Victoria A. Shuvaeva

Local structure and nature of phase transitions in KNbO3

Abstract The direction and magnitude of Nb displacements from the center of oxygen octahedron in orthorhombic, tetragonal and cubic phases of KNbO 3 were determined by means of polarized X-ray absorption fine structure (polarized XAFS). The XAFS spectra for Nb K -edge were obtained from plate-like single crystals in a transmission mode. It is found that in the orthorhombic phase the direction of the Nb displacement is very close to the polar axis. However, the disorder of Nb positions grows gradually with increasing temperature. All phase transitions are governed by both displacive and order-disorder mechanisms. The rhombohedral-to-orthorhombic phase transition is considered as essentially a displacive type, while the order-disorder component is dominant in the tetragonal-to-cubic phase transition.

The local structure of PbIn1/2Nb1/2O3

The local structure of ordered and disordered PbIn1/2Nb1/2O3 has been investigated as a function of temperature using X-ray absorption fine structure (XAFS) technique. Both EXAFS (extended XAFS) and near edge structure of the spectra were analysed. Radial distribution of oxygen atoms around Nb appeared to be similar for the both modifications and independent of the temperature. Presence of the pre-edge peak in the spectra indicated, that the value of the Nb displacement is quite large both below and above the phase transition point. The local structure of PbIn1/2O3 is discussed in comparison with the other Nb-containing perovskite compounds.

Polarized XAFS Study of KNbO 3 Local Structure

Nb positions in KNbO 3 crystal were determined at room temperature via polarized Nb K-XAFS measurements. It has been revealed that the direction of Nb displacement in orthorhombic phase is very close to polar axis direction. This refute eight-site model which assumes that Nb atoms are disordered among several equivalent positions and phase transition are of order-disorder type. Analysis of polarized XAFS data showed that only slight disorder among close positions is possible. The distance between these positions is of an order of temperature atomic displacements, so the disorder, if any, is likely of dynamic nature. Results obtained are in perfect agreement with X-ray diffraction data.

X-ray diffraction study of domain reversal in relaxor Pb(Zn1/3Nb2/3)O3

The behavior of domain reversal in relaxor Pb(Zn 1/3 Nb 2/3 )O 3 single crystals was studied using X-ray diffraction measurements. After the electric field was switched instantaneously, the {222} B...

The regular approach for identifying the atomic displacements in ABO3 crystals and its application to high-temperature phases of KNbO3.

The approach for identifying the atomic displacements from the ideal positions in perovskite structure is proposed, based: on characteristic features in the Fourier Transforms (FT) for different models of atomic displacements and on the results of EXAFS calculations lot alternative models of ideal structure distortions, generated using the value of averaged atomic displacement provided by diffraction methods. The approach is applied to study the local structure distortions in tetragonal and cubic phases of KNbO3. The effect of anharmonic motion of atoms on the obtained structure information is simulated through the cumulant expansion.

Local Distortions of Ideal Perovskite Structure in KNbO3 Revealed by EXAFS

The approach for extracting information on the directions and values of atomic displacements from the ideal positions in perovskite type compounds is proposed using EXAFS spectra. It is shown that reliable information can be obtained analysing the discrepancy factor between theoretical and experimental Fourier Transformants (FT) for different models of atomic displacements, generated using the value of averaged atomic displacement, provided by X-ray diffraction. By this approach, the rhombohedral local distortion in cubic phase of KNbO3 is revealed. In tetragonal phase of KNbO3 the minimum of discrepancy factor is obtained in assumption that Nb is shifted by 0.19 A along [211] direction. These results are shown to be stable under the effects of atoms anharmonicity motion, simulated trough the cumulant expansion by variation of the C4-coeficient value. It is shown that strong difference in Im(FT) remains under all possible C4 values, which makes Im(FT) to be reliable probe for B atom displacement, independent of radial distribution function asymmetry.

Deglitching procedure for XAFS.

A computer program for eliminating artifacts such as glitches and Bragg peaks of the polarized XAFS spectra is presented. It permits to easily locate and to eliminate from the EXAFS spectra the additional signals originated from the Bragg scattering by crystalline samples. The test of the procedure on the spectra with specially introduced artifacts and on the experimental polarized XAFS spectra showed its high effectiveness.

EXAFS studies of KBr1-x(NO2)x mixed crystals.

EXAFS studies of KBr1-x(NO2)x mixed crystals (x<0.4) were performed to elucidate the local structure of their dipole glass. The local structure, especially the nearest bromine-potassium pair-distribution, is not affected by replacing bromine ions with nitrite ones even below the glass transition temperature. The mean-square displacement of the bromine-potassium pair-distribution obeys the Debye approximation, while that of average structure derived from previous X-ray diffraction studies deviates largely from the approximation. The results indicate that a cluster with only short-range ferroelectric correlation is induced by the short-range order of nitrite ions within the distorted or fluctuated lattice caused by their reorientational freezing.

Average and local structures of KBr1-x(NO2)x mixed crystals

Abstract The average and local structures of a dipole glass, KBr1-x(NO2)x, were clarified using X-ray diffraction and extended X-ray absorption fine structure (EXAFS). The average structure exhibits the distorted or fluctuated lattice below the glass transition temperature Tg . The mean square displacement of potassium and bromine ions largely deviates from the Debye approximation and the deviation is characterized by the Edwards—Anderson order parameter. On the other hand, the local structure shows no anomalous behavior and the mean square displacement of the pair distribution of potassium—bromine ions obeys the Debye approximation even below Tg . The results indicate that a cluster with only short-range correlation induced by the short-range ferroelectric order of nitrite ions exists within the distorted or fluctuated lattice caused by the reorientational freezing of nitrite ions.