Polarized XAFS study of high-temperature phases of NaNbO3

Temperature dependence of the Nb displacement relative to the center of oxy gen octahedron in NaNbO3 has been studied by polarized Nb K XAFS. Spectra w ere measured at two orientations of a single crystalline sample. Room tempe rature EXAFS data are in a good agreement with earlier X-ray diffraction da ta: Nb antiferroelectric displacements were found to be orthogonal to the b axis. Analysis of the temperature dependent EXAFS data didn't reveal any a brupt changes of Nb-O distances in the phase transition points. In all high -temperature paraelectric phases Nb appeared to be displaced to the off-cen ter positions. Displacements, orthogonal to b axis, remained almost constan t, while displacement along b axis gradually increased with temperature, so that in the cubic phase the displacements along all axes became equal. Thi s shows, that disorder plays an important role in the high temperature phas es. The above results are supported also by the analysis of the pre-edge st ructure, - the integral intensity of the pre-edge peak was temperature-inde pendent when the polarization vector of the X-rays was orthogonal to b axis and gradually increased with temperature when the polarization was paralle l to b.