Browsing by Author "Kennedy, BJ"
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- ItemAnion Disorder in Lanthanoid Zirconates Gd2-xTbxZr2O7(American Chemical Society., 2013-08-05) Reynolds, EM; Blanchard, PER; Kennedy, BJ; Ling, CD; Liu, S; Avdeev, M; Zhang, Z; Cuello, GJ; Tadich, A; Jang, LYThe pyrochlore?defect fluorite order?disorder transition has been studied for a series of oxides of the type Gd2?xTbxZr2O7 by a combination of diffraction and spectroscopy techniques. Synchrotron X-ray diffraction data suggest an abrupt transition from the coexistence of pyrochlore and defect fluorite phases to a single defect fluorite phase with increasing Tb content. However neutron diffraction data, obtained at ? ≈ 0.497 Å for all Gd-containing samples to minimize absorption, not only provide evidence for independent ordering of the anion and cation sublattices but also suggest that the disorder transition across the pyrochlore?defect fluorite boundary of Ln2Zr2O7 is rather gradual. Such disorder was also evident in X-ray absorption measurements at the Zr L3-edge, which showed a gradual increase in the effective coordination number of the Zr from near 6-coordinate in the pyrochlore rich samples to near 7-coordinate in the Tb rich defect fluorites. These results indicate the presence of ordered domains throughout the defect fluorite region, and demonstrate the gradual nature of the order?disorder transition across the Gd2?xTbxZr2O7 series. © 2013, American Chemical Society.
- ItemAnomalous lattice parameter increase in alkali earth aluminium substituted tungsten defect pyrochlores(Elsevier, 2009-03) Thorogood, GJ; Kennedy, BJ; Peterson, VK; Elcombe, MM; Kearley, GJ; Hanna, JV; Luca, VThe structures of the defect pyrochlores AAl(0.33)W(1.67)O(6) where A = K, Rb or Cs have been investigated using X-ray and neutron powder diffraction methods as well as the ab initio modelling program VASP The three cubic pyrochlores exhibit a non-linear increase in lattice parameter with respect to ionic radius of the A cation as a consequence of displacive disorder of the A-type cations. Solid state Al-27 MAS NMR studies of this pyrochlore system reveal shifts in the delta-21-22 ppm range that are indicative of pseudo-5 coordinate Al environments and emanate frorn distorted Al octahedral with one abnormally long Al. O bond. Solid state K-39, Rb-85, Rb-87 and Cs-133 MAS and static NMR Studies reflect the local cation disorder demonstrated in the structural Studies. © 2008, Elsevier Ltd.
- ItemAnomalous thermal expansion in orthorhombic perovskite SrIrO3: Interplay between spin-orbit coupling and the crystal lattice(Americal Physical Society, 2014-06-16) Blanchard, PER; Reynolds, EM; Kennedy, BJ; Kimpton, JA; Avdeev, M; Belik, AAThe structure of the orthorhombic (Pbnm) polytype of SrIrO3 has been investigated between 3 and 1100 K using a combination of synchrotron and neutron diffraction methods. The orthorhombic structure persists to 1100 K, the highest temperature available in this work. This is a consequence of the larger than expected octahedral tilting estimated from the neutron diffraction studies. We postulate that the strong spin-orbit coupling of the Ir4+ cation, which splits the t2g band, introduces additional strain on the lattice. This introduces unusual thermal expansion of the cell. SrIrO3 was characterized by resistivity, magnetization, and specific heat measurements. Metallic conductivity was observed between 2 and 300 K without indication of the previously reported metal-insulator transition. The Sommerfeld constant γ was 3.12(2) mJ mol−1 K−2, and a Fermi-liquid behavior was observed between 2 and 30 K with positive magnetoresistence of up to 2% (at 70 kOe and between 2 and 50 K).© 2014, American Physical Society.
