Polarized XAFS study of high-temperature phases of NaNbO₃

Abstract

Temperature dependence of the Nb displacement relative to the center of oxygen octahedron in NaNbO 3 has been studied by polarized Nb K XAFS. Spectra were measured at two orientations of a single crystalline sample. Room temperature EXAFS data are in a good agreement with earlier X-ray diffraction data: Nb antiferroelectric displacements were found to be orthogonal to the b axis. Analysis of the temperature dependent EXAFS data didn't reveal any abrupt changes of Nb-O distances in the phase transition points. In all high-temperature paraelectric phases Nb appeared to be displaced to the off-center positions. Displacements, orthogonal to b axis, remained almost constant, while displacement along b axis gradually increased with temperature, so that in the cubic phase the displacements along all axes became equal. This shows, that disorder plays an important role in the high temperature phases. The above results are supported also by the analysis of the pre-edge structure, -the integral intensity of the pre-edge peak was temperature-independent when the polarization vector of the X-rays was orthogonal to b axis and gradually increased with temperature when the polarization was parallel to b.

Displacive Order–Disorder Behavior and Intrinsic Clustering of Lattice Distortions in Bi‐Substituted NaNbO3

“…The PDD for Nb is nearly Gaussian but anisotropic with larger magnitudes of displacements in the a-b plane. This result is consistent with existing data for the structures of orthorhombic NaNbO3 [30] and KNbO3 [31] , which indicate that the magnitudes of the locally correlated components of Nb displacements in the c-direction that give rise to the (001) diffuse sheets are relatively small. Sectioning the Nb PDD indicates that the maximum probability is near the average position.…”

“…These local shifts are disordered, with split-site/hollow Bi PDDs. A similar effect exists for Nb which is known to be locally off-centered in the cubic phases of alkaline niobates [30,31] . As mentioned in the Introduction, octahedral rotations mitigate the underbonding of Bi and thus are expected to couple to its displacements.…”

Low‐temperature Raman spectroscopic studies in NaNbO3

“…Extensive studies on the various aspects of phase transitions in NaNbO 3 have been reported using a variety of experimental techniques including X-ray absorption fine structure, [3] transmission electron spectroscopy, [4] electron paramagnetic resonance, [5] nuclear magnetic resonance, [6] etc. Extensive Raman studies combined with X-ray diffraction (XRD) and dielectric measurements confirmed the presence of an incommensurate phase in NaNbO 3 in the temperature interval 410-460 K. [2] Low wavenumber Raman studies on NaNbO 3 single crystal revealed the occurrence of a quasi-elastic peak, and the phase transition was attributed to the order-disorder process due to the relaxation response of Nb ion in the temperature interval 513-713 K. [7] Shen et al, [8] on the basis of the temperature variation of phonon modes during cooling and heating cycles of measurement, suggested that both FE and AFE phases coexist in the temperature range 40-180 K during cooling cycle.…”

Competing antiferroelectric and ferroelectric interactions inNaNbO3: Neutron diffraction and theoretical studies

“…1. Experimental studies of NaNbO 3 using a wide variety of techniques including x-ray and neutron diffraction [8][9][10][11][12][13][14][15] , Raman and infrared spectroscopy [16][17][18][19][20][21] , XAFS 22 , transmission electron microscopy (TEM) 23 , electron paramagnetic resonance (EPR) 24 , nuclear magnetic resonance (NMR) 25 , etc. have been reported and the crystal structure [8][9][10][11][12][13][14][15] , phonon spectrum [16][17][18][19][20][21]26 , dielectric and piezoelectric behavior 4,6,2 6-30 , temperature and pressure driven phase transitions and critical behavior of the order parameter 24,25 studied.…”