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CALCULATIONS OF PEROVSKITE SURFACE RELAXATION E.Heifetsa,b, E.A.Kotominc,d, R.I.Eglitisc, and R.E.Cohena,b a Carnegie Institution of Washington, Washington , D.C.20015 b Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125 c Fachbereich Physik, Universität Osnabrück, D-49069 Osnabrück, Germany d Institute of Solid State Physics, University of Latvia, 8 Kengaraga, Riga LV-1063, Latvia ABSTRACT The (100) and (110) surface relaxations are calculated for SrTiO3 and BaTiO3 perovskite thin films by means of a semi-empirical shell model (SM) for different surface terminations. Our SM results for the (100) surface structure are in good agreement with our present ab initio HartreeFock calculations with electron correlation corrections, previous ab initio pseudopotential calculations and LEED experiments. The surface energy for the Ba-, Sr-, TiO- terminated (110) surfaces is found much larger than that for the (100) one. In contrast, the surface energy for the asymmetric O-termination, where outermost O atoms are strongly on-plane displaced, is the lowest for all (110) terminations and thus the most stable. INTRODUCTION Perovskite ferroelectric ABO3 thin films are important for many high tech applications including high capacity memory cells, catalysis, optical waveguides, integrated optics applications, substrates for the high Tc cuprate superconductor growth, etc. [1-4] where surface structure and quality are of primary importance. In this paper, we calculate the atomic structure of the SrTiO3 and BaTiO3 (100) and (110) surfaces for the ideal cubic phases. It should be noted that at all temperatures bulk SrTiO3 exhibits paraelectric properties, despite the antiferrodistortive (AFD) transition at 105 K to a tetragonal phase in which the oxygen octahedra have rotated in opposite directions in neighboring unit cells [5]. In contrast, isostructural BaTiO3 undergoes several phase transitions from paraelectric to ferroelectric phases as the temperature decreases. The SrTiO3 (100) surface relaxation has been characterized by means of low energy electron diffraction (LEED), reflection high-energy electron diffraction (RHEED), and medium energy ion scattering (MEIS) measurements [6-10]. Recently, several ab initio [11-17] and shell model (SM) [18-20] studies were published for the (100) surface of BaTiO3 and SrTiO3 crystals. Here we perform much more detailed SM studies for both crystals with different terminations, supported by ab initio calculations. The (110) surface only recently became a subject of intensive experimental investigations, focusing mainly on SrTiO3, and using STM, UPS, XPS, and Auger spectroscopies, and LEED [21]. In this paper we report the first simulations of (011) surfaces of BaTiO3 and SrTiO3 crystals. We performed these simulations using SM. METHOD In the present study we have studied a periodic two-dimensional slab of cubic SrTiO3 and BaTiO3 crystals by means of the SM [22] as realized in the MARVIN computer code [23]. To study the surface relaxation, we have optimized the atomic