- ItemAntiferromagnetism in a technetium oxide. Structure of CaTcO3(American Chemical Society, 2011-01-16) Avdeev, M; Thorogood, GJ; Carter, ML; Kennedy, BJ; Ting, J; Singh, DJ; Wallwork, KSThe technetium perovskite CaTcO3 has been synthesized. Combining synchrotron X-ray and neutron diffraction, we found that CaTcO3 is an antiferromagnetic with a surprisingly high Neel temperature of ~800 K. The transition to the magnetic state does not involve a structural change, but there is obvious magnetostriction. Electronic structure calculations confirm the experimental results. © 2011, American Chemical Society
- ItemThe application of synchrotron radiation induced X-ray emission in the measurement of zinc and lead in Wistar rat ameloblasts(Elsevier, 2007-10) Arora, M; Kennedy, BJ; Ryan, CG; Boadle, RA; Walker, DM; Harland, CL; Lai, B; Cai, ZH; Vogt, S; Zoellner, H; Chan, SWYThe development of analytical techniques for the measurement of trace elements in cellular compartments of developing teeth remains an important methodological issue in dental research. Recent advances in third generation synchrotron facilities have provided high brilliance X-ray sources that can be effectively used to study trace element distributions in small spatial regions with low detection limits. The present study describes for the first time the application of synchrotron radiation induced X-ray emission (SRIXE) in measuring the distribution of zinc and lead in the ameloblasts of developing Wistar rat teeth. Wistar rats were fed a standard rat diet, containing the normal dietary requirements of zinc, ad libitum and exposed to 100ppm of lead in drinking water. Resin embedded sections of first mandibular molars were analysed using a 13.3keV incident monochromatic X-ray beam focussed to a 0.2μm spot. Characteristic X-rays arising from the entire thickness of the sample were measured using an energy dispersive detector for quantitative analysis of elemental concentrations. The results showed that intranuclear concentrations of zinc were greater than levels in the cytoplasm. Furthermore, nuclear and cytoplasmic concentrations of zinc in the maturation stage (742±27 and 424±25ppm, respectively) were significantly higher than the zinc levels observed in the nucleus and cytoplasm of presecretory stage ameloblasts (132±10 and 109±10ppm, respectively) (p<0.05). A clear lead signal above the background was not detected in the ameloblasts and lead concentrations could only be reliably measured in the developing enamel. Overall, SRIXE was an effective method of studying the spatial distribution of zinc in the cells of developing teeth and offered a unique combination of sub-micron spatial resolution and parts-per-million detection limits (0.8–1 and 0.6–1ppm for zinc and lead, respectively). © 2007, Elsevier Ltd.
- ItemAverage and local ordering of Yb2(Ti2-xYbx)O7-x/2 ‘stuffed’ pyrochlores: the development of a robust structural model(Elsevier, 2021-10-01) Mullens, BG; Zhang, Z; Avdeev, M; Brand, HEA; Cowie, BCC; D'Angelo, AM; Múzquiz, MS; Kennedy, BJThe long-range (average) and short-range (local) structures in the Yb2(Ti2-xYbx)O7-x/2 (x = 0.00–0.67) series were studied using a combination of diffraction and spectroscopic techniques. The average structure, established from synchrotron X-ray and neutron powder diffraction data, shows the development of multiphase regions from x = 0.134 and the formation of anti-site disorder from x = 0.335. The local structure, established from X-ray absorption near-edge structure (XANES), shows a gradual evolution of short-range disorder. The crystal field splitting energy of the Ti4+ ions decreases from 2.15 to 1.91 eV with increasing Yb3+ content, reflecting the increase in coordination number from 6 to predominantly 7. Electrochemical impedance spectroscopic studies show an increase in oxygen ionic conductivity by almost a factor of 3 upon doping with small amounts of Yb3+ (x = 0.067). These results imply that the disordering across the anion and cation sublattices are different and inducing small amounts of disorder into the pyrochlore structure may lead to applications in solid-oxide fuel cells. © 2021 Elsevier Inc.
