Browsing by Author "Evans, IR"
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- ItemBi(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, IROxide 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.
- ItemBi1−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, IRA 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.
- ItemEvidence of continuous pottery production during the late Byzantine period in the Studenica Monastery, a UNESCO World Heritage Site(Elsevier, 2019-05) Stojanović, S; Bikić, V; Miličić, L; Evans, IR; Scarlett, NVY; Brand, HEA; Damjanović-Vasilić, LA collection of 63 pottery shards excavated at the Studenica Monastery, Serbia, originating from two distinct cultural strata (late 12th until the late 13th century, and the 14th and the first half of 15th century) was subject of this work. Mineralogical and chemical composition of body and glaze and production technology of investigated pottery were determined combining optical microscopy, inductively coupled plasma-optical emission and wavelength dispersive X-ray fluorescence spectrometry, Fourier transform infrared and micro-Raman spectroscopy, high-resolution synchrotron powder X-ray diffraction and multivariate statistical analysis. In addition, clay rod with traces of glaze from the kiln found within the Monastery complex was investigated. The firing temperature was estimated at 600–700 °C for the most of cookware and at 800–1000 °C for tableware. Pottery, made of non-calcareous clay, was covered with transparent lead based glaze and copper and iron were used as colorants. Combining results of all used techniques no significant differences in mineralogical and chemical composition among samples from two cultural strata were identified indicating continuous pottery production process from 13th until 15th century in Studenica. © 2019 Elsevier B.V
- ItemExpanded chemistry and mixed ionic-electronic conductivity in vanadium-substituted variants of γ-Ba4Nb2O9(International Union of Crystallography, 2021-08-14) Brown, AJ; Schwaighofer, B; Avdeev, M; Johannessen, B; Evans, IR; Ling, CDTwo new compositional series with the previously unique γ-Ba4Nb2O9 type structure, γ-Ba4VxTa2-xO9 and γ-Ba4VxNb2-xO9 (x = 0-2/3), have been synthesised via solid-state methods. Undoped Ba4Ta2O9 forms a 6H-perovskite type phase, but with sufficient V doping the γ-type phase is thermodynamically preferred and possibly more stable than γ-Ba4Nb2O9, forming at a 200 °C lower synthesis temperature. This is explained by the fact that Nb5+ ions in γ-Ba4Nb2O9 simultaneously occupy 4-, 5- and 6-coordinate sites in the oxide sublattice, which is less stable than allowing smaller V5+ to occupy the former and larger Ta5+ to occupy the latter. We characterised the structures of the new phases using a combination of X-ray and neutron powder diffraction. All compositions hydrate rapidly and extensively (up to 1/3 H2O per formula unit) under ambient conditions, like the parent γ-Ba4Nb2O9 phase, and show moderate but improved mixed-ionic electronic conduction. At lower temperatures the ionic conduction is predominately protonic, while at higher temperatures it is dominated by oxide and electron-hole conduction.
- ItemExpanded chemistry and proton conductivity in vanadium-substituted variants of γ-Ba4Nb2O9(American Chemical Society, 2021-09-09) Brown, AJ; Schwaighofer, B; Avdeev, M; Johannessen, B; Evans, IR; Ling, CDWe have substantially expanded the chemical phase space of the hitherto unique γ-Ba4Nb2O9 type structure by designing and synthesizing stoichiometric ordered analogues γ-Ba4V1/3Ta5/3O9 and γ-Ba4V1/3Nb5/3O9 and exploring the solid-solution series γ-Ba4VxTa2–xO9 and γ-Ba4VxNb2–xO9. Undoped Ba4Ta2O9 forms a 6H-perovskite type phase, but with sufficient V doping the γ-type phase is thermodynamically preferred and possibly more stable than γ-Ba4Nb2O9, forming at a 200 °C lower synthesis temperature. This is explained by the fact that Nb5+ ions in γ-Ba4Nb2O9 simultaneously occupy 4-, 5-, and 6-coordinate sites in the oxide sublattice, which is less stable than allowing smaller V5+ to occupy the former two and larger Ta5+ to occupy the latter. The x = 1/3 phase γ-Ba4V1/3Ta5/3O9 shows greatly improved ionic conduction compared to the x = 0 phase 6H-Ba4Ta2O9. We characterized the structures of the new phases using a combination of X-ray and neutron powder diffraction. All compositions hydrate rapidly and extensively (up to 1/3 H2O per formula unit) in ambient conditions, like the parent γ-Ba4Nb2O9 phase. At lower temperatures, the ionic conduction is predominately protonic, while at higher temperatures it is likely other charge carriers make increasing contributions.© 2021 American Chemical Society
- ItemLocal structure, dynamics, and the mechanisms of oxide ionic conduction in Bi26Mo10O69(American Chemical Society, 2012-12-11) Ling, CD; Miiller, W; Johnson, MR; Richard, D; Rols, S; Madge, J; Evans, IRWe report the results of a computational and experimental study into the stabilized fluorite-type delta-Bi(2)O(3)-related phase Bi(26)Mo(10)O(69) aimed at clarifying the local and average structure, for which two distinct models have previously been proposed, and the oxide ionic diffusion mechanism, for which three distinct models have previously been proposed. Concerning the structure, we propose a new model in which some molybdenum atoms have higher coordination numbers than 4; that is, some MoO(5) trigonal bipyramids coexist with MoO(4) tetrahedra. This accounts for the additional oxygen required to achieve the nominal composition (a tetrahedrononly model gives Bi(26)Mo(10)O(68)) without invoking a previously proposed unbonded interstitial site, which we found to be energetically unfavorable. All these MoO(x) units are rotationally disordered above a first-order transition at 310 degrees C, corresponding to a first-order increase in conductivity. Concerning oxide ionic diffusion above that transition temperature, we found excellent agreement between the results of ab initio molecular dynamics simulations and quasielastic neutron scattering experiments. Our results indicate a mechanism related to that proposed by Holmes et al. (Chem. Mater. 2008, 20, 3638), with the role previously assigned to partially occupied interstitial oxygen sites played instead by transient but stable MoO(5) trigonal bipyramids and with more relaxed requirements in terms of the orientation and timing of the diffusive jumps. © 2012, American Chemical Society.
