Browsing by Author "Jang, LY"

Now showing 1 - 17 of 17
  • No Thumbnail Available
    Item
    Anion 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, LY
    The 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.
  • No Thumbnail Available
    Item
    Chemical, 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, LY
    The 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
  • No Thumbnail Available
    Item
    Does local disorder occur in the pyrochlore zirconates?
    (American Chemical Socity, 2012-12-17) Blanchard, PER; Clements, R; Kennedy, BJ; Ling, CD; Reynolds, EM; Avdeev, M; Stampfl, APJ; Zhang, Z; Jang, LY
    The zirconates Ln2Zr2O7 (Ln = lanthanoid) have been studied using a combination of Zr L-edge X-ray absorption near edge structure (XANES) and synchrotron X-ray and neutron powder diffraction methods. These studies demonstrate that as the size of the lanthanoid cation decreases, the local structure evolves smoothly from the ideal pyrochlore toward the defect fluorite rather than undergoing an abrupt transformation. The Zr L-edge spectrum is found to be extremely sensitive to changes in the local coordination environment and demonstrates an increase in local disorder across the pyrochlore oxides. The sensitivity of the XANES measurements enables us to identify the progressive nature of the transition that could not be detected using bulk diffraction techniques. © 2012, American Chemical Society.
  • No Thumbnail Available
    Item
    Gradual structural evolution from pyrochlore to defect-fluorite in Y2Sn2-xZrxO7: average vs local structure
    (ACS Publications, 2013-11-25) Zhang, ZM; Middleburgh, SC; de los Reyes, M; Lumpkin, GR; Kennedy, BJ; Blanchard, PER; Reynolds, EM; Jang, LY
    We have studied the long-range average and local structures in Y2Sn2−xZrxO7 (x = 0−2.0) using synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, respectively, and by theoretical methods. While the diffraction data indicate a clear phase transition from ordered pyrochlore to disordered defectfluorite at x ∼ 1.0−1.2, X-ray absorption near-edge structure (XANES) results at the Zr L3- and Y L2-edges reveal a gradual structural evolution across the whole compositional range. These findings provide experimental evidence that the local disorder occurs long before the pyrochlore to defect-fluorite phase boundary, as determined by X-ray diffraction, and the extent of disorder continues to develop throughout the defectfluorite region. The Zr and Y L-edge spectra are very sensitive to changes in the local structure; such sensitivity enables us to reveal the progressive nature of the phase transition. Experimental results are supported by ab initio atomic scale simulations, which provide a mechanism for disorder to initiate in the pyrochlore structure. Further, the coordination numbers of the cations in both the defect-fluorite and pyrochlore structures are predicted, and the trends agree well with the experimental XANES results. The calculations predict that the coordination of cations in the Y2Zr2O7 defect-fluorite (normally considered to be 7 for all cations) varies depending on the species with the average coordination of Y and Zr being 7.2 and 6.8, respectively. © 2013, American Chemical Society.
  • No Thumbnail Available
    Item
    Investigating the order-disorder phase transition in Nd2-xYxZr2O7via diffraction and spectroscopy
    (Royal Society of Chemistry, 2013-01-01) Blanchard, PER; Liu, S; Kennedy, BJ; Ling, CD; Zhang, Z; Avdeev, M; Cowie, BCC; Thomsen, L; Jang, LY
    The pyrochlore-defect fluorite phase transition in the mixed-metal zirconate Nd2-xYxZr2O7 (0 <= x <= 2) solid solution was investigated using synchrotron X-ray and neutron diffraction, as well as X-ray absorption spectroscopy. Diffraction analysis revealed a two-phase region between 1.0 <= x <= 1.2. In the pyrochlore phase, Zr L-3-edge XANES analysis demonstrated a gradual change in the local coordination environment of the B site with increasing Y content that was consistent with an increase in disorder. Although Y L-3-edge XANES analysis suggested that the Y cations remained in an ordered coordination environment in the pyrochlore phase, disorder did gradually increase once the fluorite phase formed. It was found that Y cations prefer an ordered coordination environment near the phase boundary whereas Zr cations prefer a disordered coordination environment. © 2013, Royal Society of Chemistry
  • No Thumbnail Available
    Item
    Iron-monosulfide oxidation in natural sediments: resolving microbially mediated S transformations using XANES, electron microscopy, and selective extractions
    (American Chemical Society, 2009-05-01) Burton, ED; Bush, RT; Sullivan, LA; Hocking, RK; Mitchell, DRG; Johnston, SG; Fitzpatrick, RW; Raven, M; McClure, S; Jang, LY
