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Sr 2 Zn 1−x Ni x WO 6 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 P 2 1 /n space group with 0 ≤ x ≤ 0.18, a tetragonal structure with a I 4/ m space group with x = 0.25, and a tetragonal structure with a I 4/ mmm space group with 0.5 ≤ x ≤ 1. Further in-situ Raman study at elevated temperature (up to 555 °C) shows the pristine monoclinic phase (0 ≤ x ≤ 0.18) follows the phase path: monoclinic ( P 2 1 /n ) → tetragonal ( I 4/ m ) → tetragonal ( I 4/ mmm ) → cubic ( Fm 3 ¯ m ); for the compound Sr 2 Zn 0.75 Ni 0.25 WO 6 , it involves later two phase transitions as tetragonal ( I 4/ m ) → tetragonal ( I 4/ mmm ) → cubic ( Fm 3 ¯ m ); and for the 0.5 ≤ x ≤ 1 compounds, only one phase transition I 4/ mmm to Fm 3 ¯ m was observed. The combined investigation of composition and temperature provides us a complete understanding of chemical substitution and temperature effect on structural phase transition. [ABSTRACT FROM AUTHOR]

[Ba 2 CaWO 6 ] 1− x [Sr 2 CaWO 6 ] 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 ̅ m ) to tetragonal ( I 4/ m ) to monoclinic structural phases ( I2/m , P2 1 /n ). These transitions have been confirmed by Raman studies Fm 3 ¯ m x = 0 → I 4 / m 0.1 ≤ x ≤ 0.2 → I 2 / m 0.3 ≤ x ≤ 0.5 → P 2 1 / n 0.6 ≤ x ≤ 1 Furthermore, increasing the temperature for the compositions [Ba 2 CaWO 6 ] 1− x [Sr 2 CaWO 6 ] x (0.1≤ x <1), manifests the P 2 1 / n to I 2/ m , the I 2/ m to I 4/ m and the I 4/ m to Fm 3 ̅ m phase transitions. For the compositions (0.1≤ x ≤0.2) the tetragonal to cubic phase transition is well illustrated. For the room temperature I 2/ m monoclinic compositions, two phase transitions were observed for all the compositions with x ranging from 0.3 to 0.5: from the monoclinic ( I 2/ m ) to tetragonal ( I 4/ m ), and from I 4/ m to Fm 3 ̅ m structures. Finally, for the room temperature P 2 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. [ABSTRACT FROM AUTHOR]

Structures of Sr 2−x Ca 1+x TeO 6 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 P 2 1 / n as space group and a triclinic system with P 1 ¯ 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 Ca 3 TeO 6 . For this compound, considerable changes in the temperature dependence of the modes were well illustrated. [ABSTRACT FROM AUTHOR]

Highlights: [•] Synthesis and characterization of new double perovskites Ba2−x Sr x MgTeO6 (0⩽ x ⩽2). [•] High temperature studies of Ba2−x Sr x MgTeO6 double perovskite using Raman spectroscopy. [•] Temperature and compositions induced phase transitions in these materials. [•] Structural determination/refinement of these compounds as a function of composition. [Copyright &y& Elsevier]

A series of perovskites Sr 3– x Ca x Fe 2 TeO 9 (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 I 4/ 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: I 4/ m → Fm 3 ¯ m . This phase transition occurs at high–temperature. Analysis of Raman spectra recorded at several temperatures shows that this phase transition appears near ∼375 °C for ( x = 0), ∼435 °C for ( x = 0.5) and ∼451 °C for ( x = 1). A proportional and gradual increase of temperature phase transition is observed as function of the calcium amount. [ABSTRACT FROM AUTHOR]

Highlights: [•] Synthesis and characterization of Ba2−xSr x MWO6 (M=Ni, Co and 0⩽ x ⩽2) double perovskites. [•] High temperature studies of these double perovskite using Raman spectroscopy. [•] Temperature and compositions induced phase transitions in these materials. [•] Structural determination/refinement of these compounds as a function of composition. [ABSTRACT FROM AUTHOR]