Browsing by Author "Igartua, JM"

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    Crystal structure and phase transitions of Sr2CdWO6.
    (Elsevier, 2007-08) Gateshki, M; Igartua, JM; Faik, A
    The crystal structure of Sr2CdWO6, prepared by solid state reaction, was determined by high-resolution X-ray diffraction at different temperatures. At room temperature, this compound has a monoclinic structure (space group P2(1)/n) with a = 5.7463(1), b = 5.8189(1), c = 8.1465(1), β = 90.071(1). At 1105 K the structure is converted to tetragonal (space group I4/m). Diffraction data also suggest that a cubic phase exists above 1220K. Comparing the phase transition temperatures of Sr2CdWO6 with those of other compounds of the Sr2MWO6 family reported previously, it was observed that the transition temperatures are higher in compounds with low-tolerance factors. At the same time, the temperature range in which the intermediate tetragonal phase exists is reduced. © 2007, Elsevier Ltd.
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    Crystal structures and cation ordering of Sr2AlSbO6 and Sr2CoSbO6.
    (Elsevier, 2008-08) Faik, A; Gateshki, M; Igartua, JM; Pizarro, JL; Insausti, M; Kaindl, R; Grzechnik, A
    The two double perovskite oxides Sr2AlSbO6 and Sr2CoSbO6 were prepared and their structures studied with the X-ray powder diffraction method. At room temperature the crystal structure of Sr2AlSbO6 is cubic (Fm (3) over barm), with a = 5.6058(1) angstrom. It was found that depending on the preparation conditions, the Al3+ gild Sb5+ cations can be either entirely or partially ordered. In the case of the partially ordered Sr2AlSbO6 sample, the extension of cation ordering was estimated from the hkl-dependent broadening of the diffraction peaks and the results were interpreted as evidence of the formation of anti-phase domains in the material. Low-temperature Raman spectroscopic measurements demonstrated that the cubic phase of Sr2AlSbO6 is stable down to 79 K. The room-temperature crystal structure of Sr2CoSbO6, is trigonal (space group R (3) over bar) with a = 5.6058(1) angstrom and c = 13.6758(3) angstrom. At 470 K, however, the material undergoes a continuous phase transition and its structure is converted to cubic (space group Fm (3) over barm). The studied Sr2CoSbO6 sample was partially ordered, but unlike Sr2AlSbO6, no indication of the formation of anti-phase domains was observed. © 2008, Elsevier Ltd.
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    High-pressure study of the Sr2CoWO6 ordered double perovskite tungstate oxide.
    (Elsevier, 2008-10-15) Manoun, B; Igartua, JM; Gateshki, M; Saxena, SK
    Using synchrotron radiation and Raman spectroscopy and a diamond anvil cell we measured the pressure dependence of the lattice parameters and Raman modes of polycrystalline Sr2CoWO6. Angle-dispersive X-ray diffraction patterns were indexed and showed that at 2.2 GPa, the material transforms from the I4/m tetragonal structure to the P2(1)/n monoclinic structure. For pressure values between 2.2 and 12.7 GPa only monoclinic symmetries were found. We had difficulties with the convergence of the profile matching of the diffractogram. taken at 12.7 GPa, indicating that the second phase-transition took place. To get more informations about these structural changes, an in-situ Raman spectroscopic study was conducted to explore the pressure-induced phase-transition sequence of Sr2CoWO6 to pressures of 40.8 GPa at room-temperature. Group theory yields nine Raman-active modes for Sr2CoWO6 (I4/m), all the predicted bands are observed at ambient conditions (phase I). The experimental results indicate that structural changes are observed at 2.15 and 11.15 GPa, which we attribute to phase transitions; thus, giving rise to two new phases, named as phase II and phase III, respectively. In the 9.33-14.84 GPa interval a coexistence of phases II and III is observed. © 2007, Elsevier Ltd.
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    Synthesis, crystal structure and vibrational spectroscopy studies of the lacunar apatite NaPb2Ca2(PO4)3
    (University of Mohammed Premier Oujda in partnership with the Association CEMADES, 2015-05-21) Chari, A; Orayech, B; Faik, A; Igartua, JM; El Bouari, A
    Powder NaPb2Ca2(PO4)3 lacunar apatite was synthesized by the solid-state reaction method, and its crystal structure was investigated by Rietveld analysis. NaPb2Ca2(PO4)3 material is hexagonal apatite at room temperature, adopting the space group P63/m (ITA No. 176), a=b=9.6075(2)Å, c=7.0167(1)Å, Z=2. Rietveld refinements showed that the site 4f is shared by three cations Ca, Pb and Na; while, the 6h is occupied by the Pb and Na cations. The structure can be described as built up from the PO4tetrahedra and the sixfold coordination cavities, which delimit hexagonal tunnels along the c-axis direction. These tunnels are linked by the cations occupying the 4f sites. Raman spectroscopy analysis has been carried out. The observed frequencies were assigned and discussed on the basis of unit-cell group analysis and by comparison to other apatite-type materials.