Local atomic structure of niobates and titanates from X-ray absorption spectroscopic data

The local atomic structure of PbTiO₃, BaTiO₃, and KNbO₃ perovskite-type crystals and K _x Na_{1 ? x} NbO₃ solid solutions in different phases is investigated using the angular dependence of the pre-edge structure of the Ti and Nb K X-ray absorption spectra and the EXAFS data. In noncubic phases, a considerable deviation of the local structure from the structure determined from diffraction data is observed only for the tetragonal phase of the BaTiO₃ crystal. It is revealed that, in the cubic phase of niobates, the niobium atoms are characterized by significant displacements from the centrosymmetric positions along the threefold axes, so that they are close in the magnitude and the direction to the displacements in the low-temperatures rhombohedral phases.