Theory of Valence Transition in BiNiO$_3$

TL;DR

This paper presents a theoretical study of the valence transition in BiNiO$_3$, explaining its temperature- and pressure-driven phase changes and related valence states through an effective orbital model.

Contribution

The study introduces a new effective model accounting for valence skipping and charge transfer, elucidating the mechanisms behind valence transitions in BiNiO$_3$.

Findings

Valence transition caused by electron filling locking and charge/magnetic orderings.

Critical temperature and transition nature depend on Bi-Ni energy levels and electron interactions.

Phase diagram explains experimental valence transitions and trends in related oxides.

Abstract

Motivated by the colossal negative thermal expansion recently found in BiNiO$_3$, the valence transition accompanied by the charge transfer between the Bi and Ni sites is theoretically studied. We introduce an effective model for Bi-$6s$ and Ni-$3d$ orbitals with taking into account the valence skipping of Bi cations, and investigate the ground-state and finite-temperature phase diagrams within the mean-field approximation. We find that the valence transition is caused by commensurate locking of the electron filling in each orbital associated with charge and magnetic orderings, and the critical temperature and the nature of the transitions are strongly affected by the relative energy between the Bi and Ni levels and the effective electron-electron interaction in the Bi sites. The obtained phase diagram well explains the temperature- and pressure-driven valence transitions in BiNiO$_3$ and the systematic variation of valence states for a series of Bi and Pb perovskite oxides.