Browsing by Author "Marlton, FP"

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    Beyond the ionic radii: A multifaceted approach to understand differences between the structures of LnNbO4 and LnTaO4 fergusonites
    (Elsevier, 2023-01-05) Mullens, BG; Saura-Múzquiz, M; Marlton, FP; Avdeev, M; Brand, HEA; Mondal, S; Vaitheeswaran, G; Kennedy, BJ
    Synchrotron X-ray powder diffraction methods have been used to obtain accurate structures of the lanthanoid tantalates, LnTaO4, at room temperature. Three different structures are observed, depending on the size of the Ln cation: P21/c (Ln = La, Pr), I2/a (Ln = Nd-Ho), and P2/c (Ln = Tb-Lu). BVS analysis indicated that TaV is six-coordinate in these structures, with four short bonds and two longer bonds. Synchrotron X-ray powder diffraction methods were also used to observe the impact of Ta doping on the orthoniobates, Ln(Nb1-xTax)O4 (Ln = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Yb, and Lu). Where both the niobate and tantalate oxide were isostructural (fergusonite structure, space group I2/a), complete solid solutions were prepared. In these solid solutions, the unit cell volume decreases as the Ta content increases. The subtle interaction evident between the LnO8 and BO6 sublattices in the fergusonite-type oxides was not observed in the related pyrochlore oxides. A combined synchrotron X-ray and neutron powder diffraction study of the series Ho(Nb1-xTax)O4 was used to determine accurate atomic positions of the anions, and hence, bond lengths. This revealed a change in the (Nb/Ta)-O bond lengths, reflective of the difference in the valence orbitals of Nb(4d) and Ta(5d). Examination of the partial density of states demonstrates differences in the electronics between Nb and Ta, leading to a difference in the bandgap. This study highlights the importance of the long B-O contacts in the fergusonite structures, and its potential impact on the I2/a to I41/a phase transition. © 2022 Elsevier B.V.
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    Insight into the variations of ABO4 structures: combined experimental and computational studies
    (Australian Nuclear Science and Technology Organisation, 2021-11-23) Mullens, BG; Saura-Múzquiz, M; Marlton, FP; Brand, HEA; Avdeev, M; Kennedy, BJ
    The development of carbon-neutral energy-generation is critical to combatting climate change. One such technology is the development of next-generation ion conductors for solid-oxide fuel cells (SOFCs). SOFCs offer a more efficient method of extracting energy from hydrogen or hydrocarbon fuels than current combustion engines due to their one-step chemical process. However, a bottleneck to the large-scale uptake of SOFCs is the poor performance of the conducting electrolytes that separate the anode from the cathode. Various ABO4 structures have recently been proposed as solid electrolyte candidates in SOFCs, with increased hightemperature ionic conductivity being measured in chemically doped LaNbO4. However, the various phase transitions of these materials within the operational temperature of SOFCs makes them non-ideal. To understand the effects of chemical doping on the structure and electrochemical properties, several complex ABO4 structures have been investigated. In this work, we present the solid-solution series Ln(Nb1􀀀xTax)O4 (Ln = La-Lu). Using a combination of synchrotron X-ray and neutron powder diffraction methods, these studies have revealed several anomalies across the series. The structures appear to be sensitive to the size of the Ln cation and their synthesis conditions, with a difference in ionic conduction performance being observed. This experimental data has been further reinforced by ground state energy calculations performed using density functional theory. This is a landmark accomplishment that has not been previously used in similarly studied structures. These insights can be used in the development and engineering of novel and advanced electrolyte materials for SOFCs. © The Authors
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    Lattice disorder and oxygen migration pathways in pyrochlore and defect-fluorite oxides
    (International Union of Crystallography, 2021-08-14) Marlton, FP; Zhang, ZM; Zhang, YP; Proffen, TE; Ling, CD; Kennedy, BJ
