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Browsing by Author "Withers, RL"

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A (3 + 3)-dimensional “hypercubic” oxide-ionic conductor: type ii bi2o3–nb2o5
(ACS Publications, 2013-04-09) Ling, CD; Schmid, S; Blanchard, PER; Petříček, V; McIntyre, GJ; Sharma, N; Maljuk, A; Yaremchenko, AA; Kharton, VV; Gutmann, MJ; Withers, RL
The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated ?hypercubic? structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an ?inflated? pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system. © 2013, American Chemical Society.
Ab initio structure determinations of high-performance radioactive waste forms
(Elsevier B. V., 2006-11-15) Carter, ML; Wallwork, KS; James, M; Withers, RL
Immobilisation of high-level radioactive wastes in stable matrices for long-term storage or geological disposal is a key step in closing the nuclear fuel cycle. Titanate minerals have shown much promise in their capacity to take up radioactive elements into their crystal structures at regular lattice sites in pyrochlore and brannerite phases. Previously unreported uranyl titanates BaUTiO6 and Na2U3Ti2O11 have been synthesised under argon at 1400 and 1250 deg. C, respectively and their structures determined by ab initio techniques using a combination of electron, synchrotron and neutron powder diffraction. BaUTiO6 crystallises with a carnotite-type structure (P21/c; a=6.4463(1) A, b=8.5999(1) A, c=10.2532(1) A; >=75.936(1)o and V=551.36(1) A3); while Na2U3Ti2O11 forms an orthorhombic structure (Pnma; a=31.0421(8) A, b=7.2774(2) A, c=7.6111(2) A and V=1719.4(1) A3). Due to their potential value as a radioactive waste forms, the leach rates of uranium, barium and sodium were determined. Normalised uranium leach rates from BaUTiO6 and Na2U3Ti2O11, averaged over 7 days were orders of magnitude less than those for brannerite and for U-containing pyrochlore waste forms. Crown Copyright © 2006 Published by Elsevier B.V
Ab initio structure determinations of high-performance radioactive waste forms
(The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Carter, ML; Wallwork, KS; James, M; Withers, RL
Immobilisation of high-level radioactive wastes in stable matrices for long-term storage or geological disposal is a key step in closing the nuclear fuel cycle. Titanate minerals have shown much promise in their capacity to take up radioactive elements into their crystal structures at regular lattice sites in pyrochlore and brannerite phases. Previously unreported uranyl titanates BaUTiO6 and Na2U3Ti2O11 have been synthesised under argon at 1400 and 1250 deg. C, respectively and their structures determined by ab initio techniques using a combination of electron, synchrotron and neutron powder diffraction. BaUTiO6 crystallises with a carnotite-type structure (P21/c; a=6.4463(1) A, b=8.5999(1) A, c=10.2532(1) A; >=75.936(1)o and V=551.36(1) A3); while Na2U3Ti2O11 forms an orthorhombic structure (Pnma; a=31.0421(8) A, b=7.2774(2) A, c=7.6111(2) A and V=1719.4(1) A3). Due to their potential value as a radioactive waste forms, the leach rates of uranium, barium and sodium were determined. Normalised uranium leach rates from BaUTiO6 and Na2U3Ti2O11, averaged over 7 days were orders of magnitude less than those for brannerite and for U-containing pyrochlore waste forms. © The Authors
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.