- ItemCation antisite disorder in uranium-doped gadolinium zirconate pyrochlores(Elsevier, 2014-09-01) Gregg, DJ; Zhang, ZM; Thorogood, GJ; Kennedy, BJ; Kimpton, JA; Griffiths, GJ; Guagliardo, PR; Lumpkin, GR; Vance, ERThe incorporation of uranium into gadolinium zirconate (Gd2Zr2O7) is investigated by synchrotron X-ray powder diffraction and X-ray absorption near-edge structure (XANES) spectroscopy. The results suggest that the uranium cation is largely located on the pyrochlore B-site instead of the targeted A-site. Cation disorder in Gd2Zr2O7 and U-doped Gd2Zr2O7 is investigated by positron annihilation lifetime spectroscopy (PALS) which demonstrates the absence of cation vacancies in these systems. This work provides direct evidence for cation antisite (A- and B-site mixing) disorder in U-doped and off-stoichiometric Gd2Zr2O7 pyrochlore. © 2014, Elsevier B.V.
- ItemCation disorder in NaW2O6+δ·nH2−zO post-ion exchange with K, Rb, Sr, and Cs(Pergamon-Elsevier Science Ltd, 2011-06-01) Thorogood, GJ; Kennedy, BJ; Avdeev, M; Peterson, VK; Hanna, JV; Luca, VThe structure of the defect pyrochlore NaW2O6+delta center dot nH(2-z)O after ion exchange with K, Rb, Sr or Cs for Na has been investigated using thermal analysis, solid-state nuclear magnetic resonance, laboratory X-ray and neutron diffraction methods. Neutron diffraction studies show that both the A-type cations (Na+, K+, Rb+, and/or Cs+) and the water molecules reside within the channels that form in the 111 direction of the W2O6 framework and that these strongly interact. The analytical results suggest that the water and A-type cations compete for space in the tunnels within the W2O6 pyrochlore framework, with the total number of water molecules and cations being approximately constant in the six samples investigated. The interplay between the cations and water explains the non-linear dependence of the a lattice parameter on the choice of cation. It appears that the ion-exchange capacity of the material will be controlled by the amount of water initially present in the sample. Crown Copyright (C) 2011 Published by Elsevier Ltd. All rights reserved.
- ItemCation order and magnetic behaviour in mixed metal bismuth scheelite Bi3FeMo2O12(International Union of Crystallography, 2021-08-14) Saura-Múzquiz, M; Mullens, BG; Liu, J; Vogt, T; Maynard-Casely, HE; Avdeev, M; Kennedy, BJThe scheelites are a family of compounds with chemical formula ABO4, and a characteristic crystal structure consisting of AO8 dodecahedra and BO4 tetrahedra. This structure is flexible and can accommodate a large variety of cations with a range of atomic radii and valence combinations. Scheelite-type oxides, such as CaWO4, BiVO4 and NaLa(MoO4)2 have been extensively studied due to their diverse range of physical and electronic properties . In particular, Bi3+ containing molybdates have been found to be efficient photocatalysts due to the strong repulsive force of the 6s2 lone pair of Bi3+, resulting in distortion of the BO4 tetrahedra and alteration of the band gap [2, 3]. In 1974 Bi3FeMo2O12 (BFMO) was reported as the first scheelite-type compound containing trivalent cations on the tetrahedral sites . Interestingly, two different polymorphs of BFMO can be isolated by varying the synthesis conditions . The tetragonal scheelitetype polymorph, described by space group I41/a with a = 5.32106(13) Å and c = 11.656(4) Å, can be prepared by a sol-gel route from aqueous solution of the constituent ionic species and has a disordered arrangement of the Fe and Mo cations. When heated above 500 °C, a 2:1 ordering of the Mo and Fe cations occurs, which lowers the symmetry to monoclinic (C2/c). The corresponding superstructure has a tripling of the a axis (a = 16.9110 (3) Å, b = 11.6489(2) Å, c= 5.25630(9) Å, β = 107.1395(11)°). The two structures are illustrated in Figure 1. In the present study, both polymorphs of BFMO were synthesized and their structure and magnetic properties characterized using a combination of powder diffraction, microscopy and