Computer modeling of point defects, polarons, excitons and surfaces in perovskite ferroelectrics

We review results of our recent large-scale computer simulations of point defects, excitons and 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 are 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 (FP-LMTO) first-principles calculations. The results of the electronic structure calculations for different terminations of SrTiO₃ (100) thin films are discussed. These calculations are based on the ab initio Hartree-Fock (HF) method and Density Functional Theory (DFT). Results are compared with previous ab initio plane-wave LDA and classical Shell Model (SM) calculations. Calculated considerable increase of the Ti-O chemical bond nearby the surface is confirmed by experimental data.