Ab-initio study of the polar SrTiO 3 (1 1 0) (1 1) surfaces Franc ßois Bottin * , Fabio Finocchi, Claudine Noguera Groupe de Physique des Solides, Universit ees Paris 6 et Paris 7, UMR CNRS 7588, 2 Place Jussieu, Paris Cedex 5 75251, France Abstract Various (1 1) terminations of the SrTiO 3 (1 1 0) polar surface are considered and their electronic and structural properties computed through first-principles calculations. The relative stability of terminations that differ by stoichi- ometry is compared by taking into account the influence of the chemical environment. In contrast with the metallic stoichiometric SrTiO termination, the nonstoichiometric ones are insulating and their stability comparable to t the SrTiO 3 (1 0 0) surfaces. We thus argue that, among the (1 1) unreconstructed faces, some nonstoichiometr orientations may be obtained in suitable O-poor environments. Ó 2003 Elsevier Science B.V. All rights reserved. Keywords: Semi-empirical models and model calculations; Density functional calculations; Surface structure, morphology, rou and topography; Surface energy 1. Introduction Among the perovskites compounds, the surfaces of SrTiO 3 have recently attracted a large interest. Indeed, they can be used as a substrate for growing high-T c cuprate superconductors. They also show an aptitude to water photo-electrolysis. Recently, the possibility ofobtaining ferroelectric surface phases was discussed [1]. While the SrTiO 3 (1 0 0) face has been widely studied [2], only in the last few years the SrTiO 3 (1 1 0)surface has been the objectof experimental and theoretical investiga- tions.Their relative scarcity is connected to the polar characterof the surface:along the [1 1 0] direction,planes of either SrTiO or O 2 composi- tion alternate, with a net dipole moment per unit cell.Therefore,a dipole moment that is propor- tional to the number of bulk layers appears, which impliesa divergentsurface energy. In order to overcome this instability, modifications of the net chargedistribution atthe outermostlayersare needed, which can be achieved by anomalous fill- ing of surface states, or through nonstoichiometric reconstructions, or by adsorbing charged species [3].As a consequence, polar orientations are in generaless stable than nonpolar ones, and show an enhanced reactivity, which makes their prepa- ration and experimental study rather delicate. For the same reasons, they often represent a challenge for theory,which should be able to treat those phenomena on equal footing.In particular,STM and conductivity measurements [4,5] show that the SrTiO 3 (1 1 0) surface displays various reconstruc- tions and that it can be characterized as insulating or metallic,according to the annealing tempera- ture or the actual exposure to oxygen. As a first step towardsthe understanding of such a complex scenario, we prosecute a previous * Corresponding author. Tel.: +33-1-44274611; fax: +33-1- 435428778. E-mail address: fbottin@gps.jussieu.fr (F. Bottin). 0039-6028/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0039-6028(03)00227-9 Surface Science 532–535 (2003) 468–471 www.elsevier.com/locate/susc