Large- Scale ab initio Simulations of Fe-doped SrTiO 3 perovskites
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Large- Scale ab initio Simulations of Fe-doped SrTiO3 Perovskites R.A. Evarestova,b , R.I. Eglitisb , S. Piskunovb, E. A. Kotominb,c and G. Borstelb a Department of Quantum Chemistry, St. Petersburg University, St. Peterhof 198904, Russia b Fachbereich Physik, Universität Osnabrück, D-49069 Osnabrück, Germany c Institute for Solid State Physics, The University of Latvia, 8 Kengaraga str., LV-1063 Riga, Latvia ABSTRACT Using the Unrestricted Hartree-Fock method and supercells containing up to 160 atoms, we calculated the energy level positions in the gap and atomic geometry for the Fe4+ impurity substituting for a host Ti atom in SrTiO3. In agreement with experiment, the high spin (S=2) state is much lower in energy than the zero-spin state. The energy level positions strongly depend on the asymmetric displacement mode of the six nearest O ions which is a combination of the Jahn-Teller and breathing modes. A considerable covalent bonding between the Fe ion and four nearest O ions takes place. INTRODUCTION Properties of transition metal impurities, especially iron, in ABO3 perovskite ferroelectrics are of considerable interest due to their photochromic, photorefractive and other applications [1,2]. There were several theoretical calculations for ion impurities substituting for B atoms in KNbO3 [3,4], SrTiO3 [5,6], BaTiO3 [7] (see more in a recent book [8]). Most of these studies were semi-empirical and/or cluster calculations. The only first-principles calculations were performed recently for Fe in KNbO3 [4] using the linear muffin tin orbital method in the atomic sphere approximation (LMTO-ASA). However, no lattice relaxation around impurity was calculated, and the calculated density of states depends considerably on the parameters of the so-called LDA+U scheme. In this paper, we present and analyze results of the first ab initio calculations with a detailed treatment of lattice relaxation around iron impurity in SrTiO3. The periodic Unrestricted Hartree-Fock (UHF) calculations were performed here for the supercell model of a defective crystal. The convergence of the results with the supercell size increasing is investigated and the convergence is achieved for supercells containing 160 atoms and more than 1600 basis functions. SUPERCELL APPROACH There are two general models widely used in the theory of point defects in crystalline solids which are based on the Born-von Karman periodic boundary conditions (PBC): Supercell Model (SCM) and the Cyclic Cluster Model (CCM) [9,10]. (Note that the third, Quasi-molecular Large Unit Cell model [11] is in fact the embedded CCM model which uses the cyclic cluster embedded into the Madelung field of the surrounding crystal.) The SCM and CCM have both similarities and differences. The similarity is that in both models the basic translation vectors of a direct lattice are used for generating a Large Unit Cell (LUC) in a real space maintaining the point symmetry of the Bravais lattice. The type of the crystalline lattice composed of such LUCs may differ from the host crystalline
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