magnetometry techniques. In situ neutron powder diffraction (NPD) measurements of the structural evolution of disordered tetragonal BFMO with increasing temperature showed that no amorphization takes place prior to the formation of the ordered monoclinic phase. The lack of a structural break-down, despite the substantial cation movement required in such a transformation, suggests that a certain degree of local cation order exists in the “disordered” tetragonal phase, facilitating the direct conversion to the fully ordered monoclinic structure. Instead of the expected amorphization and recrystallization, the conversion takes place via a 1st order phase transition, with the tetragonal polymorph exhibiting negative thermal expansion prior to its conversion into the monoclinic structure. Zero-field-cooled/field-cooled and field-dependent magnetization curves of the monoclinic structure revealed the existence of a magnetic transition below 15 K. The long-range nature of the lowtemperature magnetic structure in the monoclinic polymorph was verified by high-resolution NPD data, which revealed the emergence of an incommensurate magnetic structure. There is no evidence for long-range magnetic order in the tetragonal polymorph. This is, to the best of our knowledge, the first study of the phase transition mechanism and magnetic properties of this complex system and represents a milestone in the structural understanding and targeted design of Bi3+ containing molybdates as efficient photocatalysts. © The Authors
- ItemCation order in mixed metal bismuth scheelite Bi3FeMo2O12(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Saura-Múzquiz, M; Gaza, M; Maynard-Casely, HE; Kennedy, BJThe scheelites are a family of compounds with chemical formula AB O4 where A and B can represent a variety of different cations. The highly versatile scheelite crystal structure consists of A O8 dodecahedra and B O4 tetrahedra and gives rise to a variety of interesting properties depending on the combination of cations.1 Scheelite-type oxides including CaWO4, BiVO4 and NaLa(MoO4)2 have been extensively studied for applications exploiting some of these properties including luminescence, ferroelectricity, ionic conductivity and photocatalytic activity. In particular, Bi3+ containing molybdates are efficient photocatalysts2, 3 due to the strong repulsive force of the 6s2 lone pair of Bi3+, resulting in distortion of the B O4 tetrahedra and alteration of the band gap. The compound of interest in the present study, Bi3FeMo2O12 (BFMO), was reported by Sleight et al. in 1974 as the first scheelite type compound containing trivalent cations on the tetrahedral site.4 Notably, two different polymorphs of BFMO can be isolated.5 The ideal tetragonal scheelite-type structure in space group I 41/ a (#88) can be prepared by a wet chemical route from aqueous solution of the constituent elements. Jeitschko et al. reported in 1975 that, when the tetragonal scheelite structure is heated above 600 C° for ~10 h, a 2:1 ordering of the Mo and Fe cations occurs, which lowers the symmetry to monoclinic in space group C 2/ c (#15), and gives rise to a tripling of the a axis. Here, phase pure BFMO in the disordered tetragonal structure was synthesized by a wet chemical route. The conversion from the disordered tetragonal to the ordered monoclinic structure was examined by in situ neutron powder diffraction in order to understand the temperature dependence of the phase transition and cation order in the mixed metal bismuth scheelite. The study shows no amorphization prior to the formation of the ordered monoclinic phase. Given the substantial cation movement involved in such a transformation, the lack of structural break-down suggests that a certain degree of local cation order may already exist in the tetragonal phase, facilitating the conversion into a fully ordered monoclinic structure. This hypothesis is further supported by an opening in the field-dependent magnetization curve of the tetragonal phase at 1.8 K. © The authors.
