Geometrical microstructure of ${\mathrm{Fe}}_{\mathrm{Nb}}^{3+}$-${\mathit{V}}_{\mathrm{O}}$ defects in ${\mathrm{KNbO}}_3$

Results of atomistic modeling studies are reported which concern the axial relaxation of a niobium-substituting ${\mathrm{Fe}}^{3+}$ cation relative to an associated oxygen vacancy. The employed methods are shell-model simulations and embedded-cluster calculations. These simulation studies consistently show that the iron impurity is displaced in the opposite direction to the oxygen vacancy. The present results are in contrast to known interpretations based on the superposition model (SPM). It is shown finally that slight modifications of iron-specific superposition-model 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 expected to be valid for most axial defect aggregates of this particular type.