(Ca0.37Sr0.63)TiO3 perovskite - an example of an unusual class of tilted perovskites
(Institute of Physics, 2008-04-02) Howard, CJ; Withers, RL; Knight, KS; Zhang, Z
The structure of Ca0.37Sr0.63TiO3 perovskite has been carefully investigated using electron and neutron diffraction. Electron diffraction supports the supercell and Pbcm space group previously proposed, while high resolution neutron powder diffraction data provide the basis for a new structure refinement. The distortions of the resultant structure relative to the ideal parent perovskite structure type have been decomposed into normal modes. It is found that the primary modes of distortion are octahedral tilting modes associated with the R point (k = [1/2, 1/2, 1/2]∗) and the ξ = 1/4 point on the T line of symmetry (k = [1/2, 1/2, ξ]∗). There is a smaller secondary, probably induced, mode of distortion involving antiferroelectric displacements of the Ti cations associated with the ξ = 1/4 point on the T line of symmetry (k = [0, 0, ξ]∗). This compound provides one of the very few currently known examples of a tilted perovskite structure that shows tilting modes associated with the ubiquitous k = (1/2, 1/2, ξ)∗ modulation wavevectors for a value of ξ different from 0 and 1/2. © 2008, Institute of Physics
ChemInform abstract: synthesis and structural studies of lanthanide substituted bismuth—titanium pyrochlores
(Wiley, 2009-06-12) Ting, J; Kennedy, BJ; Withers, RL; Avdeev, M
Bi2/3Ln4/3Ti2O7 (Ln: La, Nd, Eu, Ho, Yb) pyrochlores are synthesized by solid state reaction of stoichiometric mixtures of Bi2O3, TiO2, and Ln2O3 (900, 1000, and 1100 °C, 48 h each) and characterized by powder XRD. The presence of six-fold disorder, associated with the Bi 6s2 lone pair electrons, is believed to stabilize the unexpected stoichiometry. © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Composition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4-xSrxNaSb3O12
(Elsevier, 2007-11) Zhou, QD; Kennedy, BJ; Elcombe, MM; Withers, RL
A series of 25 members of the 1:3 ordered perovskite family of the type Ba4-xSrxNaSb3O12 has been synthesized and their structures determined using synchrotron X-ray and neutron powder diffraction techniques. At room temperature the sample Ba4NaSb3O12 has a cubic structure in space group Im (3) over barm with a = 8.2821(1) angstrom, where the Na and Sb cations are ordered in the octahedral sites but there is no tilting of the (Na/Sb)O-6 octahedra. As the average size of the A-site cation decreases, through the progressive replacement of Ba by Sr, tilting of the octahedra is introduced firstly lowering the symmetry to tetragonal in P4/mnc then to orthorhombic in Cmca and ultimately a monoclinic structure in P2(1)/n as seen for Sr4NaSb3O12 with a = 8.0960(2) angstrom, b = 8.0926(2) angstrom, c = 8.1003(1) angstrom and β = 90.016(2)degrees. The powder neutron diffraction studies show that the orthorhombic and tetragonal phases in Cmca and P4/mnc co-exist at room temperature for samples with x between 1.5 and 2. © 2007, Elsevier Ltd.
A correlated electron diffraction, in situ neutron diffraction and dielectric properties investigation of poled (1-x)Bi0.5Na0.5TiO3-xBaTiO3 ceramics
(American Institute of Physics, 2011-10-15) Wang, J; Liu, Y; Withers, RL; Studer, AJ; Li, Q; Norén, L; Guo, YP
A correlated electron diffraction, temperature-dependent in situ neutron diffraction, and temperature-dependent dielectric properties investigation of poled (1-x)Bi(0.5)Na(0.5)TiO(3)-xBaTiO(3) (BNTBT100x) (x = 0.04, 0.07, and 0.12) samples has been carried out. The results show that the depolarization temperature, T(d), of the rhombohedral BNTBT 4 sample is associated with the disappearance of G +/- 1/2 [111]*(p) satellite reflections and a(-)a(-)a(-) octahedral tilting while that of the BNTBT 12 sample is associated with a metrically tetragonal to metrically cubic or pseudo-cubic symmetry. In the case of the poled BNTBT 7 sample in the MPB region, the dielectric properties show a quite distinct two stage transition from a room temperature clearly metrically tetragonal phase again to a metrically cubic or pseudo-cubic symmetry above 150 degrees C. There is no apparent change in its average structure in vicinity of T(d) in BNTBT 7. Electron diffraction shows the presence of considerable octahedral tilt twin disorder in all three samples. (C) 2011 American Institute of Physics.
Coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F
(Elsevier, 2009-05) Norén, L; Withers, RL; Goossens, DJ; Elcombe, MM; Kearley, GJ
The average, as well as the cation and anion 'disordered', crystal Structure of Li4NbO4F has been carefully investigated via coupled neutron and X-ray powder diffraction studies as well as via electron diffraction studies. The existence of a spectacular highly structured diffuse intensity distribution in the latter provides strong evidence for coupled Li1+/Nb5+ and O2-/F- ordering on the Na and Cl sites of the average NaCl structure of Li4NbO4F. Bond valence sum calculations have been used to investigate local crystal chemistry as well as to Suggest plausible local crystal chemical constraints while ab initio DFT based theoretical calculations of a 2 x 2 x 2 supercell have been carried Out in order to provide additional insight into the local crystal chemistry of this compound. © 2009, Elsevier Ltd.
