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Structures of Sr2-xCa1+xTeO6 double perovskites have been studied by the profile analysis of X-ray diffraction data and Raman spectroscopy at room temperature. This series adopts a monoclinic symmetry for the compositions (0 <= x <= 0.5) with P2(1)/n as space group and a triclinic system with P (1) over bar space group for the compositions (0.5 < x <= 2). These results were confirmed by the observed tolerance factor calculated from the distances obtained from the Rietveld refinements which indicates that the true tilt system for the compositions range (0.5 < x <= 2) is the triclinic tilt system. Clear changes in the Raman modes centered at 600, 610 and 620 cm(-1) and the FWHM of O-Te-O bending vibrations, centered at 738 cm(-1) confirmed that the triclinic symmetry takes place between the compositions x = 0.5 and x = 1. Furthermore, Raman spectroscopy studies at high temperature were done for Ca3TeO6. For this compound, considerable changes in the temperature dependence of the modes were well illustrated.

A series of perovskites Sr3-xCaxFe2TeO9 (0 <= x <= 1) have been prepared in polycrystalline form by solid-state reaction method in air. These materials have been studied by X-ray powder diffraction method (XRPD) and Raman spectroscopy. An analysis of the XRD patterns at room temperature has shown that these compounds crystallize in a tetragonal system, space group I4/m. The structure contains alternating (Fe/Te)(2a)O-6 and (Fe/Te)(2b)O-6 octahedra, tilted in anti-phase in the basal ab-plane. The study of Raman spectroscopy at various temperatures shows a transition from tetragonal to cubic phase: I4/m -> Fm (3) over bar m. This phase transition occurs at high-temperature. Analysis of Raman spectra recorded at several temperatures shows that this phase transition appears near similar to 375 degrees C for (x = 0), similar to 435 degrees C for (x = 0.5) and similar to 451 degrees C for (x = 1). A proportional and gradual increase of temperature phase transition is observed as function of the calcium amount.

High-pressure structural and electrical properties of Sr2ZnWO6 double perovskite were investigated using in situ angle-dispersive synchrotron X-ray diffraction (XRD), Raman, and alternating current (AC) impedance spectroscopy. A structural transition from monoclinic (P2(1)/n) to triclinic (P (1) over bar) phase around 9 GPa was observed due to the pressure-induced distortion of (W, Zn)O-6 octahedron. In situ high-pressure Raman spectroscopy showed the increasing interaction among O-W-O in WO6 octahedron with pressure and a transition pressure consistent with the XRD results. From the AC impedance spectroscopy measurements, the resistivity increased steeply by similar to 1 order of magnitude around 11 GPa, indicating an electronic transition accompanying the symmetry change. The increase in the interaction among O-W-O enhances the attraction of O2- electrons toward W6+, thus increasing the covalence, which in turn lowers the charge transfer energy between O2- and W6+ and induces the resistivity increase under high pressure.

Sr2Zn1-xNixWO6 solid solutions (with 0 <= x <= 1) have been synthesized by solid-state reactions, and characterized by X-Ray diffraction and Raman spectroscopy. The X-Ray powder diffraction (XRD) data at room temperature shows a systematical change of structure vs. nickel content: the compound adopts a monoclinic phase with a P2(1)/n space group with 0 <= x <= 0.18, a tetragonal structure with a I4/m space group with x = 0.25, and a tetragonal structure with a I4/mmm space group with 0.5 <= x <= 1. Further in-situ Raman study at elevated temperature (up to 555 degrees C) shows the pristine monoclinic phase (0 <= x <= 0.18) follows the phase path: monoclinic (P2(1)/n) -> tetragonal (I4/m) -> tetragonal (I4/ mmm) -> cubic (Fm (3) over barm); for the compound Sr2Zn0.75Ni0.25WO6, it involves later two phase transitions as tetragonal (I4/m) -> tetragonal (I4/mmm) -> cubic (Fm (3) over barm); and for the 0.5 <= x <= 1 compounds, only one phase transition I4/mmm to Fm (3) over barm was observed. The combined investigation of composition and temperature provides us a complete understanding of chemical substitution and temperature effect on structural phase transition.

[Ba2CaWO6](1-x)[Sr2CaWO6](x) (0 <= x <= 1) materials were synthesized by the high temperature solid state reaction and firing methods, and characterized using techniques of X-ray diffraction and Raman spectroscopy. The crystal structures were determined by Rietveld refinements on the laboratory X-ray powder diffraction data. As a function of composition, upon increasing the strontium content, the samples exhibit a sequence of three phase transitions: from cubic (Fm (3) over barm) to tetragonal (I4/m) to monoclinic structural phases (I2/m, P2(1)/n). These transitions have been confirmed by Raman studies Fm (3) over barm(x=0) -> I4/m(0.1 <= x <= 0.2) -> I2/m(0.3 <= x <= 0.5) -> P2(1)/n(0.6 <= x <= 1) Furthermore, increasing the temperature for the compositions [Ba2CaWO6](1-x)[Sr2CaWO6](x) (0.1 <= x < 1), manifests the P2(1)/n to I2/m, the I2/m to I4/m and the I4/m to Fm<(3)over bar>m phase transitions. For the compositions (0.1 <= x <= 0.2) the tetragonal to cubic phase transition is well illustrated. For the room temperature I2/m monoclinic compositions, two phase transitions were observed for all the compositions with x ranging from 0.3 to 0.5: from the monoclinic (I2/m) to tetragonal (I4/m), and from I4/m to Fm (3) over barm structures. Finally, for the room temperature P2(1)/n monoclinic compositions, only two phase transitions are observed in the temperature range probed by Raman spectroscopy, the temperature was not high enough to reach the tetragonal-to-cubic phase transition.