- ItemCation order/disorder and local structures in alkaline earth pyrochlores(International Union of Crystallography (IUCr), 2008-08-23) Thorogood, GJ; Kennedy, BJ; Peterson, VK; Elcombe, MM; Kearley, GJ; Hanna, JVMaterials that form the A2-yB2O7-x pyrochlore structure have various applications including use as catalysts, fuel cells, piezoelectrics, ferroelectric devices and ferro-magnets; have a wide range of electrical and ionic conductivities, including metallic, semi and super; can be used in nuclear-waste immobilisation due to radiation toughness, and have ion exchange properties. The degree of disorder of the A-cation is important and may play the major role in the effectiveness of these materials ion exchange properties; and contribute to the high relative permittivities displayed by (Bi,Zn)2(Zn,Nb)2O7. Previous Neutron and X-ray diffraction studies suggest that the disorder involves displacement of the A-cation along the six <112> or <110> directions. Our observation of diffuse scattering in electron diffraction patterns of CsTi0.5W1.5O6 pyrochlores suggests there are strong local correlations among the disordered ions. Movement of O at 48f away from 0.375 reduces the interaction between the two networks and so may increase the amount of disorder of the A-cation. The structures of the defect pyrochlores AAl0.33W1.67O6 where A= K, Rb or Cs have been investigated using an array of advanced structural probes; X-ray and neutron diffraction methods, NMR spectroscopy as well as the ab-initio modeling using VASP. The structures do not show a simple correlation between the radius of the A-type cation and the cubic lattice parameter. Our structural studies suggest that this may reflect the degree of local disorder of the A-cation. The results of these studies will be presented in this presentation. © 2008 International Union of Crystallography
- ItemChemical, magnetic and orbital order in the substituted double perovskite Sr(1-x)Ca(x)Mn0.5Ru0.5O3(American Physical Society, 2010-03) Ricciardo, RA; Woodward, PM; Cuthbert, HL; Zhou, QD; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LYThe structural and magnetic properties of Sr(1x)Ca(x)Mn0.5Ru0.5O3 have been investigated. SrMn0.5Ru0.5O3 exhibits antiferromagnetic ordering, TN ≈200 K. Neutron powder diffraction of this perovskite indicates the presence of orbital ordering of the occupied Mn+3 dz2 orbitals, stabilizing the spin ordering corresponding to the AFM C-type structure. The substitution of smaller Ca+2 for Sr+2 on the A-site induces a change in the octahedral tilt system, (a0a0c−) to (a−b+a−) forcing a loss of this type of orbital ordering for x ≥ 0.2. This is accompanied by a crossover to a ferromagnetic ground state with 200K ≤ TC ≤ 300K even in the absence of long range chemical order. Magnetic data and neutron powder diffraction of CaMn0.5Ru0.5O3 and Sr0.5Ca0.5Mn0.5Ru0.5O3 reveal reduced magnetic moments and imply a complex magnetic behavior as well as a complete loss of orbital ordering for CaMn0.5Ru0.5O3. X-ray absorption studies show increased electron transfer from Mn to Ru. © 2021 American Physical Society
- ItemChemInform abstract: structural studies of the disorder and phase transitions in the double perovskite Sr2YTaO6(Wiley, 2010-10-07) Zhou, Q; Kennedy, BJ; Avdeev, MThe evolution of the crystal structure of the title compound is monitored from room temperature to 1250 °C by powder neutron and synchrotron XRD. At 50 °C Sr2YTaO6 crystallizes with a monoclinic superstructure in the space group P21/n, at 1000 °C in the tetragonal space group I4/m, and at 1200 °C in the cubic space group Fm equation image m. The tilting of the octahedra which exists in the room temperature structure is progressively lost upon heating. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
- ItemChemInform abstract: synthesis and structural studies of lanthanide substituted bismuth—titanium pyrochlores(Wiley, 2009-06-12) Ting, J; Kennedy, BJ; Withers, RL; Avdeev, MBi2/3Ln4/3Ti2O7 (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
