Nonstoichiometry and relative stabilities of Y2Ti2O7 polar surfaces: A density functional theory prediction

Density functional theory calculations were carried out to predict the surface structures and relative stabilities of (1 0 0) and (1 1 0) polar surfaces of a complex metal oxide, Y2Ti2O7. Based on a thermodynamic defect model, surface stabilities were evaluated as a direct function of stoichiometry and environmental factors, i.e. oxygen partial pressure and temperature. Calculations show that, as the oxygen partial pressure increases, the most stable termination of the (1 1 0) changes from Y/Ti-rich to O-rich. For the (1 0 0) surfaces, the most stable termination changes from Y/Ti-rich to stoichiometric, and then to O-rich, with increasing oxygen partial pressure. All variants of the (1 1 0) surfaces were found to be more stable than (1 0 0) surfaces. In particular, non-stoichiometric (1 1 0) surfaces are always more stable than their stoichiometric counterparts.