Electronic structure of the α and δ phases of Bi2 O3: A combined ab initio and x-ray spectroscopy study

α- Bi2 O3 is the thermodynamically stable phase of Bi2 O3 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 O-K shell x-ray absorption and emission spectroscopies. We examine the nature of bonding in α- Bi2 O3 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 δ- Bi2 O3 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 δ phase, contrary to previous force field calculations and electrostatic arguments which favor the 111 alignment.