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

Abstract

We review results of our recent computer simulations of point defects, electron and hole polarons in ABO(3) 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 congruent tol eV. We used the INDO method also for the modelling of the atomic and electronic structure of KNbxTa1-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.