Electrostatic stability of insulating surfaces: Theory and applications

TL;DR

This paper develops a theoretical framework for understanding the electrostatic stability of insulating surfaces, providing criteria for non-polar terminations and illustrating with calculations on LaAlO3 and SrTiO3 surfaces, including effects of adsorbates.

Contribution

It offers a microscopic justification for ionic-limit arguments and introduces simple criteria for constructing non-polar crystal surfaces, with practical model calculations.

Findings

Higher-index LaAlO3 surfaces are energetically more favorable than (100) surfaces.

External adsorbates like water can significantly alter surface stability.

The theory applies to solids with mixed ionic/covalent bonding.

Abstract

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/covalent character. Based on these arguments, we propose simple criteria to construct arbitrary non-polar 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 (n10) 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).