# Unusual Mott transition associated with charge-order melting in BiNiO$_3$ under pressure

## TL;DR

This study reveals a pressure-induced Mott insulator-to-metal transition in BiNiO$_3$, driven by structural changes and charge transfer, with the Ni valence state remaining constant across the transition.

## Contribution

It demonstrates the link between structural phase change and electronic transition in BiNiO$_3$, highlighting the melting of charge disproportionation as a key mechanism.

## Key findings

- BiNiO$_3$ undergoes a Mott transition at ~4.8% volume compression.
- The transition involves a change from triclinic to orthorhombic structure.
- Ni valence remains as Ni$^{2+}$ across the transition.

## Abstract

We study the electronic structure, magnetic state, and phase stability of paramagnetic BiNiO$_3$ near a pressure-induced Mott insulator-to-metal transition (MIT) by employing a combination of density functional and dynamical mean-field theory. We obtain that BiNiO$_3$ exhibits an anomalous negative-charge-transfer insulating state, characterized by charge disproportionation of the Bi $6s$ states, with Ni$^{2+}$ ions. Upon a compression of the lattice volume by $\sim$4.8\%, BiNiO$_3$ is found to make a Mott MIT, accompanied by the change of crystal structure from triclinic $P\bar{1}$ to orthorhombic $Pbnm$. The pressure-induced MIT is associated with the melting of charge disproportionation of the Bi ions, caused by a charge transfer between the Bi $6s$ and O $2p$ states. The Ni sites remain to be Ni$^{2+}$ across the MIT, which is incompatible with the valence-skipping Ni$^{2+}$/Ni$^{3+}$ model. Our results suggest that the pressure-induced change of the crystal structure drives the MIT in BiNiO$_3$.