- ItemCoexistence of ferroelectricity and magnetism in transition-metal-doped n = 3 aurivillius phases(Institute of Physics, 2008-01-16) Sharma, N; Kennedy, BJ; Elcombe, MM; Liu, Y; Ling, CDMagnetic-cation-doped three-layer Aurivillius phases Bi2-xSr2+x(Nb/Ta)(2+x)M1-xO12, x approximate to 0.5 and M = Ru4+, Ir4+ or Mn4+, are shown to have the same orthorhombic space group symmetry and similar dielectric and ferroelectric properties as their (non-magnetic) ferroelectric parent compounds Bi2-xSr2+xNb2+xTi1-xO12, x = 0, 0.5. The magnetic-cation-doped phases also show evidence for short-range ferromagnetic (M = Mn) and antiferromagnetic (M = Ru and Ir) exchange, demonstrating the potential of these naturally layered phases as templates for multiferroic (magnetoelectric) materials. © 2008, Institute of Physics
- ItemComparison of thermal expansion of Tc and Re salts(Materials Research Society (MRS), 2017-10-29) Thorogood, GJ; Kennedy, BJ; Reynolds, EM; Poineau, F; Yu, M; Injac, S99Tc is the most significant long-lived product of uranium fission, producing the largest fraction of the total long-lived radiation emissions of nuclear waste. Tc 7+ compounds are highly mobile in the environment. Relatively little is known regarding the solid state chemistry of Tc. Recently we studied the structural properties of (NH4)TcO4 (Tc7+) and confirmed that this is isostructural with (NH4)ReO4 adopting a tetragonal scheelite type structure in space group I41/a. The unit cell parameters of (NH4)TcO4 are strongly temperature dependent with the structure showing negative thermal expansion along both the a- and c-axis, albeit at different temperatures This behavior is significantly different to that previously reported for the isostructural oxide (NH4)ReO4, although we note that the data for (NH4)ReO4 was collected at much lower resolution. Nevertheless it is clear in the literature that the thermal expansion behavior of (NH4)ReO4 is highly anisotropic. Despite the difference in the thermal expansion between what we have observed for (NH4)TcO4 and that described by others for (NH4)ReO4 it is likely that the origin of the anomalous thermal expansion in is the same in both cases, namely it is a consequence of re-orientation of the ammonium ions in the surrounding cage of eight oxygen atoms. To verify this for (NH4)TcO4 requires we replace the ammonium cation with another small cation. Therefore we have compared the thermal behavior of AReO4 with ATcO4 to determine if Re oxides are suitable surrogates to predict the behavior of Tc oxides and how they may behave in the environment.
- ItemComplex 5d magnetism in a novel S= 1/2 trimer system, the 12L hexagonal perovskite ba4biir3o12(American Chemical Society, 2013-10-21) Miiller, W; Dunstan, MT; Huang, Z; Mohamed, Z; Kennedy, BJ; Avdeev, M; Ling, CDThe 12L hexagonal perovskite Ba4BiIr3O12 has been synthesized for the first time and characterized using high-resolution neutron and synchrotron X-ray diffraction as well as physical properties measurements. The structure contains Ir3O12 linear face-sharing octahedral trimer units, bridged by corner-sharing BiO6 octahedra. The average electronic configurations of Ir and Bi are shown to be +4(d5) and +4(s1), respectively, the same as for the S = 1/2 dimer system Ba3BiIr2O9, which undergoes a spin-gap opening with a strong magnetoelastic effect at T* = 74 K. Anomalies in magnetic susceptibility, heat capacity, electrical resistivity, and unit cell parameters indeed reveal an analogous effect at T* ≈ 215 K in Ba4BiIr3O12. However, the transition is not accompanied by the opening of a gap in spin excitation spectrum, because antiferromagnetic coupling among S = 1/2 Ir4+ (d5) cations leads to the formation of a S = 1/2 doublet within the trimers, vs S = 0 singlets within dimers. The change in magnetic state of the trimers at T* leads to a structural distortion, the energy of which is overcompensated for by the formation of S = 1/2 doublets. Extending this insight to the dimer system Ba3BiIr2O9 sheds new light on the more pronounced low-temperature anomalies observed for that compound. © 2013, American Chemical Society.
