Calculations of Surface Structure for SrTiO 3 Perovskite
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Calculations of Surface Structure for SrTiO3 Perovskite
E. Heifetsa, R.I. Eglitisb, E.A. Kotominb,c, and G. Borstelb Carnegie Institution of Washington, 5251 Broad Branch Rd., N.W. Washington D.C. 20015 and California Institute of Technology, MS 252-21, Pasadena CA 91125 b Department of Physics, University of Osnabrueck, D-49069 Osnabrueck, Germany c Institute for Solid State Physics, University of Latvia, 8 Kengaraga str.,Riga LV-1063, Latvia a
ABSTRACT We present and discuss results of the calculations for SrTiO3 (100) surface relaxation with different terminations (SrO and TiO2) using a semi-empirical shell model (SM) as well as ab initio methods based on Hartree-Fock (HF) and Density Functional Theory (DFT) formalisms. Using the SM, the positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized. This permits us determination of surface rumpling and surfaceinduced dipole moments (polarization) for different terminations. We also compare results of the ab initio calculations based on both HF with the DFT-type electron correlation corrections, several DFT with different exchange-correlation functionals, and hybrid exchange techniques. Our SM results for the (100) surfaces are in a good agreement with both our ab initio calculations and LEED experiments. INTRODUCTION Thin films of ABO3 perovskite ferroelectrics 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 structure of the SrTiO3 (100) surface 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].) 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 [1117] and shell model (SM) [18-20] studies were published for the (100) surface of isostructural BaTiO3 and SrTiO3 crystals. We perform here much more detailed SM Studies for SrTiO3 with different surface terminations, supported by ab initio calculations. METHOD In the present study we have studied a two-dimensional slab of cubic SrTiO3 by means of the SM [21] as realized in the MARVIN computer code [22]. To study the surface relaxation, we have optimized the atomic positions in several (varied from one to 16) near-surface planes, placed into the electrostatic field of the slab (simulated by 20 additional planes whose atoms
O9.1.1
were fixed in their perfect lattice sites). The number of these additional planes was chosen to reach a convergence of the crystalline field in the surface plane
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