Computer modelling of point defects in ABO 3 perovskites and MgO

Abstract

We present results for basic intrinsic defects: F-type electron centers (O vacancy which trapped one or two electrons) and hole polarons bound to Mg or K vacancy in ionic MgO and partly covalent KNbO 3 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 KNbO 3 characterized by close absorption energies. Our calculations confirm existence of the self-trapped electron polarons in KNbO 3, KTaO 3, BaTiO 3, and PbTiO 3 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 KNbO 3 by 0.1 eV exceeds that for the self-trapped electron polaron and equals 0.88 eV.