Computer modelling of point defects in ABO(3) perovskites and MgO

Abstract

We present results for basic intrinsic defects: F-type electron centers (0 vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO3 perovskite, respectively. We demonstrate that a considerable covalency of the perovskite chemical bonding makes the F-type centers therein much more similar to defects in partly-covalent quartz-type oxides rather than the conventional F centers in alkali halides and ionic MgO. Both one-site (atomic) and two-site (molecular) polarons are expected to coexist in KNbO3 characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO3, KTaO3, BaTiO3, and PbTiO3 crystals. The self-trapped electron is mostly localized on B-type ion due to a combination of breathing and Jahn-Teller modes of nearest six oxygen ion displacements. The relevant lattice relaxation energies are typically 0.2-0.3 eV, whereas the optical absorption energies 0.7-0.8 eV, respectively. According to our calculations, the absorption energy of a bound electron polaron in KNbO3 by 0.1 eV exceeds that for the self-trapped electron polaron and equals 0.88 eV. (C) 2004 Elsevier B.V. All rights reserved.