Electrostatic stability of insulating surfaces: Theory and applications

We analyze the electrostatic stability of insulating surfaces in the framework of the bulk modern theory of polarization. We show that heuristic arguments based on a fully ionic limit find formal justification at the microscopic level, even in solids where the bonding has a mixed ionic and covalent character. Based on these arguments, we propose simple criteria to construct arbitrary nonpolar terminations of a given bulk crystal. We illustrate our ideas by performing model calculations of several LaAlO3 and SrTiO3 surfaces. We find in the case of ideal LaAlO3 surfaces that cleavage along a higher-index (01n) direction is energetically favorable compared to the polar (100) orientation. In the presence of external adsorbates or defects, the picture can change dramatically, as we demonstrate in the case of H2O/LaAlO3(100).