    Iron-monosulfide oxidation and associated S transformations in a natural sediment were examined by combining selective extractions, electron microscopy and S K-edge X-ray absorption near-edge structure (XANES) spectroscopy. The sediment examined in this study was collected from a waterway receiving acid−sulfate soil drainage. It contained a high acid-volatile sulfide content (1031 μmol g−1), reflecting an abundance of iron-monosulfide. The iron-monosulfide speciation in the initial sediment sample was dominated by nanocrystalline mackinawite (tetragonal FeS). At near-neutral pH and an O2 partial pressure of 0.2 atm, the mackinawite was found to oxidize rapidly, with a half-time of 29 ± 2 min. This oxidation rate did not differ significantly (P < 0.05) between abiotic versus biotic conditions, demonstrating that oxidation of nanocrystalline mackinawite was not microbially mediated. The extraction results suggested that elemental S (S08) was a key intermediate S oxidation product. Transmission electron microscopy showed the S08 to be amorphous nanoglobules, 100−200 nm in diameter. The quantitative importance of S08 was confirmed by linear combination XANES spectroscopy, after accounting for the inherent effect of the nanoscale S08 particle-size on the corresponding XANES spectrum. Both the selective extraction and XANES data showed that oxidation of S08 to SO42− was mediated by microbial activity. In addition to directly revealing important S transformations, the XANES results support the accuracy of the selective extraction scheme employed here. © 2009, American Chemical Society
  • No Thumbnail Available
    Item
    Key role of Bismuth in the magnetoelastic transitions of Ba3BiIr2O9 and Ba3BiRu2O9 as revealed by chemical doping
    (America Chemical Society, 2013-12-24) Blanchard, PER; Huang, ZX; Kennedy, BJ; Liu, S; Miiller, W; Reynolds, EM; Zhou, QD; Avdeev, M; Zhang, ZM; Aitken, JB; Cowie, BCC; Jang, LY; Tan, TT; Li, S; Ling, CD
    The key role played by bismuth in an average intermediate oxidation state in the magnetoelastic spin-gap compounds Ba3BiRu2O9 and Ba3BiIr2O9 has been confirmed by systematically replacing bismuth with La3+ and Ce4+. Through a combination of powder diffraction (neutron and synchrotron), X-ray absorption spectroscopy, and magnetic properties measurements, we show that Ru/Ir cations in Ba3BiRu2O9 and Ba3BiIr2O9 have oxidation states between +4 and +4.5, suggesting that Bi cations exist in an unusual average oxidation state intermediate between the conventional +3 and +5 states (which is confirmed by the Bi L3-edge spectrum of Ba3BiRu2O9). Precise measurements of lattice parameters from synchrotron diffraction are consistent with the presence of intermediate oxidation state bismuth cations throughout the doping ranges. We find that relatively small amounts of doping (∼10 at%) on the bismuth site suppress and then completely eliminate the sharp structural and magnetic transitions observed in pure Ba3BiRu2O9 and Ba3BiIr2O9, strongly suggesting that the unstable electronic state of bismuth plays a critical role in the behavior of these materials. © 2013 American Chemical Society.
  • No Thumbnail Available
    Item
    A medium-energy photoemission and ab-initio investigation of cubic yttria-stabilised zirconia
    (AIP Scitation, 2014-03-01) Cousland, GP; Cui, XY; Smith, AE; Stampfl, CM; Wong, L; Tayebjee, M; Yu, DH; Triani, G; Evans, PJ; Ruppender, HJ; Jang, LY; Stampfl, APJ
    Experimental and theoretical investigations into the electronic properties and structure of cubic yttria-stabilized zirconia are presented. Medium-energy x-ray photoemission spectroscopy measurements have been carried out for material with a concentration of 8-9 mol. % yttria. Resonant photoemission spectra are obtained for a range of photon energies that traverse the L2 absorption edge for both zirconium and yttrium. Through correlation with results from density-functional theory (DFT) calculations, based on structural models proposed in the literature, we assign photoemission peaks appearing in the spectra to core lines and Auger transitions. An analysis of the core level features enables the identification of shifts in the core level energies due to different local chemical environments of the constituent atoms. In general, each core line feature can be decomposed into three contributions, with associated energy shifts. Their identification with results of DFT calculations carried out for proposed atomic structures, lends support to these structural models. The experimental results indicate a multi-atom resonant photoemission effect between nearest-neighbour oxygen and yttrium atoms. Near-edge x-ray absorption fine structure spectra for zirconium and yttrium are also presented, which correlate well with calculated Zr- and Y-4d electron partial density-of-states and with Auger electron peak area versus photon energy curve. © 2014, AIP Publishing LLC.
