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A series of double-perovskite oxides Sr2Sr1-xCaxTeO6 (0 ≤ x ≤ 1) samples have been prepared in order to study the effect of composition on phase formation and the related optical properties. The objective of this work is to study the possibility of calcium insertion with different percentages since calcium in known to enhance the electrical properties. In this work a well detailed structure and phase transitions investigation of the compounds were conducted and probed by X-ray diffraction and Raman spectroscopy techniques at room temperature. Both Rietveld refinements and Raman studies revealed that two phase transitions took place as the calcium amount x increases; a first from a triclinic to a hexagonal in the range 0.1 < x < 0.25 and a second from the hexagonal to a monoclinic structure in the range 0.5 < x < 0.6. The optical absorption α(λ) was also determined as a function of calcium content in the series samples using UV-vis analysis. It was revealed that a remarkable change occured in the cut-off wavelength, confirming the sequence of phase transitions C1 → R 3 ¯ m → P21/n. The optical band gap measurements showed that the triclinic Sr3TeO6 and the monoclinic Sr2CaTeO6 phases have band gap values of 2.75 and 2.81 eV, respectively. When the calcium increased to x = 0.25 (Sr2.75Ca0.25TeO6), remarkable decrease of the band gap value was observed, with Eg = 2.627 eV. [ABSTRACT FROM AUTHOR]

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]