Browsing by Author "Gateshki, M"

Now showing 1 - 7 of 7
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    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.
  • No Thumbnail Available
    Item
    Influence of the surface roughness on the properties of Au films measured by surface plasmon resonance and x-ray reflectometry.
    (Elsevier, 2011-01-31) Velinov, T; Ahtapodov, L; Nelson, A; Gateshki, M; Bivolarska, M
    Thickness and refractive index of Au films thermally evaporated onto glass substrates and with an underlayer of Cr are determined from surface plasmon resonance. The results for the thickness are found to agree very well with those from X-ray reflectivity when a simple model of layers with flat interfaces is used. Plasmon propagation along thin films is influenced by radiative damping due to scattering from surface roughness. To study this influence the surface roughness of the glass substrate, Cr an Au layers are measured by X-ray reflectometry and the results used to introduce three intermediate layers with effective refractive indices and thicknesses corresponding to the roughness. Then Fresnel's equations are used to fit the reflectivity and to deduce the layer properties. It is found that the roughness affects to a great extent the optical parameters of the Au films even when it is smaller than 1 nm. In particular, the absolute value of real part of the dielectric constant decreases while its imaginary part increases when those effects are not taken into account. © 2011, Elsevier Ltd.
  • No Thumbnail Available
    Item
    PbZr1-xTixO3 by soft synthesis: structural aspects
    (American Physical Society, 2007-07) Pradhan, SK; Gateshki, M; Niederberger, M; Ren, Y; Petkov, V
    The structural aspects of a soft synthetic route employed to obtain fine crystallite PbZr1−xTixO3 (x=0,0.5,1) powders are revealed by total x-ray diffraction and atomic pair distribution function analysis. It is found that the atomic-scale structure of the intermediate, highly disordered phase the route passes through resembles but is not exactly of the targeted, perovskite-type structure. It is suggested that future synthesis efforts are directed toward closing the observed “structure gap” as much as possible. © 2007, American Physical Society
  • No Thumbnail Available
    Item
    Structural, magnetic and electronic properties of LaNi0.5Fe0.5O3 in the temperature range 5-1000 K
    (Elsevier, 2008-08) Gateshki, M; Suescun, L; Kolesnik, S; Świerczek, K; Short, SM; Dabrowski, B; Mais, J
    The structure, magnetism, transport and thermal expansion of the perovskite oxide LaNi0.5Fe0.5O3 were studied over a wide range of temperatures. Neutron time-of-flight data have shown that this compound undergoes a first-order phase transition between similar to 275 and similar to 310K. The structure transforms from orthorhombic (Pbnm) at low temperatures to rhombohedral (R (3) over barc) above room temperature. This phase transition is the cause for the previously observed co-existence of phases at room temperature. The main structural modification associated with the phase transition is the change of tilting pattern of the octahedra from a(+)b(-)b(-) at low temperatures to a(-)a(-)a(-) at higher. Magnetic data strongly suggests that a spin-glass magnetic state exists in the sample below 83 K consistent with the absence of magnetic ordering peaks in the neutron data collected at 30K. At high temperatures the sample behaves as a small polaron electronic conductor with two regions of slightly different activation energies of 0.07 and 0.05 eV above and below 553 K, respectively. The dilatometric data show an average thermal expansion coefficient of 14.7 x 10(-6) K-1 which makes this material compatible with frequently used electrolytes in solid oxide fuel cells. © 2008, Elsevier Ltd.
  • Thumbnail Image
    Item
    Structure of nanosize materials by high-energy x-ray diffraction: study of titanate nanotubes
    (Oldenbourg Wissenschaftsverlag, 2007-11) Gateshki, M; Chen, Q; Peng, LM; Chupas, P; Petkov, V
    High-energy X-ray diffraction and atomic Pair Distribution Function analysis are employed to determine the atomic-scale structure of titanate nanotubes. It is found that the nanotube walls are built of layers of Ti-O6 octahedra simular to those observed in crystalline layered titanates. In the nanotubes, however, the layers are bent and not stacked in perfect registry as in the crystal. © 2007, Oldenbourg Wissenschaftsverlag