Crystal defect structures and phase stability in Ba hexaaluminates

Computer atomistic simulation techniques have been used to investigate the crystal chemistry, defect structures, and phase relationships in Ba hexaaluminates. Equilibrated lattice energies for several structural models, proposed to explain the nonstoichiometries of Ba hexaaluminates, have been calculated and suggest that Ba hexaaluminate in the magnetoplumbite structure is unstable compared to nonstoichiometric beta-alumina type structures. The lack of superstructure in phase I is explained by the very small difference in lattice energies between the structures in which the defect complex of this phase is differently arranged. Our calculations also suggest that the most appropriate structural model for phase II is one which includes both triple Reidinger defects and barium interstitials inside the spinel blocks from the viewpoint of both energy and crystal symmetry. (C) 1996 Academic Press, Inc.