- ItemNew 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, IRThe 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
- ItemOxide ion and proton conductivity in highly oxygen-deficient cubic perovskite SrSc0.3Zn0.2Ga0.5O2.4(American Chemical Society, 2020-04-27) Fuller, CA; Berrod, Q; Frick, B; Johnson, MR; Avdeev, M; Evans, JSO; Evans, IRA series of Zn-substituted compounds, Sr2Sc1–xZnxGaO5–0.5x, based on the brownmillerite-type oxide ion conductor Sr2ScGaO5 have been synthesized, and a single-phase region has been identified at 0.4 ≤ x < 0.6. The structure and dynamics of Sr2Sc0.6Zn0.4GaO4.8 were investigated by X-ray and neutron diffraction, neutron total scattering and pair distribution function (PDF) analysis, impedance spectroscopy, and neutron spectroscopy. The material was found to be a highly disordered cubic perovskite with a remarkable level of oxygen deficiency across a large temperature range. These structural properties lead to an increase of oxide ion conductivity of about two orders of magnitude relative to the parent Sr2ScGaO5. The presence of proton conductivity and some water uptake was suggested by the impedance data and corroborated by thermogravimetric analysis (TGA), solid state nuclear magnetic resonance (NMR), variable temperature X-ray diffraction, and neutron spectroscopy. Both proton and oxide ion conductivity produced a measurable quasi-elastic neutron scattering (QENS) signal, and the onset of each dynamic process could be observed by monitoring the temperature dependence of the elastic and inelastic scattering intensities measured in fixed window scans. Neutron total scattering and PDF studies revealed a local structure that is markedly different from the perovskite average structure, and we propose that Sr2Sc0.6Zn0.4GaO4.8 contains a rare one-coordinate or terminal oxygen site. © 2020 American Chemical Society
- ItemOxide 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, IRWe 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
- ItemStructural 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, IRBismuth 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.
- ItemStructure and dynamics studies of the short strong hydrogen bond in the 3,5-dinitrobenzoic acid-nicotinic acid molecular complex(Royal Society of Chemistry, 2013-01-01) Ford, SJ; McIntyre, GJ; Johnson, MR; Evans, IRThe molecular complex between 3,5-dinitrobenzoic acid and nicotinic acid (35DBNA) has been studied by variable temperature single crystal X-ray and neutron diffraction (30 to 300 K) and ab initio molecular dynamics, in order to investigate the dynamics and any proton migration in this system, which exhibits structural similarities with the well-known proton migration material 3,5-dicarboxylic acid. The refined structures clearly indicate a significant degree of proton transfer in the short NHO hydrogen bond, contrary to the previous description of 35DBNA as an organic adduct without proton transfer. This behaviour is consistent with the difference between the pKa values of 3,5-dinitrobenzoic acid and the ring nitrogen atom in nicotinic acid. Complementary ab initio MD simulations at 400 K show the key proton hopping across the NHO short hydrogen bond, spending short periods along the trajectory (8% of the simulation time) bonded to the O atom. Similar simulations performed on 3,5-dicarboxylic acid and 3,4-dicarboxylic acid show that the MD calculations correlate well with the experimental observations (or absence) of proton migration, and therefore suggest that they could be used as a predictive tool for investigating this phenomenon in short strong hydrogen bonds. © 2013, Royal Society of Chemistry.
- ItemSynthesis 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, IRLone-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