V. A. Shuvaeva

Crystal structure of the electric-fieldinduced ferroelectric phase of NaNbO3

The structure of electric-field-induced ferroelectric phase of NaNbO3 has been studied by means of X-ray diffraction method. Crystal cell is orthorhombic (P21 ma) with parameters: a = 5.569. b = 7.790, c = 5.518 A, V = 239.38 A3, Z = 4. Field-induced phase transition mainly results in reversal of atomic shift directions relative to the cubic perovskite atomic positions in one half of initial orthorhombic cell. Increasing of NbO6 octahedra tilts and additional displacements of some oxygen atoms are accounted for electric field effect.

The local structure of mixed-ion perovskites

The temperature-dependent Nb K-edge absorption spectra of several mixed-ion Pb-containing perovskite compounds were analysed to determine the Nb displacement and to trace its changes through the phase transitions. Both extended x-ray absorption fine structure (EXAFS) and the pre-edge region of the spectra were involved in the analysis. The results show that, in the compounds studied, Nb occupies an off-centre position with symmetry lower than that implied by macroscopic symmetry. The magnitude and direction of the Nb off-centre displacement do not display any noticeable temperature change and are not affected by the change in macroscopic symmetry. The Nb–O distribution and its temperature evolution do not show any distinct dependence on the degree of compositional ordering and properties of the samples.

Formation of ferroelectric phases in KNbO3 and other niobates with perovskite structure

The Curie-Weiss temperatures TCW of NaNbO3 and AgNbO3 are determined for the first time by extrapolating the temperature dependence of the reciprocal permittivity from the cubic phase. It is established that the values of TCW of ANbO3 perovskites (A = Na, Ag, K) are practically equal, i.e., that the cubic phases of all the compounds are potentially unstable to an almost equal extent with respect to polarization. From an analysis of the EXAFS spectra of the NaNbO3 and KNbO3 cubic phases, it follows that the potential-energy surfaces of the Nb cation in both compounds are similar and exhibit eight minima shifted by 0.16 and 0.19 Å, respectively, from the oxygen-octahedron center along the [111]-type directions. Based on these facts, it is inferred that the ANbO3 instability with respect to the occurrence of polarization is due to the tendency of the Nb cations to be ordered over the eight minima and that the phase transitions induced by this instability are of the order-disorder type. A statistical model capable of describing the sequence of ferroelectric phase transitions occurring in KNbO3 and in (Ba, Sr)TiO3 is proposed and studied.

X-Ray, Optical and Dielectric Studies of Diffused Phase Transitions in NaNbO3-Based Solid Solution Crystals

In flux-grown (1−x)NaNbO3-(x)Gd1/3NbO3 solid solution crystals a diffusion of the dielectric permittivity ϵ maximum grows with x and increases dramatically when x exceeds a threshold value x0 ≈ 0.1. Optical studies by the rotating-polarizer method show that Gd-doping results in a very small mean size of twins and the distribution of the birefringence image becomes very messy. In the x < x0 range the changes of lattice parameters and birefringence corresponding to the ϵ(T) anomaly were revealed. The results obtained are in line with the assumption that phase transition diffusion in NaNbO3 is a result of the local strains stemmed from the impurities.

An x-ray diffraction and EXAFS study of the electric-field-induced ferroelectric phase

A structural study of a electric-field-induced ferroelectric phase has been performed using single-crystal x-ray diffraction and EXAFS techniques. The polar axis was found to be orthogonal to the direction of atomic displacements in the initial antiferroelectric phase. The ferroelectric properties of the phase are due to a Pb displacement of about 0.17 A relative to the oxygen framework along the polar axis. The Zr atoms are disordered between two symmetry-related positions and shifted mainly orthogonally to the polar axis. The results obtained by the two methods of crystal structure investigation are in good agreement.

Structure and dielectric properties of Bi6 − xSrxTi2 − xNb2 + xO18 (x = 0–2) solid solutions

The structural and electrophysical characteristics of a series of solid solutions of layered perovs-kite-like oxides Bi6 − xSrxTi2 − xNb2 + xO18 (x = 0, 0.25, 0.5, 1.0, 1.5, 2.0) have been studied. The temperature dependences of the relative permittivity ɛ/ɛ0(T) and dielectric loss tangent tanδ have been measured. The dependences of the maximum of the permittivity ɛ/ɛ0, Curie temperature TC, lattice parameters, and the unit cell volume on x have been obtained. The structural parameter a, which corresponds to the polar direction, and the value of the orthorhombic distortion of the unit cell have been found to demonstrate noticeable negative deviations from the Vegard’s law. It has been established that the variations of the orthorhombic distortion correlate with the variations of the permittivity maximum; however, they do not markedly influence the Curie temperature that varies linearly over entire range of changes in x.

