Browsing by Author "Reynolds, EM"

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    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.
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    Anomalous 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, AA
    The 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.
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    Comparison 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, S
    99Tc 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.
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    Crystal structure and phase transitions in the uranium perovskite, Ba2SrUO6
    (Elsevier, 2013-02-01) Reynolds, EM; Kennedy, BJ; Thorogood, GJ; Gregg, DJ; Kimpton, JA
    The structure of one of the oxides proposed to be present in the grey phase of irradiated mixed oxide fuel, the double perovskite Ba2SrUO6 has been investigated from room temperature to 1300 K using synchrotron X-ray powder diffraction methods. The divalent strontium and hexavalent uranium are found to be fully ordered in the double-perovskite arrangement of alternating octahedra sharing corner oxygen atoms. At room temperature Ba2SrUO6 adopts a monoclinic structure in space group P21/n. Heating to above 900 K induces a first order transition to a rhombohedral structure, and further heating to above 1200 K results in a continuous transition to a cubic structure. The sequence of structures is associated with the progressive loss of cooperative tilting of the corner sharing SrO6 and UO6 octahedra. © 2012, Elsevier B.V.
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    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.
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    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.
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    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.
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    Peroxide defect formation in zirconate perovskites
    (Royal Society of Chemistry, 2014) Middleburgh, SC; Karatchevtseva, I; Kennedy, BJ; Burr, PA; Zhang, ZM; Reynolds, EM; Grimes, RW; Lumpkin, GR
    Atomic scale modelling suggests that excess oxygen can be accommodated in the group II perovskite zirconates by the formation of peroxide ion defects. This is unprecedented given the lack of charge compensating defects required for standard excess oxygen accommodation. The solution energy of O2 was predicted to be close to zero for BaZrO3, accommodating the peroxide ion defect more easily than in SrZrO3 or CaZrO3. This was experimentally examined by exposing SrZrO3 and BaZrO3 to hydrogen peroxide solution and then carrying out Raman spectroscopy measurements to look for a peak indicative of peroxide ions. A peak was observed at ∼1000 cm−1 in both compositions, suggesting the theoretically predicted peroxide ion is present. © 2014, The Royal Society of Chemistry.
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    The role of π-bonding on the high temperature structures in the uranium double perovskites Ba2CaUO6 and BaSrCaUO6
    (Royal Society of Chemistry, 2015-08-12) Reynolds, EM; Thorogood, GJ; Avdeev, M; Brand, HEA; Gu, QF; Kennedy, BJ
    The high temperature structural behaviour of the uranium perovskites Ba2CaUO6 and BaSrCaUO6 has been investigated using a combination of synchrotron X-ray and neutron powder diffraction. Ba2CaUO6 undergoes a complex sequence of structures associated with the progressive loss of cooperative octahedral tilting: P21/n → I2/m → I2/m → I4/m → Fm[3 with combining macron]m. The observation of the intermediate tetragonal structure, I4/m, in this, contrasts with the previously reported rhombohedral R[3 with combining macron] intermediate formed by the Ba2SrUO6 oxide. The importance of π-bonding in determining the structural sequence is discussed. © The Royal Society of Chemistry 2015
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    Structural and spectroscopic studies of Ba2Y1−δUO6+x
    (Elsevier B.V., 2016-11-01) Reynolds, EM; Kennedy, BJ; Avdeev, M; Thorogood, GJ; Zhang, Z; Brand, HEA
    A combination of S-XRD and NPD demonstrate the structure of Ba2Y0.879UO6+x to be monoclinic in space group I2/m. That the U is hexavalent is evident from the U L2-edge XANES measurements. This appears to be a rare example of a double perovskite containing vacancies at the octahedral B-sites and interstitial oxygen defects, which combine to stabilise hexavalent U and appears to be a consequence of the preparation of the sample in air. The Y vacancies, coupled with anion disorder, results in a distortion of the BO6 octahedra. © 2016 Elsevier B. V.