Critical role of the coupling between the octahedral rotation and a-site ionic displacements in PbZrO3-based antiferroelectric materials investigated by in situ neutron diffraction
(American Physical Society, 2017-12-21) Lu, T; Studer, AJ; Yu, DH; Withers, RL; Feng, Y; Chen, H; Islam, SS; Xu, Z; Liu, Y
This in situ neutron-diffraction study on antiferroelectric (AFE) Pb0.99(Nb0.02Zr0.65Sn0.28Ti0.05)O3 polycrystalline materials describes systematic structural and associated preferred orientation changes as a function of applied electric field and temperature. It is found that the pristine AFE phase can be poled into the metastable ferroelectric (FE) phase at room temperature. At this stage, both AFE and FE phases consist of modes associated with octahedral rotation and A-site ionic displacements. The temperature-induced phase transition indicates that the octahedral rotation and ionic displacements are weakly coupled in the room-temperature FE phase and decoupled in the high-temperature FE phase. However, both temperature and E-field-induced phase transitions between the AFE and high-temperature FE phase demonstrate the critical role of coupling between octahedral rotation and A-site ionic displacements in stabilizing the AFE structure, which provides not only experimental evidence to support previous theoretical calculations, but also an insight into the design and development of AFE materials. Moreover, the associated preferred orientation evolution in both AFE and FE phases is studied during the phase transitions. It is found that the formation of the preferred orientation can be controlled to tune the samples’ FE and AFE properties. ©2017 American Physical Society - Open access
The crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8
(Institute of Physics, 2009-03-25) Hudspeth, JM; Goossens, DJ; Studer, AJ; Withers, RL; Norén, L
The crystal and magnetic structures of LaCa2Fe3O8 and NdCa2Fe3O8 have been established using a combination of x-ray, neutron and electron diffraction. It was already considered likely that LaCa2Fe3O8 and NdCa2Fe3O8 were made up of stacked perovskite-like layers of FeO6 octahedra, with every third layer being replaced by a layer of tetrahedrally coordinated Fe, rather like a variation on the Brownmillerite (Ca2Fe2O5) structure type. We have gone further and determined a likely space group for this Grenier phase and determined the magnetic structure of the compounds at room temperature. The space group is found to be P2(1)ma (b axis as the long axis), and the crystal structure has been refined, subject to the stacking faulting along the long axis that is apparent in electron diffraction patterns. The magnetic structure of LaCa2Fe3O8 is shown to consist of antiferromagnetically ordered Fe3+ ions on a collinear G-type antiferromagnetic structure, with the magnetic moments most likely (anti) parallel with the c axis, and of magnitude 3.4 +/- 0.2 mu(B) (3.6 +/- 0.2 mu(B) for NdCa2Fe3O8). The result is reasonable given the magnetic structures of the end members of the La1-xCaxFeO3 series, LaFeO3 (x = 0) and Ca2Fe2O5 (x = 1). © 2009, Institute of Physics
Crystal structures and phase transitions in a-site deficient perovskites Ln1/3TaO3
(American Chemical Society, 2008-11-11) Zhou, QD; Saines, PJ; Sharma, N; Ting, J; Kennedy, BJ; Zhang, Z; Withers, RL; Wallwork, KS
The synthesis and structures of the perovskites Ln1/3TaO3 are described. As the size of the Ln cation is reduced, the compounds display a sequence of structure: P4/mmm/La → Cmmm/Ce−Gd → Pmma/Tb, Dy → Pmc21/Ho, Er. Although apparently tetragonal in P4/mmm, electron diffraction patterns of Tm1/3TaO3 reveal this has a complex incommensurate structure. Likewise Gd1/3TaO3 appears metrically tetragonal, but electron diffraction and synchrotron X-ray powder diffraction demonstrate this is actually orthorhombic. The suppression of the spontaneous orthorhombic strain in Gd1/3TaO3 is thought to be due to the proximity to the first-order Cmmm−Pmma transition. Variable temperature studies show both Tb1/3TaO3 and Dy1/3TaO3 undergo a first-order Cmmm−Pmma transition upon heating. © 2008, American Chemical Society
Diffuse scattering in the cesium pyrochlore CsTi0.5W1.5O6
(Elsevier, 2008-04-01) Thorogood, GJ; Saines, PJ; Kennedy, BJ; Withers, RL; Elcombe, MM
The structure of the defect pyrochlore CsTi0.5W1.5O6 has been investigated using electron, synchrotron X-ray and neutron diffraction methods. The material is cubic a = 10.2773 angstrom with displacive disorder of the Cs cations along the < 1 1 1 > direction. The local structure, revealed by the diffuse structure in the electron diffraction patterns shows there is correlated displacement of the heavy Cs atoms along the < 1 1 0 > directions. The thermal expansion of the material is also described. © 2008, Elsevier Ltd.
