Local distortions of ideal perovskite structure in KNbO3 revealed by EXAFS

The approach for extracting information on the directions and values of ato mic displacements from the ideal positions in perovskite type compounds is proposed using EXAFS spectra. It is shown that reliable information can be obtained analysing the discrepancy factor between theoretical and experimen tal Fourier Transformants (FT) for different models of atomic displacements , generated using the value of averaged atomic displacement, provided by X- ray diffraction. By this approach, the rhombohedral local distortion in cub ic phase of KNbO3 is revealed. In tetragonal phase of KNbO3 the minimum of discrepancy factor is obtained in assumption that Nb is shifted by 0.19 Ang strom along [211] direction. These results are shown to be stable under the effects of atoms anharmonicity motion, simulated trough the cumulant expan sion by variation of the C-4-coeficient value. It is shown that strong diff erence in Im(FT) remains under all possible C-4 values, which makes Im(FT) to be reliable probe for B atom displacement, independent of radial distrib ution function asymmetry.