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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 alpha(lambda) 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) over bar m -> P2(1)/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.

Sr(1.50)Ca(1.50)Fe(2.25)Mo(0.75)O(9-delta)and Sr(1.92)Ca(1.08)Fe(1.04)W(0.96)O(9-delta)double perovskites are synthesized in the polycrystalline form by a solid-state reaction route in air and studied at room temperature using the of PXRD, Raman and Mossbauer spectroscopy techniques. The Rietveld refinement analysis reveals that both compounds adopt a tetragonal system with the space groupI4/m and lattice parametersa=b= 5.5176(1) A andc= 7.8065(2) A for Sr(1.50)Ca(1.50)Fe(2.25)Mo(0.75)O(9-delta)anda=b= 5.5453(1) A andc= 7.8388(1) A for Sr1.92Ca1.08Fe2.04W0.96O9-delta. Raman spectra are consistent with the group theoretical analysis predicted for tetragonal symmetryI4/m(point group C4h5).Fe-57 Mossbauer spectra recorded at room temperature show a paramagnetic behavior for Sr1.50Ca1.50Fe2.25Mo0.75O9-delta, and magnetic ordering for Sr1.92Ca1.08Fe2.04W0.96O9-delta. The isomer shift (delta) and quadrupole splitting (Delta) values are characteristic of high-spin Fe(3+)in a distorted octahedral coordination.

Polycrystalline samples of the series of triple perovskites Sr3-xPbxFe2TeO9 (0 <= x <= 2.25) (SPFTO) were synthesized using solid state reaction. These materials have been studied by a combination of X-ray powder diffraction (XRPD), Mossbauer spectrometry, Raman and UV-Vis spectroscopies. The crystal structures were resolved by the Rietveld refinement method, and revealed that this Sr3-xPbxFe2TeO9 (0 <= x <= 2.25) system shows one space group change from tetragonal I4/m (0 <= x <= 1) to another tetragonal form I4/mmm (1.25 <= x <= 1.88) and a second transition to hexagonal R3 over bar m (2.08 <= x <= 2.25). An anti-site disordering of Fe and Te on the B sites has been detected indicating the presence of a partial amount of Te at Fe positions and vice versa. The valence state of iron in the Fe site was determined to be Fe(III) by Mossbauer spectrometry, which also revealed two sites in a concordance with the XRPD measurements. Fe-57 Mossbauer spectra measurements show paramagnetic and magnetic ordering behaviors. The observed Raman spectra as a function of composition show obvious changes on the positions (wavenumbers), the FWHM and the intensities of the modes confirming the phase transformations observed by the XRPD results. These structural transitions led to a distinct change in the optical band gap energy, varying from 2.14 to 1.85 eV.

Ceramics of Sr2CaFe2WO9 and Sr2PbFe2TeO9 double perovskites have been prepared in polycrystalline form by solid-state technique, in the air. The crystalline structure was analyzed using X-ray powder diffraction (XRPD) at room temperature. Rietveld analysis of XRPD) patterns show that both compounds adopt a tetragonal structure with space group I4/m, with unit cell parameters a = 5.5453(1) angstrom, c = 7.8389(1) angstrom for Sr(2)CaFc(2)WO(9), and a = 5.5994(15) angstrom, c = 7.8979(30) angstrom for Sr2PbFe2TeO9. A certain degree of anti-site disordering of W and/or Te and Fe on the B -sites have been detected, indicating the presence of a partial amount of W and/or Te at Fe positions and vice versa.

Sr2ZnTeO6 double-perovskite oxide has been synthesized using solid state chemistry. As synthesized, the crystalline structure of Sr2ZnTeO6, refined using x-ray diffraction, is monoclinic having the space group I2/m. Structural phase transitions are studied using Raman spectroscopy in the temperature range 25 degrees C-567 degrees C. It is found that two of three observed bending Raman vibrations merge together at a temperature of around 120 degrees C, indicating a Sr2ZnTeO6 phase transition from the monoclinic (I2/m) to the tetragonal (I4/m). Furthermore, a temperature-dependence change-rate of external and O-Te-O bending modes and stretching modes are detected at 270 degrees C, which is interpreted as a phase transition from the tetragonal (I4/m) to the cubic (Fm-3m) structure.

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.

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.

Synthesis of apatites, Na1-xKxCaPb3(PO4)(3) 0 <= x <= 1, with anion vacancy were carried out using solid state reactions. The solid solution of apatite-type structure crystallize in the hexagonal system, space group P6(3)/m (No. 176). Rietveld refinements showed that around 90% of Pb2+ cations are located in the (6h) sites, the left amount of Pb2+ cations are located in the (4f) sites; 27-31% of Ca2+ cations are located in the (6h) sites, the left amount of Ca2+ cations are located in the (4f) sites. The ninefold coordination sites (4f) are also occupied by the K+ and Na+ monovalent ions. The structure can be described as built up from [PO4](3-) tetrahedra and Pb2+/Ca2+ of sixfold coordination cavities (6h positions), which delimit void hexagonal tunnels running along [001]. These tunnels are connected by cations of mixed sites (4f) which are half occupied by Pb2+/Ca2+ and half by Na+/K+ mixed cations. The assignment of the observed frequencies in the Raman and infrared spectra is discussed on the basis of a unit cell group analysis and by comparison with other apatites. Vibrational spectra of all the compositions are similar and show some linear shifts of the frequencies as a function of the composition toward lower values due the substitutions of Na+ by K+ with a larger radius.

[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.

Double-perovskite oxide Sr2MgTeO6 has been synthetized, and its crystal structure was probed by the technique of X-ray diffraction at room temperature. The structure is monoclinic, space group I2/m. Temperature-induced phase transitions in this compound were investigated by Raman spectroscopy up to 550 °C. Two low-wavenumber modes corresponding to external lattice vibrations merge at temperature of around 100 °C, indicating a phase transition from the monoclinic (I2/m) to the tetragonal (I4/m) structure. At 300 °C, changes in the slopes of temperature dependencies of external and O–Te–O bending modes are detected and interpreted as a second phase transition from the tetragonal (I4/m) to the cubic (Fm-3m) structure.