Colossal Negative Thermal Expansion Induced by Valence Skipping Elements―Stagehand into Main Player―

Abstract

Motivated by the colossal negative thermal expansion recently found in BiNiO₃ , 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 BiNiO₃ and the systematic variation of valence states for a series of Bi and Pb perovskite oxides. We also report recent spectroscopic experimental results in the Bi and Pb oxides.