GEOMETRICAL MICROSTRUCTRE OF FE-NB(3-VO DEFECTS IN KNBO3())

Results of atomistic modeling studies are reported which concern the a xial relaxation of a niobium-substituting Fe3+ cation relative to an a ssociated oxygen vacancy. The employed methods are shell-model simulat ions and embedded-cluster calculations. These simulation studies consi stently show that the iron impurity is displaced in the opposite direc tion to the oxygen vacancy. The present results are in contrast to kno wn interpretations based on the superposition model (SPM). It is shown finally that slight modifications of iron-specific superposition-mode l parameters are sufficient in order to achieve agreement between SPM- based interpretations and shell-model as well as embedded-cluster-type calculations. The relaxation pattern found in this special case is ex pected to be valid for most axial defect aggregates of this particular type.