X-ray diffraction and Raman spectroscopy studies of Sr-substituted double perovskite compounds Ba2-xSrxMgTeO6 with (0 <= x <= 2) were carried out. At room temperature, both Rietveld refinements and Raman studies showed that two phase transitions occurred as a function of increased strontium content; from cubic to tetragonal structure, at x similar to 1.5(1), and from the tetragonal to monoclinic structure at x similar to 1.9(1). Furthermore, upon temperature increase up to 430 degrees C, the monoclinic and tetragonal perovskites manifest transformations to the tetragonal and cubic symmetries, and to the cubic symmetry, respectively. For this series, the transitions show considerable changes in the compositional and temperature dependencies of the modes, the spectral positions of which exhibit a linear behavior. The symmetry transformations between the encountered three crystal systems is revealed by significant changes of the slopes. (C) 2014 Elsevier B. V. All rights reserved.

X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba2-xSrxMWO6 with (M = Ni, Co and 0 <= x <= 2) were investigated. The samples show a transition from cubic to tetragonal phase as a function of composition while increasing strontium amount; both Rietveld refinements and Raman studies showed that this transition occurs between x = 1.2 and 1.4. Furthermore, increasing temperature for the tetragonal compositions (1.4 <= x <= 2), manifest the tetragonal to cubic phase transition. For this series, the transition from tetragonal (I4/m)/to cubic (Fm-3m) shows considerable changes in the composition and temperature dependence of the modes: all the Raman modes show a linear behavior when temperature is increased, then the slope change dramatically indicating the symmetry change from tetragonal to cubic. To better view this phase transition, the full width at half maximum (FWHM) temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached the transition temperature, a drop in the width of this mode was observed.

X-ray diffraction and Raman spectroscopy studies of Sr doped double perovskites compound Ba2-xSrxZnWO6 with (0 <= x <= 2) were investigated. As a function of composition while increasing strontium amount, the samples show two phase transitions from cubic to tetragonal phase and from tetragonal to monoclinic structure. Both Rietveld refinements and Raman studies showed that the first phase transition occurs between x = 1 and 1.2; the second phase transition is observed around x = 2. Furthermore, increasing the temperature for the compositions (1.2 <= x <= 2), manifests the tetragonal to cubic phase transition. For x = 2 two phase transitions were observed as a function of temperature. For this series, the phase transitions are illustrated by considerable changes in the composition and temperature dependence of the modes. All the Raman modes show a linear behavior when the composition or temperature is increased, then the slope change indicating the symmetry change.

In this work we report on the crystal structure and Raman spectroscopy of BaSrMWO6 (M=Ni, Co, Mg) double perovskite oxides. The results showed that the Ni, Co and Mg containing compounds crystallize in a cubic system with the space group Fm-3 in. The double perovskite structure can be represented as a three-dimensional network of alternating MO6 and WO6 octahedra, with Ba- and Sr-atoms occupying the interstitial spaces. The Raman spectra of this crystalline perovskite oxides are interpreted by means of factor group analysis in terms of space group Fm-3 m. Assignments of the W-O vibrational stretching and bending modes have been made. High temperature Raman spectroscopy of BaSrNiWO6 showed that all lattice modes illustrate a linear decrease in wavenumbers while temperature is increased. No discontinuities, in the temperature changes, in wavenumber observed for all modes recorded: indicating that the structure remains cubic and stable up the highest temperature studied here.

In this paper, we report on high temperature Raman spectroscopy studies (up to 550 °C) of the phase transitions in Sr2BWO6 (B = Ni, Mg) double perovskite oxides. For both compounds, the transition from the tetragonal (I4/m) phase to the cubic (Fm-3m) phase is accompanied by considerable changes in the temperature dependence of the modes. To better view this phase transition, the FWHM temperature dependence of the most intense modes were studied; by this study, we have obtained a sensitive guide to the onset of the phase transition. A large increase of the FWHM with increasing temperature is observed for all modes in the spectrum, clear transitional effects were observed: the width behaves in a linear way as a function of temperature; when the temperature reached around 300 °C, a drop in the width of this mode was observed; thus, showing the phase transition occurrence for both compounds. This drop is much more illustrated in Ni-containing compound than in Sr2MgWO6.