Local atomic structure of niobates and titanates from X-ray absorption spectroscopic data

The local atomic structure of PbTiO3, BaTiO3, and KNbO3 perovskite-type crystals and KxNa1 − xNbO3 solid solutions in different phases is investigated using the angular dependence of the pre-edge structure of the Ti and Nb K X-ray absorption spectra and the EXAFS data. In noncubic phases, a considerable deviation of the local structure from the structure determined from diffraction data is observed only for the tetragonal phase of the BaTiO3 crystal. It is revealed that, in the cubic phase of niobates, the niobium atoms are characterized by significant displacements from the centrosymmetric positions along the threefold axes, so that they are close in the magnitude and the direction to the displacements in the low-temperatures rhombohedral phases.

Studies of Domain and Twin Patterns in NaNbO3-Gd1/3NbO3 Solid Solution Crystals

Studies of (1-x)NaNbO3-xGd1/3NbO3 crystals in polarized light revealed the independence of the temperature of rotational phase transition into the cubic phase on x. This correlates with the similarity of Na+ and Gd3+ ionic radii. Increase of Gd content leads to the diffusion of permittivity maxima and a dramatic decrease of a mean size of twins (domains). This makes both crystallooptic studies and rotating polarizer method ineffective. Multifractal analysis of false-colour image patterns obtained by rotating polarizer method, using the Metripol microscope system enables to detect the changes in domain (twin) pattern in the temperature range of diffuse phase transition.

Determining the speciation of Zn in soils around the sediment ponds of chemical plants by XRD and XAFS spectroscopy and sequential extraction

For a correct assessment of risk of polluted soil, it is crucial to establish the speciation and mobility of the contaminants. The aim of this study was to investigate the speciation and transformation of Zn in strongly technogenically transformed contaminated Spolic Technosols for a long time in territory of sludge collectors by combining analytical techniques and synchrotron techniques. Sequential fractionation of Zn compounds in studied soils revealed increasing metal mobility. Phyllosilicates and Fe and Mn hydroxides were the main stabilizers of Zn mobility. A high degree of transformation was identified for the composition of the mineral phase in Spolic Technosols by X-ray powder diffraction. Technogenic phases (Zn-containing authigenic minerals) were revealed in Spolic Technosols samples through the analysis of their Zn K-edge EXAFS and XANES spectra. In one of the samples Zn local environment was formed by predominantly oxygen atoms, and in the other one mixed ZnS and ZnO bonding was found. Zn speciation in the studied technogenically transformed soils was due to the composition of pollutants contaminating the floodplain landscapes for a long time, and, second, this is the combination of physicochemical properties controlling the buffer properties of investigated soils. X-ray spectroscopic and X-ray powder diffraction analyses combined with sequential extraction assays is an effective tool to check the affinity of the soil components for heavy metal cations.

Structure and dielectric properties of solid solutions Bi7Ti4 + xWxNb1 − 2xO21 (x = 0–0.5)

The structural and electrophysical characteristics of a series of solid solutions of layered perovskite-type oxides Bi7Ti4 + xWxNb1 − 2xO21 (x = 0–0.5) have been investigated. According to X-ray powder dif- fraction data, all the studied compounds are single-phase and have the structure of Aurivillius phases (m = 2.5) with an orthorhombic crystal lattice (space group I2cm, Z = 2). The changes in the tetragonal and orthorhombic distortions of perovskite-like layers in the compounds have been considered depending on their chemical composition. The temperature dependences of the relative permittivity ε(T) have been measured. It has been shown that the Curie temperature TC of the perovskite-type oxides Bi7Ti4 + xWxNb1 − 2xO21 (x = 0–0.5) decreases linearly with an increase in the parameter x. The activation energies of charge carriers have been obtained in different temperature ranges. It has been found that there are three temperature regions with very different activation energies due to different natures of charge carriers in the studied compounds.