Polarized XAFS technique has been applied for the determination of the directions and the values of the Nb displacements relative to the centers of oxygen octahedra in KNbO3. The temperature dependence of the both EXAFS and XANES spectra were analyzed in order to reveal the role of disorder in the phase transitions of this compound. The polarized XAFS appeared to be enough sensitive to detect the delicate balance between orderdisorder and displacive mechanisms of the phase transitions in KNbO3 The magnitude of the pre-edge peak on the Nb K-XAFS spectra was found to be highly correlated with the value of the Nb off-center displacement, obtained by fitting of the EXAFS spectra.
Determination of atomic displacements in perovskite-type compounds is a long standing problem. Accuracy of X-ray diffraction technique in many cases is not sufficient due to strong correlation between temperature and position parameters. Besides there are a lot of evidences that local structure in a number of perovskite crystals differs from the averaged one. Even structure of well known BaTiO3 and KNbO3 crystals is still under discussion since eight site model has been worked out on the basis of the results of diffuse scattering investigation [1]. According to this model Ti and Nb atoms are disordered among several equivalent positions and the phase transitions are of order-disorder type.We performed polarized XAFS study of Nb positions in KNbO3. The spectra were obtained from a single crystal sample for two orthogonal orientations of the crystal polar axis relative to vector of X-ray polarization. Apparent difference of Fourier transforms of the spectra for these two. orientations, presented at Figure as well as results of data fitting indicated that direction of Nb off-center displacement is very close to polar axis direction. Displacement in the orthogonal direction was estimated to be not more than temperature atomic displacements. So it has been shown that eight site model is not valid for KNbO3 and phase transition from the rhombohedral to the orthorhombic phase is essentially of displacive type.Polarized XAFS technique has been proved to be very powerfull tool for investigation of atomic displacements in perovskite compounds.
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