title: Theory of Valence Transition in BiNiO_{3}.
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 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.
authors.affiliation: Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama 351-0198, Japan.|Department of Physics, Tohoku University, Sendai, Miyagi 980-8578, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Naka
authors.affiliation: Center for Emergent Matter Science (CEMS), RIKEN, Wako, Saitama 351-0198, Japan.|Condensed Matter Theory Laboratory, RIKEN, Wako, Saitama 351-0198, Japan.
authors.forename: Hitoshi
authors.initials: H
authors.lastname: Seo
authors.affiliation: Department of Applied Physics, University of Tokyo, Bunkyo, Tokyo 113-8656, Japan.
authors.forename: Yukitoshi
authors.initials: Y
authors.lastname: Motome