Abstract
The results of quantum chemical modelling of perovskite solid solutions KNb/sub x/Ta/sub 1-x/O/sub 3/ are presented and compared with recent full-potential LMTO calculations. We show that Nb atoms in KTaO/sub 3/ 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 KTaO/sub 3/ are able for self-ordering and reveal a cooperative behaviour. This probably triggers the KTN ferroelectric phase transition. In contrast, Ta impurities in KNbO/sub 3/ 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.
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