Large scale computer modelling of point defects in ABO₃ perovskites

We present results for basic intrinsic defects: F-type electron centers, free and bound electron and hole polarons in ABO₃ perovskites. Both one-site (atomic) and two-site (molecular) hole polarons are expected to coexist, characterized by close absorption energies. Shell Model (SM) and intermediate neglect of differential overlap (INDO) calculations of the F center diffusion indicate that the relevant activation energy is quite low, ca. 0.8 eV. Further INDO calculations support the existence of self-trapped electron polarons in PbTiO₃, BaTiO₃, KNbO₃, and KTaO₃ crystals. The relevant lattice relaxation energies are typically 0.2 eV, whereas the optical absorption energies are around 0.8 eV. We also used the INDO method for modelling the atomic and electronic structure of KNb_xTa_1-xO₃ (KTN) perovskite solid solutions. The Nb impurities in KTaO₃ reveal off-center displacements starting with the smallest calculated concentrations. The calculated magnitude of the displacement is close to the EXAFS observation. Using the calculated energy gain for several concentrations of Nb in KTN, we construct the Ginzburg-Landau-type functional for the excess energy. The coefficients of this functional are concentration-dependent. This dependence allows us to define the type of concentration-induced phase transition in KTaO₃ alloyed by Nb.