  • No Thumbnail Available
    Item
    Phase and valence transitions in Ba(2)LnSn(x)Sb(1-x)O(6-delta) (Ln = Pr and Tb)
    (Elsevier, 2008-11) Saines, PJ; Kennedy, BJ; Elcombe, MM; Harris, HH; Jang, LY; Zhang, Z
    Compounds in the double perovskites series Ba(2)LnSn(x)Sb(1-x)O(6-delta) (Ln = Pr and Tb) have been synthesised and structurally characterised using synchrotron X-ray and neutron powder diffraction. It was found that the two end-members of the Ba2PrSnxSb1-xO6-delta series both adopt rhombohedral symmetry but the antimonate is a fully ordered double perovskite while the stannate has no B-site cation ordering. X-ray absorption near-edge structure (XANES) and near-infrared spectroscopy indicate that the Pr cations gradually change oxidation state from Pr3+ to Pr4+ with increasing x and that this is likely to be the cause of the loss of B-site ordering. Similarly, both Ba2TbSbO6 and Ba2TbSnO6-delta are cubic with B-site ordering present in the former but absent in the latter due to the oxidation state change of the Tb from Tb3+ to Tb4+. Multiple linear regression analysis of the Pr and Tb L-III-edge XANES indicates that the rate of Ln(3+) transforming to Ln(4+) is such that there are no oxygen vacancies in Ba2PrSnxSb1-xO6-delta but in Ba2TbSnxSb1-xO6-delta there is a small amount of oxygen vacancies, with a maximum of delta approximate to 0.05 present. © 2008, Elsevier Ltd.
  • No Thumbnail Available
    Item
    The pyrochlore to defect fluorite phase transition in Y2Sn2-xZrxO7
    (Royal Society of Chemistry, 2013-01-30) de los Reyes, M; Whittle, KR; Zhang, ZM; Ashbrook, SE; Mitchell, MR; Jang, LY; Lumpkin, GR
    The system Y2Sn2−xZrxO7 (0.0 ≤ x ≤ 2.0) undergoes a phase transformation from ordered pyrochlore (Fdm) to defect fluorite (Fmm) actuated by the substitution of Zr for Sn. X-ray diffraction patterns map the retention of the pyrochlore structure up to x = 1.2. For samples with x = 1.4–2.0 the structure can be described as defect fluorite in broad terms. Electron diffraction patterns are consistent with this interpretation; however, they also demonstrate that the defect fluorite phase exhibits a strain driven compositional/displacive modulation that changes gradually with increasing Zr content. Raman spectra are consistent with gradual anion disorder up to x = 1.0 and highly disordered anion distributions inferred for x > 1.4, but the spectra also suggest the presence of residual order due to the modulated structure. The phase transformation in this system occurs at a higher Zr content than predicted by classical radius ratio models, consistent with the covalent character of Sn–O bonding. An unusual finding of this work comes from 119Sn MAS NMR and Sn L3-edge XANES analyses, indicating that Sn4+ prefers to occupy lattice sites with a 6-fold local coordination environment throughout the series. These results suggest that the incorporation of Sn or other metal cations having significant covalent bonding or a strong preference for octahedral coordination in pyrochlore-based materials may have a detrimental effect on ionic conductivity. © 2013, The Royal Society of Chemistry.