Disordered structures and low temperature dielectric relaxation properties of two misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems
(Elsevier, 2007-09) Nguyen, HB; Norén, L; Liu, Y; Withers, RL; Wei, XY; Elcombe, MM
The disordered structures and low temperature dielectric relaxation properties of Bi1.667Mg0.70Nb1.52O7 (BMN) and Bi1.67Ni0.75Nb1.50O7 (BNN) misplaced-displacive cubic pyrochlores found in the Bi2O3–MIIO–Nb2O5 (M=Mg, Ni) systems are reported. As for other recently reported Bi-pyrochlores, the metal ion vacancies are found to be confined to the pyrochlore A site. The B2O6 octahedral sub-structure is found to be fully occupied and well-ordered. Considerable displacive disorder, however, is found associated with the O' A2 tetrahedral sub-structure in both cases. The A-site ions were displaced from Wyckoff position 16d (1/2, 1/2, 1/2) to 96 h (1/2, 1/2-éA, 1/2 + éA while the O' oxygen was shifted from position 8b (3/8, 3/8, 3/8) to Wyckoff position 32e (3/8 + éọ, 3/8 + éọ, 3/8 + éọ). The refined displacement magnitudes off the 16d and 8b sites for the A and O' sites were 0.408 Å/0.423 Å and 0.350 Å/0.369 Å for BMN/BNN, respectively. © 2007, Elsevier Ltd.
Effect of electric field and temperature on average structure and domain wall motion in 0.93Bi(0.5)Na(0.5)TiO(3)-0.07BaTiO(3) ceramic
(Hindawi Publishing Corporation, 2013-06-24) Wang, J; Liu, Y; Studer, AJ; Norén, L; Withers, RL
In situ neutron powder diffraction patterns and dielectric spectra of 0.93Bi0.5Na0.5TiO3-0.07BaTiO3 ceramic were investigated under different electrical fields and temperatures. An electric-field-induced phase transition from metrically cubic to metrically tetragonal, associated with strong domain wall motion, occurs. Such induced phase and domain wall motion are unchanged until the high-temperature phase transition occurs from metrically tetragonal to metrically cubic. All these changes are irrelevant to the observed depolarization temperature (75°C). The depolarization behaviour is thus suggested to be associated with the local structure caused by the octahedral tilt twinning disorder. © 2013, Jian Wang et al.
Energy and temperature dependence of rigid unit modes in AlPO4-5
(Royal Society of Chemsitry, 2015-07-17) Berlie, A; Kearley, GJ; Liu, Y; Yu, DH; Mole, RA; Ling, CD; Withers, RL
For structures that can be treated as networks of rigid, corner-connected polyhedra, the dominant distortion modes can be described by so-called rigid unit modes that are close to zero frequency. This type of behaviour is common in zeolitic/zeotypic materials such as the AlPO4 family of compounds and has been suggested by some authors to play a significant role in molecular diffusion within the pores of such compounds. We explore the energy and temperature dependence of these modes in AlPO4-5 using inelastic neutron scattering and heat capacity measurements. Ab initio based computational modelling is also used to assign the observed dynamic behaviour to rigid unit modes. We observe that these rigid unit modes persist down to very low temperatures and show no signs of freezing out.© 2015, the Owner Societies.
