The temperature-dependent Nb K-edge absorption spectra of several mixed-ion Pb-containing perovskite compounds were analysed to determine the Nb displacement and to trace its changes through the phase transitions. Both extended x-ray absorption fine structure (EXAFS) and the pre-edge region of the spectra were involved in the analysis. The results show that, in the compounds studied, Nb occupies an off-centre position with symmetry lower than that implied by macroscopic symmetry. The magnitude and direction of the Nb off-centre displacement do not display any noticeable temperature change and are not affected by the change in macroscopic symmetry. The Nb–O distribution and its temperature evolution do not show any distinct dependence on the degree of compositional ordering and properties of the samples.

Несмотря на многолетние исследования атомного строения сегнетоэлектрических кристаллов со структурой перовскита, проблема определения величин и направлений малых смещений атомов в различных фазах этих кристаллов не утратила своей актуальности. Основные трудности здесь связаны с тем, что точность определения положений атомов традиционными методами структурного анализа недостаточно высока из-за сильной корреляции между величинами малых смещений атомов и тепловыми параметрами.

The information on the structure of tetragonal KNbO3 is extracted from the temperature dependence of the third shell's Fourier-peak of Nb-EXAFS. It is shown that this dependence can be explained by two effects: 1) rigid rotations of the O6-octahedron and 2) increase of the third shell's DW-parameter. However, diffraction data on the temperature dependence of the DW-parameter permit us to conclude that the last effect only is sufficient and the O6-octahedron rotation doesn't occur in the high-temperature phases of KNbO3. Using EXAFS it is revealed that in the neighboring cells Nb-atoms are parallel shifted closely to [211] direction in tetragonal phase.

The approach for extracting information on the directions and values of atomic 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 experimental 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 cubic 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 Å along [211] direction. These results are shown to be stable under the effects of atoms anharmonicity motion, simulated trough the cumulant expansion by variation of the C4-coeficient value. It is shown that strong difference in Im(FT) remains under all possible C4 values, which makes Im(FT) to be reliable probe for B atom displacement, independent of radial distribution function asymmetry.

Предложенный нами новый способ определения направления смещения атомов В из центросимметричных позиций а АВО3-кристаллах применяется для исследования локальной атомной структуры КNB03 в орторомбической фазе.

Ab initio simulations of Nb K-XAFS in crystals with well known structures - LiNbO3, NaNbO3 at T=300 K, rhombohedral KNbO3 and the analysis of their Fourier-transformants (FT) permit to propose the probe for identification of Nb displacements from the centre of oxygen-octahedron for two typical types of the ideal perovskite structure distortions. The stability of the obtained probe under the variations of DW-factors is revealed. The application of this probe provides the regular approach to choose the model of radial atomic distribution function used then for precise determination of structure parameters. With its help the Nb displacement in orthorhombic phase of KNbO3 is identified removing the contradiction between X-ray diffraction and EXAFS data.