Order in the disordered state: local structural entities in the fast ion conductor Ba2In2O5

The structural features of Ba2In2O5 at high temperatures are discussed based on a thorough study of the full energy hypersurface of a 36 atoms supercell by periodic density functional theory. The results obtained for this cell are furthermore used for considering stacking-sequences and connectivity-patterns present only in larger supercells. The distribution of oxygen vacancies is far from random and relatively few configurations, associated with different arrangements of tetrahedral InO4, square pyramidal InO5 and octahedral InO6 entities, are thermally accessible at most temperatures. Our results call into the question of commonly used defect models for such grossly disordered materials in which oxygen vacancies are distributed at random over a number of lattice sites. The energetic preference for certain structural entities also has important implications for ionic transport due to restraints imposed by local symmetry.