- ItemComposition- and temperature-dependent phase transitions in 1:3 ordered perovskites Ba4-xSrxNaSb3O12(Elsevier, 2007-11) Zhou, QD; Kennedy, BJ; Elcombe, MM; Withers, RLA 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.
- ItemControlling oxygen defect formation and its effect on reversible symmetry lowering and disorder-to-order phase transformations in nonstoichiometric ternary uranium oxides(American Chemical Society, 2019-04-09) Murphy, GL; Wang, CH; Zhang, Z; Kowalski, PM; Beridze, G; Avdeev, M; Muránsky, O; Brand, HEA; Gu, QF; Kennedy, BJIn situ synchrotron powder X-ray diffraction measurements have demonstrated that the isostructural AUO4–x (A = alkaline earth metal cation) oxides CaUO4–x and α-Sr0.4Ca0.6UO4–x undergo a reversible phase transformation under reducing conditions at high temperatures associated with the ordering of in-plane oxygen vacancies resulting in the lowering of symmetry. When rhombohedral (space group R3̅m) CaUO4–x and α-Sr0.4Ca0.6UO4–x are heated to 450 and 400 °C, respectively, in a hydrogen atmosphere, they undergo a first-order phase transformation to a single phase structure which can be refined against a triclinic model in space group P1̅, δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x, where the oxygen vacancies are disordered initially. Continued heating results in the appearance of superlattice reflections, indicating the ordering of in-plane oxygen vacancies. Cooling ordered δ-CaUO4–x and δ-Sr0.4Ca0.6UO4–x to near room temperature results in the reformation of the disordered rhombohedral phases. Essential to the transformation is the generation of a critical amount of oxygen vacancies. Once these are formed, the transformation can be accessed continuously through thermal cycling, showing that the transformations are purely thermodynamic in origin. Stoichiometric structures of both oxides can be recovered by heating oxygen deficient CaUO4–x and α-Sr0.4Ca0.6UO4–x under pure oxygen to high temperatures. When heated in air, the amount of oxygen vacancy defects that form in CaUO4–x and α-Sr0.4Ca0.6UO4–x are found to correlate with the A site composition. The inclusion of the larger Sr2+ cation on the A site reduces defect–defect interactions, which increases the amount of defects that can form and lowers their formation temperature. The relative difference in the amount of defects that form can be understood on the basis of oxygen vacancy and U5+ disordering as shown by both ab initio calculations and estimated oxygen vacancy formation energies based on thermodynamic considerations. This difference in defect–defect interactions consequently introduces variations in the long-range ordered anionic lattice of the δ phases despite the isostructural relationship of the α structures of CaUO4–x and Sr0.4Ca0.6UO4–x. These results are discussed with respect to the influence the A site cation has upon anion defect formation and ordering and are also compared to δ-SrUO4–x, the only other material known to be able to undergo a reversible symmetry lowering and disorder-to-order transformation with increasing temperature. © 2019 American Chemical Society
- ItemCorrigendum to “Synthesis, structures, and phase transitions of barium bismuth iridium oxide perovskites Ba2BiIrO6 and Ba3BiIr2O9” [J. Solid State Chem. 183 (2010) 727–735](Elsevier, 2010-11-01) Ling, CD; Kennedy, BJ; Zhou, Q; Spencer, JR; Avdeev, MThe authors wish to draw the reader’s attention to an unfortunate series of typographical errors in this article. The space group of Ba2BiIrO6 at room temperature was incorrectly referred to throughout the text as R3 , The correct space group is R3c . which was used for all the Rietveld-refinements and to generate all the relevant tables and figures. © 2010 Elsevier Inc.