Browsing by Author "Tate, ML"

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    Bi(III)-containing lanthanum germanium apatite-type oxide ion conductors and their structure-property relationships
    (Australian Institute of Physics, 2016-02-04) Tate, ML; McIntyre, GJ; Evans, IR
    Oxide ion conductors are used in a wide variety of applications, including oxygen sensors and separation membranes, but are undergoing significant study for their use in solid oxide fuel cells (SOFCs), which allow for the direct conversion of chemical to electrical energy. Apatite-type silicates and germanates, La9.33+x(TO4)6O2+3x/2 (T = Si, Ge), have exhibited high oxide ion conductivities, potentially allowing for their use in SOFCs. Apatite-type compounds have the general formula, [AI4][AII6][TO4]6X2±δ, (A = alkaline or rare earth metal, or Pb; T = Ge, Si, P, V; X = O, OH, halides) and can be thought of as comprised of a framework of AI4(TO4)6 with flexible cavities containing AII6X2 units. The structures of apatite-type materials are primarily hexagonal, with the remainder being monoclinic, with several triclinic examples known. The origin of the triclinic structure is thought to be partly due to the size differences between the units comprising the framework and those within the cavities. The inclusion of interstitial oxide ions have been shown to promote the triclinic distortion, potentially caused by further expansion of the framework. Three novel Bi(III)-containing lanthanum germanium apatite compounds (Bi2La8[(GeO4)6]O3, Bi4Ca4La2[(VO4)2(GeO4)4]O2, and Bi4Ca2La4[(GeO4)6]O2) were synthesised by a solid state synthetic method, before undergoing AC impedance spectroscopy experiments to study their electrical properties. The Bi2La8[(GeO4)6]O3 compound has been identified as being the first bismuth containing apatite with a triclinic structure, whilst the Bi4-containing compounds possess hexagonal structures. All samples show high levels of conductivity, with the triclinic sample possessing higher conductivity values than the hexagonal samples at high temperature.
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    Bi1−xNbxO1.5+x (x=0.0625, 0.12) fast ion conductors: structures, stability and oxide ion migration pathways
    (Elsevier, 2015-05) Tate, ML; Hack, J; Kuang, X; McIntyre, GJ; Withers, RL; Johnson, MR; Evans, IR
    A combined experimental and computational study of Bi1−xNbxO1.5+x (x=0.0625 and 0.12) has been carried out using laboratory X-ray, neutron and electron diffraction, impedance measurements and ab-initio molecular dynamics. We demonstrate that Bi0.9375Nb0.0625O1.5625, previously reported to adopt a cubic fluorite-type superstructure, can form two different polymorphs depending on the synthetic method: a metastable cubic phase is produced by quenching; while slower cooling yields a stable material with a tetragonal √2×√2×1 superstructure, which undergoes a reversible phase transition into the cubic form at ~680 °C on subsequent reheating. Neutron diffraction reveals that the tetragonal superstructure arises mainly from ordering in the oxygen sublattice, with Bi and Nb remaining disordered, although structured diffuse scattering observed in the electron diffraction patterns suggests a degree of short-range ordering. Both materials are oxide ion conductors. On thermal cycling, Bi0.88Nb0.12O1.62 exhibits a decrease in conductivity of approximately an order of magnitude due to partial transformation into the tetragonal phase, but still exhibits conductivity comparable to yttria-stabilised zirconia (YSZ). Ab-initio molecular dynamics simulations performed on Bi0.9375Nb0.0625O1.5625 show that oxide ion diffusion occurs by O2− jumps between edge- and corner-sharing OM4 groups (M=Bi, Nb) via tetrahedral □M4 and octahedral □M6 vacancies. © 2015 Elsevier Inc.
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    New apatite‐type oxide ion conductor, Bi2La8[(GeO4)6]O3: structure, properties, and direct imaging of low‐level interstitial oxygen atoms using aberration‐corrected scanning transmission electron microscopy
    (Wiley, 2017-02-23) Tate, ML; Blom, DA; Avdeev, M; Brand, HEA; McIntyre, GJ; Vogt, T; Evans, IR
    The new solid electrolyte Bi2La8[(GeO4)6]O3 is prepared and characterized by variable‐temperature synchrotron X‐ray and neutron diffraction, aberration‐corrected scanning transmission electron microscopy, and physical property measurements (impedance spectroscopy and second harmonic generation). The material is a triclinic variant of the apatite structure type and owes its ionic conductivity to the presence of oxide ion interstitials. A combination of annular bright‐field scanning transmission electron microscopy experiments and frozen‐phonon multislice simulations enables direct imaging of the crucial interstitial oxygen atoms present at a level of 8 out of 1030 electrons per formula unit of the material, and crystallographically disordered, in the unit cell. Scanning transmission electron microscopy also leads to a direct observation of the local departures from the centrosymmetric average structure determined by diffraction. As no second harmonic generation signal is observed, these displacements are non‐cooperative on the longer length scales probed by optical methods. © 2017 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim
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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.
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    Synthesis and characterisation of new Bi (III)-containing apatite-type oxide ion conductors: the influence of lone pairs
    (Royal Society of Chemistry, 2017-09-04) Tate, ML; Fuller, CA; Avdeev, M; Brand, HEA; McIntyre, GJ; Evans, IR
    Lone-pair cations are known to enhance oxide ion conductivity in fluorite- and Aurivillius-type materials. Among the apatite-type phases, the opposite trend is found for the more widely studied silicate oxide ion conductors, which exhibit a dramatic decrease in conductivity on Bi(III) incorporation. In this work, the influence of lone-pair cations on the properties of apatite-type germanate oxide ion conductors has been investigated by preparing and characterising seven related compositions with varying Bi(III) content, by X-ray and neutron powder diffraction and impedance spectroscopy. All materials are very good oxide ion conductors (with conductivities of up to 1.29 × 10−2 S cm−1 at 775 °C). Increasing Bi(III) content leads to increases in conductivity by up to an order of magnitude, suggesting significant differences in the oxide-ion conduction mechanisms between lone-pair-containing apatite-type germanate and silicate solid electrolytes. © The Royal Society of Chemistry 2017