Computer modelling of KTN solid solutions

The results of quantum chemical modelling of perovskite solid solutions KNb_xTa_1-xO₃ are presented and compared with recent full-potential LMTO calculations. We show that Nb atoms in KTaO₃ reveal off-center displacements along both the [100] and [111] axes at as low concentrations as x = 0.125, which is in much better agreement with XAFS experiments than full potential LMTO calculations. The analysis of the optimized atomic and electronic structure clearly demonstrates that several nearest Nb impurities in KTaO₃ are able for self-ordering and reveal a cooperative behaviour. This probably triggers the KTN ferroelectric phase transition. In contrast, Ta impurities in KNbO₃ reveal no self-ordering. The qualitatively different behaviour of Nb and Ta atoms could be caused by a small difference in their chemical bonding with other atoms, first of all oxygens.