalency on the total value of the crystal field splitting is stronger than the Coulomb interaction (which would lead to the opposite level order). We also show that the states in conduction band are made predominantly of $p-$states of ligands (Cl), with small admixture of $d-$states of Au.