Polarized XAFS study of the atomic displacements and phase transitions in KNbO_3

摘要

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 KNbO_3. 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 order-disorder and displacive mechanisms of the phase transitions in KNbO_3. 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.