Large-scale computer modelling of point defects, polarons and perovskite solid solutions

We review results of our recent computer simulations of point defects, electron and hole polarons in ABO₃ perovskite crystals, focusing mostly on KNbO₃ and KTaO₃ as representative examples. We have calculated the atomic and electronic structure of defects, their optical absorption and defect-induced electron density redistribution. The majority of results is obtained using the quantum chemical method of the intermediate neglect of differential overlap (INDO) based on the Hartree-Fock formalism. The main findings are compared with results of ab initio Density Functional Theory (DFT) first-principles calculations. The migration energies for the O vacancy in KNbO₃ perovskite obtained using the classical shell model (SM) and INDO method are close (0.68 eV and 0.79 eV, respectively) and also agree with the experimental estimate of ≅1 eV. We used the INDO method also for the modelling of the atomic and electronic structure of KNb_xTa _1-xO₃ (KTN) perovskite solid solutions. Nb impurities in KTaO₃ reveal clear off-center displacement starting with the smallest calculated concentrations. The magnitude of the Nb off-center displacement is close to the EXAFS observation (0.27 a.u.). In contrast, Ta impurities in KNbO₃ always remain on-center, due to higher ionicity of Ta compared to Nb. Using the calculated energy gain due to off-center displacement of Nb atoms for several concentrations in KTN, we exploit the non-empirical Ginzburg-Landau-type functional for the excess energy with concentration- dependent coefficients. This allows us to define the type of the concentration-induced phase transition in KTaO₃ doped with Nb.