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    Structure and magnetism in Sr A_xTcO 3 perovskites: importance of the A-site cation
    (American Physical Society, 2017-02-27) Reynolds, EM; Avdeev, M; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kimpton, JA; Yu, M; Kayser, P; Kennedy, BJ
    The Sr1−xBaxTcO3 (x=0, 0.1, 0.2) oxides were prepared and their solid-state and magnetic structure studied as a function of temperature by x-ray and neutron powder diffraction. The refined Tc moments at room temperature and Néel temperatures for Ba0.1Sr0.9TcO3 and Ba0.2Sr0.8TcO3 were 2.32(14)μβ and 2.11(13)μβ and 714∘C and 702∘C, respectively. In contrast to expectations, the Néel temperature in the series Sr1−xAxTcO3 decreases with increasing Ba content. This observation is consistent with previous experimental measurements for the two series AMO3 (M=Ru, Mn; A=Ca, Sr, Ba) where the maximum magnetic ordering temperature was observed for A=Sr. Taken with these previous results the current work demonstrates the critical role of the A-site cation in the broadening of the π∗ bandwidth and ultimately the magnetic ordering temperature.©2017 American Physical Society
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    Studies of the antiferrodistortive transition in EuTiO3
    (Institute of Physics, 2014-11-12) Kennedy, BJ; Murphy, GL; Reynolds, EM; Avdeev, M; Brand, HEA; Kolodiazhnyi, T
    Structural studies of EuTiO3, conducted using synchrotron x-ray powder diffraction, reveal the sensitivity of this material to defects. The presence of a single tetragonal–cubic (I4/mcm–Pm-3m) transition is confirmed. Neutron diffraction measurements show EuTiO3 to have a G-type antiferromagnetic arrangement. Examinations of the symmetry-adapted tetragonal strains demonstrate that the stoichiometry impacts on the continuous nature of this. The impact of defects on the electrical conductivity and magnetodielectric effect is also described. © 2014 IOP Publishing Ltd
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    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.
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    Thermal expansion behavior in TcO2. Toward breaking the Tc–Tc bond
    (American Chemical Society, 2017-07-24) Reynolds, EM; Zhang, Z; Avdeev, M; Thorogood, GJ; Poineau, F; Czerwinski, KR; Kimpton, JA; Kennedy, BJ
    The structure of TcO2 between 25 and 1000 °C has been determined in situ using X-ray powder diffraction methods and is found to remain monoclinic in space group P21/c. Thermal expansion in TcO2 is highly anisotropic, with negative thermal expansion of the b axis observed above 700 °C. This is the result of an anomalous expansion along the a axis that is a consequence of weakening of the Tc–Tc bonds. © 2017 American Chemical Society
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    Thermal expansion of ammonium pertechnetate and ammonium perrhenate
    (Elsevier, 2019-06) Reynolds, EM; Yu, M; Thorogood, GJ; Brand, HEA; Poineau, F; Kennedy, BJ
    The thermal expansion of NH4TcO4 and NH4ReO4 between 97 and 500 K has been established using variable temperature synchrotron X-ray diffraction. Both oxides display a tetragonal scheelite-type structure over the temperature range. The observed anomalous thermal expansion is associated with disorder in the orientation of the NH4 groups and the presence of anisotropic, equatorial and axial, interactions between the H atoms and the BO4 tetrahedra. The importance of the hydrogen interactions was verified by a study of the isostructural compound NaReO4 over a similar temperature range. The differences in the details of the Negative Thermal Expansion (NTE) behaviour of NH4TcO4 and NH4ReO4 is a consequence of differences in the H-BO4 interactions due to changes in the B-O bond lengths within the BO4 groups. Both ammonium salts decompose when heated above ∼500 K. ©2019 Elsevier Inc.
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    An unconventional method for measuring the Tc L3-edge of technetium compounds
    (Journal of Synchrotron Radiation, 2014-01-01) Blanchard, PER; Reynolds, EM; Kennedy, BJ; Ling, CD; Zhang, Z; Thorogood, GJ; Cowie, BCC; Thomsen, L
    Tc L3-edge XANES spectra have been collected on powder samples of SrTcO3 (octahedral Tc4+) and NH4TcO4 (tetrahedral Tc7+) immobilized in an epoxy resin. Features in the Tc L3-edge XANES spectra are compared with the pre-edge feature of the Tc K-edge as well as other 4d transition metal L3-edges. Evidence of crystal field splitting is obvious in the Tc L3-edge, which is sensitive to the coordination number and oxidation state of the Tc cation. The Tc L3 absorption edge energy difference between SrTcO3 (Tc4+) and NH4TcO4 (Tc7+) shows that the energy shift at the Tc L3-edge is an effective tool for studying changes in the oxidation states of technetium compounds. The Tc L3-edge spectra are compared with those obtained from Mo and Ru oxide standards with various oxidation states and coordination environments. Most importantly, fitting the Tc L3-edge to component peaks can provide direct evidence of crystal field splitting that cannot be obtained from the Tc K-edge. © International Union of Crystallography.