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- Ezzahi, A., et al.
(författare)
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X-ray diffraction and Raman spectroscopy studies of BaSrMWO6 (M=Ni, Co, Mg) double perovskite oxides
- 2011
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Ingår i: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 985:2-3, s. 339-345
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 5. |
- Manoun, B., et al.
(författare)
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Synthesis, Rietveld refinements and Raman spectroscopic studies of tricationic lacunar apatites Na1-xKxPb4(AsO4)(3) (0 <= x <= 1)
- 2011
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Ingår i: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 986:1-3, s. 1-9
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Tidskriftsartikel (refereegranskat)abstract
- Three compounds from the solid solution of lead lacunar ionic apatites Na1-xKxPb4(AsO4)(3), with x = 0, 0.5 and 1, were successfully synthesized as single phases by solid state method at 700 degrees C. The samples were characterized by X-ray diffraction, the site of the metal ions (Na+, K+ and Pb2+) in the solid solutions was analyzed with the Rietveld method. A variation of the a and c lattice parameters in the solid solutions was observed, with an increase of a and c parameters, related to the radius of the corresponding substituted ions. It was found that Pb(II) ions in the solid solutions preferentially occupied the M(1) and M(2) sites in the lacunar anionic apatite structure. The structure contains channels running along the c axis and centred at (0 0 z). The channels are most probably occupied by the lone electron pairs of the Pb2+ cations. Raman spectra of all the compositions are similar and show some linear shifts, especially for the bending modes, in band positions as a function of the composition toward lower values due the substitutions of Na+ by K+ with a larger radius. No considerable changes in the temperature dependence of the Raman modes and the corresponding FWHM are observed and thus no temperature induced phase transition is observed in Na0.5K0.5Pb4(AsO4)(3) up to 630 K.
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- Manoun, Bouchaib, et al.
(författare)
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X-ray diffraction and Raman spectroscopy studies of temperature and composition induced phase transitions in Ba2-xSrxZnWO6 (0 <= x <= 2) double perovskite oxides
- 2012
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 533, s. 43-52
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Manoun, B., et al.
(författare)
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High temperature induced phase transitions in Sr2ZnWO6 and Sr2CoWO6 double perovskite oxides : Raman spectroscopy as a tool
- 2012
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Ingår i: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 1029, s. 81-85
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Tidskriftsartikel (refereegranskat)abstract
- In this paper we report high-temperature (up to 550 °C) Raman spectroscopy of phase transitions in Sr2MWO6 (M = Zn, Co) double perovskite oxides. For Sr2ZnWO6 modes centered at 137 cm−1, 152 cm−1, merge around 80 °C. The same behavior was observed for the 436 cm−1 and 447 cm−1 modes. The mode centered at 854 cm−1 shows a change in the slope at the same temperature, thus indicating a first order phase transition from the monoclinic (P21/n) to tetragonal (I4/m) structure. With increasing temperature, a little change in the slope of the temperature dependence of mode centered at 855 cm−1 is observed around 480 °C which might be a sign of the second phase transition in Sr2ZnWO6 (tetragonal (I4/m) to cubic (Fm-3m)). The temperature dependence of the full width at half maximum (FWHM) of the most intense modes in Sr2ZnWO6 spectra was studied. The width behaves linearly as a function of temperature with a break observed in the slope of the width of the modes at 80 °C; thus, showing the first phase transition occurrence. For Sr2CoWO6 the transition from tetragonal (I4/m) structure to cubic (Fm-3m) phase is accompanied by considerable changes in the temperature dependence of modes centered at 438 cm−1 and 845 cm−1. The transition was observed around 420 °C.
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- Manoun, B., et al.
(författare)
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Synthesis, structure, and high temperature Mössbauer and Raman spectroscopy studies of Ba1.6Sr1.4Fe2WO9 double perovskite
- 2011
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 509:1, s. 66-71
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Tidskriftsartikel (refereegranskat)abstract
- Ba1.6Sr1.4Fe2WO9 has been prepared in polycrystalline form by solid-state reaction method in air, and has been studied by X-ray powder diffraction method (XRPD), and high temperature Mössbauer and Raman spectroscopies. The crystal structure was resolved at room temperature by the Rietveld refinement method, and revealed that Ba 1.6Sr1.4Fe2WO9 crystallizes in a tetragonal system, space group I4/m, with a = b = 5.6489(10), c = 7.9833(2) and adopts a double perovskite-type A3B′2B″O 9 (A = Ba, Sr; B′ = Fe/W, and B″ = Fe/W) structure described by the crystallographic formula (Ba1.07Sr 0.93)4d(Fe0.744W0.256) 2a(Fe0.585W0.415)2bO6. The structure contains alternating [(Fe/W)2aO6] and [(Fe/W)2bO6] octahedra. Mössbauer studies reveal the presence of iron in the 3+ oxidation state. The high temperature Mössbauer measurements showed a magnetic to paramagnetic transition around 405 ± 10 K. The transition is gradual over the temperature interval. The decrease in isomer shift is in line with the general temperature dependence. While the isomer shift is rather linear over the whole temperature range, the quadratic dipolar ΔE temperature dependence shows an abrupt change at 405 K. The latter results allow concluding that a temperature-induced phase transition had occurred. The high temperature Raman study confirms the Mössbauer results on the magnetic to paramagnetic transition.
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| 9. |
- Manoun, Bouchaib, et al.
(författare)
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X-ray diffraction and Raman spectroscopy studies of temperature and composition induced phase transitions in Ba2-xSrxMWO6 (M = Ni, Co and 0 <= x <= 2) double perovskite oxides
- 2013
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Ingår i: Journal of Molecular Structure. - : Elsevier BV. - 0022-2860 .- 1872-8014. ; 1045, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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