    Pyrochlore oxides, with the general formula A2B2O7, are of considerable interest as catalysts for the oxygen evolution reaction[1-5], where A2Ru2O7-δ pyrochlores have recently emerged as state-of-the-art materials, and as photocatalysts for hydrogen evolution[6-8]. Fundamental to their reactivity is the local-scale vacancy ordering and mobility, which can be tailored through cation substitution[4]. The chemical and structural flexibility of pyrochlore oxides gives them a diverse range of physical and chemical properties leading to technological applications including as fast-ion conductors[9, 10], ferroelectrics[11], magnetism[12], oxide heterostructures[13, 14], and host matrices for the immobilization of actinide-rich nuclear wastes[15]. Atomic-scale disorder plays an important role in the chemical and physical properties of oxide materials. The structural flexibility of pyrochlore-type oxides allows for crystal-chemical engineering of these properties. Compositional modification can push pyrochlore oxides towards a disordered defect-fluorite structure with anion Frenkel pair defects that facilitate oxygen migration. The local structure of the long-range average cubic defect-fluorite was recently claimed to consist of randomly arranged orthorhombic weberitetype domains[16]. In this work we show, using low-temperature neutron total-scattering experiments, that this is not the case for Zrrich defect-fluorites. By analyzing data from the pyrochlore/defect-fluorite Y2Sn2-xZrxO7 series using a combination of neutron pair distribution function and big-box modelling, we have differentiated and quantified the relationship between anion sub-lattice disorder and Frenkel defects. These details directly influence the energy landscape for oxygen migration and are crucial for simulations and design of new materials with improved properties. © The Authors
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    Synthesis and crystal structures of two polymorphs of Li4–2xMg1+ xTeO6
    (Elsevier, 2020-07-01) Brown, AJ; Liu, JT; Marlton, FP; Avdeev, M; Kennedy, BJ; Ling, CD
    Two polymorphs of lithium magnesium tellurate Li4–2xMg1+xTeO6 have been prepared by solid-state reactions and their crystal structures characterised by powder X-ray and neutron diffraction. For x ​≈ ​0, a monoclinic C2/m phase is obtained, structurally similar to other O3 type honeycomb layered tellurate and antimonate compounds. The basic structure consists of [Mg2TeO6]3− honeycomb layers alternating with Li layers, with some anti-site disorder of Li and Mg between layers, analogous to the structure of Li4ZnTeO6. For 0 ​< ​x ​< ​~0.5 (specifically, x ​= ​0.33) an orthorhombic Fddd phase is obtained, with a rock-salt superstructure containing disordered Li/Mg cation sites surrounding ordered TeO6 octahedra, analogous to the structure of Li3Co2TaO6.© 2020 Elsevier Inc.
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    Understanding disorder in the Y2Sn2-xZrxO7 pyrochlore oxides
    (Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Marlton, FP; Kennedy, BJ; Ling, CD; Zhang, ZM
    Solid oxide fuel cells (SOFCs) offer clean alternatives to current carbon emitting energy sources; however, reducing their operation temperature is a major challenge for widespread application. Hence, the development of novel electrolyte materials with high room temperature ionic conductivity is crucial. Pyrochlore oxides, of the general chemical formula A2B2O7, exhibit chemical and structural flexibility, resulting in a diverse range of physical properties and technological applications, such as host matrices in the immobilization of actinide-rich nuclear wastes. In particular, they have gained interest as fast-ion conductors for electrolytes in SOFCs. The pyrochlore structure is highly ordered, which limits long-range oxygen diffusion. This can be altered via disordering, which results in the formation of oxygen Frenkel pairs that improve conductivity. The pyrochlore structure can adopt the disordered-fluorite structure via changes in composition, temperature, pressure and radiation. The disordered-fluorite exhibits lower formation energy of the Frenkel defect; however, this increase in structural disorder can increase the activation energies needed for long-range migration, which results in optimal conductivity occurring in partially disordered materials. Hence, the interplay between disorder and order in the atomic structure is key to the physical properties of these materials. There have been many studies dedicated to understanding the structural order and disorder in pyrochlore and disordered-fluorite oxides, with a recent study claiming the local structure of the disordered-flourite to be weberite. It was proposed that the overall structure of a disordered-fluorite consists of randomly arranged orthorhombic weberite domains that result in the long-range cubic disordered-fluorite.In this study we use low temperature (15 K) neutron pair distribution function (PDF) and big box modelling to understand the local-scale structure of the Y2Sn2-xZrxO7 system. We show that the local structure of the Y2Zr2O7 disordered fluorite does not contain ordered weberite domains, which emphasizes the importance of low temperature measurements in the local structure analysis of disordered oxide materials. Our analysis of the Y2Sn2-xZrxO7 series serves as a direct method for quantifying disorder and Frenkel defects. Understanding and quantifying these atomic-scale distortions is essential in simulations and design as it directly influences the energy landscape for anion mobility. These techniques can be used in the development and engineering of novel and advanced electrolyte materials for SOFCs. , © The authors.