The formation of defect-pairs for highly efficient visible-light catalysts
(Wiley, 2017-01-23) Sun, QB; Cortie, DL; Zhang, SY; Frankcombe, TJ; She, GW; Gao, J; Sheppard, LR; Hu, WB; Chen, H; Zhuo, SJ; Chen, DH; Withers, RL; McIntyre, GJ; Yu, DH; Shi, WS; Liu, Y
Highly efficient visible-light catalysts are achieved through forming defect-pairs in TiO2 nanocrystals. This study therefore proposes that fine-tuning the chemical scheme consisting of charge-compensated defect-pairs in balanced concentrations is a key missing step for realizing outstanding photocatalytic performance. This research benefits photocatalytic applications and also provides new insight into the significance of defect chemistry for functionalizing materials. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Highly efficient visible light catalysts driven by Ti3+-VO-2Ti4+-N3− defect clusters
(Wiley, 2018-10-13) Sun, QB; Zhang, SY; Cortie, DL; Langley, J; Cox, N; Frankcombe, TJ; Gao, J; Chen, H; Withers, RL; Kremer, F; Yu, DH; Brink, F; Shi, WS; Liu, Y
Local defect structures play significant roles on material properties, but they are seriously neglected in the design, synthesis, and development of highly efficient TiO2-based visible light catalysts (VLCs). Here, we take anatase TiO2 nanocrystals that contain (Ti3+, N3−) ions and have the complicated chemical formula of (Ti1-x4+Tix3+)(O2-2-y-zNy3-□z) as an example, and point out that the formation of Ti3+-VO-2Ti4+-N3− local defect clusters is a key missing step for significantly enhancing VLC properties of host TiO2 nanocrystals. Experimental and theoretical investigations also demonstrate the emergent behaviors of these intentionally introduced defect clusters for developing highly efficient VLCs. This research thus not only provides highly efficient visible light catalysts for various practical applications but also addresses the significance of local defect structures on modifying material properties. © 2019 Wiley-VCH Verlag GmbH & Co.
Highly efficient visible light catalysts driven by Ti3+‐VO‐2Ti4+‐N3− defect clusters
(Wiley, 2018-10-13) Sun, QB; Zhang, SY; Cortie, DL; Langley, J; Cox, N; Frankcombe, TJ; Gao, J; Chen, H; Withers, RL; Kremer, F; Yu, DH; Brink, F; Shi, WS; Liu, Y
Local defect structures play significant roles on material properties, but they are seriously neglected in the design, synthesis, and development of highly efficient TiO2‐based visible light catalysts (VLCs). Here, we take anatase TiO2 nanocrystals that contain (Ti3+, N3−) ions and have the complicated chemical formula of ()(□z) as an example, and point out that the formation of Ti3+‐VO‐2Ti4+‐N3− local defect clusters is a key missing step for significantly enhancing VLC properties of host TiO2 nanocrystals. Experimental and theoretical investigations also demonstrate the emergent behaviors of these intentionally introduced defect clusters for developing highly efficient VLCs. This research thus not only provides highly efficient visible light catalysts for various practical applications but also addresses the significance of local defect structures on modifying material properties. © 1999-2024 John Wiley & Sons, Inc or related companies.
In-situ neutron diffraction study of Pb(In1/2Nb1/2)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 single crystals under uniaxial mechanical stress
(American Institute of Physics, 2012-04-26) Li, Q; Liu, Y; Luzin, V; Studer, AJ; Wan, YH; Li, ZR; Norén, L; Withers, RL; Xu, Z
In this paper, we report the phase transition behavior of ternary relaxor ferroelectric single crystals of 0.25Pb(In1/2Nb1/2)O-3-0.44Pb(Mg1/3Nb2/3)O-3-0.31PbTiO(3) subject to a uniaxial mechanical stress up to 400 MPa. The resultant in situ neutron diffraction data are interpreted in terms of the polarization rotation theory and provide direct structural evidence for the stress-induced polarization rotation pathway deduced from studies of macroscopic physical properties under stress. It is suggested that an intermediate, metastable orthorhombic phase is induced above a critical pressure of similar to 75 MPa. This critical stress level appears to be unaffected by sample poling although the ground states (at zero stress) for the poled and unpoled crystals are different. The critical stress level, however, does decrease with increasing temperature. The elastic behavior of the intermediate phases is also studied based on a calculation of the associated lattice strains. (C) 2012 American Institute of Physics.

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