  • No Thumbnail Available
    Item
    Structure and properties of Sr1-xCaxMn0.5Ru0.5O3 perovskites: using chemical pressure to control Mn/Ru mixed valency
    (American Chemical Society, 2010-05-10) Ricciardo, RA; Cuthbert, HL; Woodward, PM; Zhou, QD; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LY
    The structure and properties of Sr1−xCaxRu0.5Mn0.5O3 compositions have been investigated. Both bond distances and X-ray absorption measurements reveal Mn3+ + Ru5+ ↔ Mn4+ + Ru4+ mixed valency across the entire series. Despite a complete lack of Ru/Mn chemical order, all samples magnetically order between 220 and 300 K. The characteristics and type of magnetic order are sensitive to the occupation and ordering of the Mn eg orbitals, which can be manipulated by changes in chemical pressure, via the Sr/Ca ratio. Sr-rich samples are tetragonally distorted by a cooperative Jahn−Teller distortion (CJTD) that leads to an elongation of the c-axis as well as antiphase rotations of the octahedra about the c-axis (a0a0c− tilt system). The CJTD results from orbital ordering involving occupied dz2 orbitals on Mn3+, which stabilize C-type antiferromagnetic order. For Sr-rich samples, the various oxidation states contribute in approximately equal proportions (i.e., Ru+4.5 and Mn+3.5). Substituting Ca2+ for Sr2+ triggers additional rotations of the octahedra (a−b+a− tilt system) that result in orthorhombic symmetry for Sr1−xCaxMn0.5Ru0.5O3 samples with x ≥ 0.3. The crossover to orthorhombic symmetry is accompanied by the loss of orbital order and the emergence of an itinerant electron ferrimagnetic state. X-ray absorption near edge structure (XANES) measurements show that as the Ca2+ content increases there is a shift in the valence degeneracy toward Mn4+ + Ru4+. This helps to explain the absence of orbital ordering in Ca-rich samples, as well as the crossover from antiferromagnetism to ferrimagnetism. Neutron diffraction and ac susceptibility measurements show that CaRu0.5Mn0.5O3 undergoes magnetic phase separation into a disordered ferrimagnetic state (TC ≈ 230 K) and a G-type antiferromagnetic state (TN ≈ 95 K). © 2010, American Chemical Society
  • No Thumbnail Available
    Item
    Studying the effects of Zr-doping in (Bi0.5Na0.5)TiO3 via diffraction and spectroscopy
    (Royal Society of Chemistry, 2014-10-13) Blanchard, PER; Liu, S; Kennedy, BJ; Ling, CD; Zhang, Z; Avdeev, M; Jang, LY; Lee, JF; Pao, CW; Chen, JL
    The structural properties of (Bi0.5Na0.5)Ti1−xZrxO3 (where 0 ≤ x ≤ 0.7) have been investigated using powder diffraction and X-ray absorption spectroscopy. Diffraction measurements on (Bi0.5Na0.5)TiO3 confirm that both monoclinic Cc and rhombohedral R3c phases are present at room temperature. Doping small amounts of Zr into the B site of (Bi0.5Na0.5)TiO3 initially stabilizes the rhombohedral phase before the orthorhombic Pnma phase begins to form at x = 0.5. Analysis of the Ti K-edge and Zr L3-edge XANES spectra show that the crystallographic phase change has very little effect on the local structure of Ti4+/Zr4+ cations, suggesting that there is little change in the cation off-center displacement within the BO6 octahedra with each successive phase change. © The Royal Society of Chemistry 2014
  • No Thumbnail Available
    Item
    Synthesis and structural studies of the transition-metal-doped Rh perovskites LaMn0.5Rh0.5O3 and LaCu0.5Rh0.5O3
    (American Chemical Society, 2010-03-09) Ting, J; Kennedy, BJ; Zhang, Z; Avdeev, M; Johannessen, B; Jang, LY
    The two orthorhombic perovskites LaMn0.5Rh0.5O3 and LaCu0.5Rh0.5O3 have been characterized using X-ray absorption near edge structure (XANES) measurements and powder synchrotron X-ray and neutron diffraction. The Mn and Rh in LaMn0.5Rh0.5O3 are essentially trivalent and disordered onto the same crystallographic site. The absence of an appreciable Jahn−Teller (JT) distortion of the BO6 octahedra is a consequence of this disorder and large magnitude of the octahedral tilting. XANES measurements show that the Rh in LaCu0.5Rh0.5O3 has a valence state of around +3.5 as a consequence of extensive charge delocalization between the Cu and Rh cations, that are also disordered onto a single site. These two effects combine to further reduce the distortion of the octahedra in LaCu0.5Rh0.5O3 relative to that of LaMn0.5Rh0.5O3. © 2010, American Chemical Society
  • No Thumbnail Available
    Item
    Synthesis, structural and magnetic studies of the double perovskites Ba2CeMO6 (M = Ta, Nb)
    (American Chemical Society, 2012-07-16) Zhou, QD; Blanchard, PER; Kennedy, BJ; Reynolds, EM; Zhang, Z; Miiller, W; Aitken, JB; Avdeev, M; Jang, LY; Kimpton, JA
    Two Ce(3+) containing double perovskites Ba(2)CeMO(6) (M = Nb and Ta) have been prepared through the use of mildly reducing conditions, and the Ce valence state has been shown to be +3 through Ce L-edge X-ray absorption measurements. Both oxides adopt a monoclinic structure in I2/m at room temperature and undergo two phase transitions upon heating, a discontinuous I2/m -> R (3) over bar and a continuous R (3) over bar -> Fm (3) over barm transition. Analysis of the first order I2/m -> R (3) over bar transitions is impeded by the complex peak shapes and diffuse scattering evident in the synchrotron powder diffraction data because of domain wall effects. © 2012, American Chemical Society.
