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 and hole polarons - in KNbO₃ perovskite crystals are used for the interpretation of available experimental data. The results of quantum chemical calculations for perovskite KNb_xTa_1-xO₃ solid solutions are presented for x = 0, 0.125, 0.25, 0.75, and 1. An analysis of the optimized atomic and electronic structure clearly demonstrates that several nearest Nb atoms substituting for Ta in KTaO₃ - unlike Ta impurities in KNbO₃ - reveal a self-ordering effect, which probably triggers the ferroelectricity observed in KNb_xTa_1-xO₃. Lastly, the (110) surface relaxations are calculated for SrTiO₃ and BaTiO₃ perovskites. The positions of atoms in 16 near-surface layers placed atop a slab of rigid ions are optimized using the classical shell model. Strong surface rumpling and surface-induced dipole moments perpendicular to the surface are predicted for both the O-terminated and Ti-terminated surfaces.