Electronic structure of theαandδphases ofBi2O3: A combinedab initioand x-ray spectroscopy study

Abstract

$\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is the thermodynamically stable phase of ${\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ at room temperature. We have performed a theoretical and experimental investigation of its electronic structure using a combination of gradient corrected density functional theory (DFT), along with x-ray photoemission and $\mathrm{O}\text{\ensuremath{-}}K$ shell x-ray absorption and emission spectroscopies. We examine the nature of bonding in $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ and in particular explore the nature of the stereochemically active Bi electron lone pair. The Bi $6s$ states are found to be concentrated at the bottom of the valence band but the states contributing to the lone pair on Bi are derived from the top of the valence band. Mixing between O $2p$ and Bi $6s$ states is found to be crucial in producing the asymmetric density on Bi. The role of the lone pair in the fast ion conductor $\ensuremath{\delta}\text{\ensuremath{-}}{\mathrm{Bi}}_{2}{\mathrm{O}}_{3}$ is also investigated, through calculation of the electronic structure with ⟨100⟩, ⟨110⟩, and ⟨111⟩ alignment of oxygen vacancies. Alignment of the vacancies along ⟨100⟩ results in the most energetically favorable configuration of the $\ensuremath{\delta}$ phase, contrary to previous force field calculations and electrostatic arguments which favor the ⟨111⟩ alignment.