Polarized XAFS study of the atomic displacements and phase transitions in KNbO₃

Abstract

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.

Temperature dependent XAFS studies of local atomic structure of the perovskite-type zirconates

“…This estimation is in agreement with the diffraction data for PZ at room temperature, which gives r 0 Ϸ 0.075 Å within the assumption that local and global structures are close to each other at room temperature as they are for the KN crystal. 7,8 Taking into account the results presented above, one can conclude that the PEFS data for the perovskite-type oxides suffering structural phase transitions, such as KN, NN, PZ, and PZT, make plausible a spherical model of their local atomic structure. According to this model, an atom B ͑Nb, Zr, Ti͒ is situated near the surface of a sphere of small radius r 0 ͑"central" sphere͒ at all temperatures.…”

“…The last statement is supported by the results of the PEFS studies performed for KN and NN single crystals. 7,8,19 The spherical model is a generalization of the n-site models that are often considered. 3,4 It is supposed in the n-site models that on the surface of the central sphere there are several sites, whose positions, contrary to the spherical model, do not vary depending on temperature and only their occupancies are temperature dependent.…”

“…However, analysis of the polarized XAFS performed for single crystals showed that in the low-temperature phases the displacive model is in good agreement with the experimental data, whereas the eight-site model provides better agreement with the local atomic structure in the cubic phase. 7,8 The idea that different models of local atomic structure are appropriate in different phases was recently supported by first-principles calculations 9 and NMR studies 10 of the atomic structure of the BaTiO 3 crystal.…”

Experimental evidence of thermal fluctuations on the x-ray absorption near-edge structure at the aluminumKedge

“…For instance in some perovskite crystals, significant spectral changes in the K pre-edge region are induced by temperature, and are interpreted in relation to phase transition. The literature reports temperature-dependent x-ray absorption spectra (including pre-edge) in perovskite titanates at the Ti K-edge, [16][17][18][19][20][21] in perovskite manganites at the Mn K-edge 22,23 and at the O K-edge, 24,25 in zirconates at the Zr K-edge, 26 niobiates at the Nb K-edge, [27][28][29][30][31] in the La 1−x Sr x FeO 3−δ system at the Fe K-edge, 32 in a La(Fe,Ni)O 3 solid oxide fuel cell cathode at the O K-edge, 33 and more recently in the Pr 0.5 Ca 0.5 CoO 3 cobaltite at the Ca L 2,3 -edges and the Pr L 3 and M 4,5edges. 34 Phase transition studies using temperaturedependent XANES are not restricted to the perovskite structure.…”

X-Ray synchrotron radiation in physicochemical studies

“…283,284 Analysis of polarisation-dependent EXAFS spectra for potassium niobate (perovskite type) made it possible to determine crystallographic directions of niobium atom shifts out of centres of octahedra formed by oxygen atoms in the phase transition. 285 Studies of distortions in planar square CuO 4 fragments in single crystals of high-temperature superconducting phases gave additional information on the mechanism of superconductivity. 286,287 In quantum mechanical description of polarised light properties, the term photon spin is often used.…”