Theory of Valence Transition in BiNiO_{3}

Area of Science:

• Materials Science
• Condensed Matter Physics
• Solid-State Chemistry

Background:

• Bismuth Nickelate (BiNiO3) exhibits significant negative thermal expansion.
• Valence transitions and charge transfer are key phenomena in perovskite oxides.

Purpose of the Study:

• To theoretically investigate the valence transition and charge transfer in BiNiO3.
• To understand the mechanisms behind negative thermal expansion in this material.

Main Methods:

• Development of an effective model for Bi-6s and Ni-3d orbitals.
• Inclusion of Bi cation valence skipping.
• Investigation using mean-field approximation for phase diagrams.

Main Results:

• Valence transition arises from commensurate locking of electron filling, linked to charge and magnetic ordering.
• Critical temperature and transition nature depend on Bi/Ni energy levels and Bi electron-electron interactions.
• The model successfully explains temperature- and pressure-driven transitions in BiNiO3.

Conclusions:

• The study provides a theoretical framework for understanding valence transitions in BiNiO3.
• The findings offer insights into the systematic variation of valence states in related perovskite oxides.
• This work contributes to the understanding of materials with negative thermal expansion properties.