Computer modeling of point defects, impurity self-ordering effects and surfaces in advanced perovskite ferroelectrics

The calculated optical properties of basic point defects - F-type centers a nd hole polarons - in KNnO(3) perovskite crystals are used for the interpre tation of available experimental data. The results of quantum chemical calc ulations for perovskite KNbxTa1-xO3 solid solutions are presented for x = 0 , 0.125, 0.25, 0.75, and 1. An analysis of the optimized atomic and electro nic structure clearly demonstrates that several nearest Nb atoms substituti ng for Ta in KTaO3 - unlike Ta impurities in KNbO3 - reveal a self-ordering effect, which probably triggers the ferroelectricity observed in KNbxTa1-x O3. Lastly, the (110) surface relaxations are calculated for SrTiO3 and BaT iO3 perovskites. The positions of atoms in 16 near-surface layers placed at op a slab of rigid ions are optimized using the classical shell model. Stro ng surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces .