Browsing by Author "Kuang, XJ"

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    Oxide ion conductivity, phase transitions, and phase separation in fluorite-based Bi38−xMo7+xO78+1.5x
    (American Chemical Society, 2010-08-10) Kuang, XJ; Li, YD; Ling, CD; Withers, RL; Evans, IR
    We present, for the first time, the ionic conductivity properties of two different, but closely related, bismuth molybdates: Bi38Mo7O78 and Bi37.5Mo7.5O78.75. Both are good oxide ion conductors, with the latter being comparable to yttria-stabilized zirconia. We show that the structure of Bi38Mo7O78 is more complex than previously reported, and that this compound is a 5 × 3 × 6 fluorite superstructure with slight monoclinic distortion. In addition to being a good oxide ion conductor, the material is noncentrosymmetric-polar and second harmonic generation (SHG) active. The second phase, orthorhombic Bi37.5Mo7.5O78.75, reported for the first time, is an excellent oxide ion conductor. The materials have been characterized by impedance spectroscopy, variable-temperature synchrotron, neutron and laboratory powder X-ray diffraction, electron diffraction, and SHG measurements. © 2010, American Chemical Society
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    Structural distortion and dielectric permittivities of KCoO2-type layered nitrides Ca1–xSrxTiN2
    (American Chemical Society, 2020-07-03) Lu, SL; Wang, YH; Lu, FQ; Feng, J; Lin, K; Xu, DM; Avdeev, M; Liu, LJ; Kuang, XJ; Xing, XR
    Among the KCoO2-type phases, the orthorhombic layered nitride CaTiN2 is a newly reported high dielectric permittivity material (εr ∼ 1300–2500 within 104–106 Hz from 80 to 450 K) while the tetragonal SrTiN2 is reported to display an unintentional metallic conduction property. In this work, a Ca1–xSrxTiN2 solid solution was synthesized, in which the insulating SrTiN2 end member and some Sr-doped CaTiN2 samples were successfully obtained, and therefore, the dielectric properties of the Ca1–xSrxTiN2 solid solution were investigated. The Sr substitution for Ca drove an orthorhombic-to-tetragonal phase transformation in Ca1–xSrxTiN2, which reduced the dielectric permittivity significantly. The tetragonal SrTiN2 displays a much lower dielectric permittivity (εr ∼ 20–70 in 105–106 Hz and 10–300 K) than that of CaTiN2. The comparison on the dielectric permittivities and structures of CaTiN2 and SrTiN2 indicates that the structural distortion arising from the splitting of N planes between Ti layers within the TiN2 pyramidal layers could be a plausible structural origin of the high bulk dielectric permittivity of CaTiN2. © 2020 American Chemical Society
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    Structural properties of the Nb-doped bismuth oxide materials, Bi1-xNbxO1.5+x
    (Australian Institute of Physics, 2015-02-03) Tate, ML; Hack, J; Kuang, XJ; McIntyre, GJ; Withers, RL; Johnson, MR; Evans, IR
    Bismuth oxide (Bi2O3) exists in five polymorphs, and possesses excellent oxide ion conductivity when in the cubic fluorite structure type, due to its intrinsic oxide ion vacancies. However, this cubic structure is only stable over a small high-temperature range. Introducing niobium into the bismuth oxide structure stabilises the highly conductive cubic and tetragonal phases to room temperature, allowing for high oxide ion conductivity at lower temperatures. In addition to stabilising the high temperature structure types, doping with niobium also introduces interstitial oxygen atoms into the material in order to maintain a charge balance. Niobium-doped bismuth oxide samples, Bi1-xNbxO1.5+x (x = 0.0625, 0.12), were synthesised by a solid state synthetic method, before undergoing AC impedance spectroscopy experiments to study their electrical properties. Both samples showed excellent oxide ion conductivities, with the cubic sample (x = 0.12) possessing higher conductivity values than the tetragonal sample (x = 0.0625). The tetragonal sample does not exhibit a loss in conductivity on thermal cycling, unlike the cubic sample, where the conductivity decreases due to a phase transformation from the cubic to the tetragonal phase. Variable temperature X-ray powder diffraction elucidated the structural transformations which the tetragonal bismuth niobate undergoes; from being tetragonal at room temperature, to cubic above 680 °C, then returning to the tetragonal phase upon cooling. To locate the interstitial oxygen atom positions in the tetragonal phase, powder neutron diffraction has been undertaken.