Equilibrium structure of δ-Bi2O3 from first principles

Using ab initio calculations, we have systematically studied the structure of delta-Bi2O3 (fluorite prototype, 25% oxygen vacancies) probing < 100 > and combined < 110 > and < 111 > oxygen vacancy ordering, random distribution of oxygen vacancies with two different statistical descriptions as well as local relaxations. We observe that the combined < 110 > and < 111 > oxygen vacancy ordering is the most stable configuration. Radial distribution functions for these configurations can be classified as discrete (ordered configurations) and continuous (random configurations). This classification can be understood on the basis of local structural relaxations. Up to 28.6% local relaxation of the oxygen sublattice is present in the random configurations, giving rise to continuous distribution functions. The phase stability obtained may be explained with the bonding analysis. Electron lone-pair charges in the predominantly ionic Bi-O matrix may stabilize the combined < 110 > and < 111 > oxygen vacancy ordering.