  • No Thumbnail Available
    Item
    Valence changes of manganese and structural phase transitions in Sr1-xPrxMnO3 (0.1 <= x <= 0.6)
    (Elsevier, 2013-05-01) Tan, TY; Martin, N; Zhou, Q; Kennedy, BJ; Gu, QF; Kimpton, JA; Zhang, Z; Jang, LY
    Fifteen perovskite manganites Sr1−xPrxMnO3, x=0.1−0.6 in steps as fine as 0.025, have been synthesized by solid state methods and their crystal structures determined using synchrotron X-ray powder diffraction. At room temperature two first order phase transitions associated with changes in the orbital ordering and the tilting of the corner shared MnO6 octahedra are evident cubic/Pm3¯m⟹x≈0.15tetragonal/(I4/mcm)⟹x≈0.45orthorhombic/Imma, with two phase regions associated with both transitions. Heating the Pr rich oxide Sr0.5Pr0.5MnO3 provides a rare example where chemical pressure and temperature result in a different sequence of transitions Imma –R3¯c–Pm3¯m, reflecting the importance of the Pr 4f electrons. X-ray absorption near-edge structure (XANES) measurements at the Mn K and Pr L3 edges show the valence state of the Mn linearly decreases with increasing Pr content whereas the Pr remains +3 across the entire series. Magnetic susceptibility measurements for selected samples are presented. © 2013, Elsevier Inc.
  • No Thumbnail Available
    Item
    X-ray absorption near edge structure and crystallographic studies of the mixed valence oxide SrRu(0.8)Ni(0.2)O(3)
    (IOP Publishing LTD, 2011-10-13) Qasim, I; Kennedy, BJ; Zhang, Z; Avdeev, M; Jang, LY
    Neutron diffraction methods are used to refine the structure of SrRu(0.8)Ni(0.2)O(3) at room temperature and 4 K. X-ray absorption measurements at the Ru L-edge demonstrate that partial oxidation of the Ru(4+) to Ru(5+) occurs upon introducing Ni into the SrRuO(3) structure to form SrRu(0.8)Ni(0.2)O(3). Whilst the diffraction measurements show that SrRu(0.8)Ni(0.2)O(3) is isostructural with SrRuO(3), the Ni doped compound exhibits an unusual first order orthorhombic-cubic phase transition near 670 K as revealed by x-ray diffraction. The Curie temperature in SrRu(0.8)Ni(0.2)O(3) is lower than that found in SrRuO(3) as a consequence of local distortions reducing the p-d hybridization. (c) 2011 IOP Publishing LTD
  • No Thumbnail Available
    Item
    X-ray absorption near edge structure and crystallographic studies of the mixed valence oxides CaRu1-xMnxO3
    (American Chemical Society, 2009-09-22) Zhou, QD; Kennedy, BJ; Zhang, Z; Jang, LY; Aitken, JB
    Eleven members of the series CaRu1-xMnxO3 With 0 <= x <= 1 have been prepared and characterized using a combination of Mn K-edge and Ru L-3,L-2-edge X-ray absorption near edge structure spectroscopy and high resolution synchrotron X-ray powder diffraction. A complex distribution of four Mn3+/4+ and Ru4+/5+ oxidation states was observed and quantified. When incorporated into CaRuO3 at low doping levels, the Mn is present as Mn3+ and Mn4+ with the nominal Mn oxidation state increasing with x (from 3.6 at x = 0.1 to 4 at x = 1). The Ru exists predominantly as Ru5+ in CaMnO3 with the nominal Ru valency as 4.8 at x = 0.9. The formation of Mn3+/4+-Ru4+/5+ redox pairs accounts for the unusual chemical composition dependence of the cell volume, evident from the diffraction studies. The coexistence of mixed valence redox pairs is also believed to contribute to the phase separation observed at 0.1 <= x <= 0.6. © 2009, American Chemical Society