Browsing by Author "Wang, CH"
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- Item3D transition metal ordering and Rietveld stacking fault quantification in the new oxychalcogenides La2O2Cu2–4xCd2xSe2(American Chemical Society, 2016-04-04) Ainsworth, CM; Lewis, JW; Wang, CH; Coelho, AA; Johnston, HE; Brand, HEA; Evans, JSOA number of LnOCuCh (Ln = La-Nd, Bi; Ch = S, Se, Te) compounds have been reported in the literature built from alternating layers of fluorite-like [Ln2O2]2+ sheets and antifluorite-like [M2Se2]2- sheets, where M is in the +1 oxidation state leading to full occupancy of available MSe4/2 tetrahedral sites. There is also a family of related LnOM0.5Se (Ln = La & Ce, M = Fe, Zn, Mn & Cd) compounds built from alternating layers of [Ln2O2]2+ sheets and [MSe2]2- sheets, where M is in the +2 oxidation state with half occupancy of available tetrahedral sites and complex ordering schemes in two dimensions. This paper reports a new family of compounds containing both +1 and +2 metal ions in the La2O2Cu2-4xCd2xSe2 family. We show how Cu1+ and Cd2+ ions segregate into distinct fully occupied and half occupied checkerboard-like layers respectively, leading to complex long-range superstructures in the third (stacking) dimension. To understand the structure and microstructure of these new materials we have developed and implemented a new methodology for studying low and high probability stacking faults using a Rietveld-compatible supercell approach capable of analyzing systems with thousands of layers. We believe this method will be widely applicable. © 2016 American Chemical Society.
- 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
- ItemCrystal structure and magnetic modulation in β−Ce2O2FeSe2(American Physical Society, 2017-08-11) Wang, CH; Ainsworth, CM; Champion, SD; Stewart, GA; Worsdale, MC; Lancaster, T; Blundell, SJ; Brand, HEA; Evans, JSOWe report a combination of x-ray and neutron diffraction studies, Mössbauer spectroscopy, and muon spin relaxation (μ+SR) measurements to probe the structure and magnetic properties of the semiconducting β-Ce2O2FeSe2 oxychalcogenide. We report a structural description in space group Pna21 which is consistent with diffraction data and second harmonic generation measurements and reveal an order-disorder transition on one Fe site at TOD≈330K. Susceptibility measurements, Mössbauer, and μ+SR reveal antiferromagnetic ordering below TN=86K and more complex short range order above this temperature. 12 K neutron diffraction data reveal a modulated magnetic structure with q=0.444bN∗. © 2017 American Physical Society.
- ItemInfinitely adaptive transition-metal ordering in Ln2O2MSe2-type oxychalcogenides(American Chemical Society, 2015-04-30) Ainsworth, CM; Wang, CH; Johnston, HE; McCabe, EE; Tucker, MG; Brand, HEA; Evans, JSOA number of Ln2O2MSe2 (Ln = La and Ce; M = Fe, Zn, Mn, and Cd) compounds, built from alternating layers of fluorite-like [Ln2O2]2+ sheets and antifluorite-like [MSe2]2– sheets, have recently been reported in the literatures. The available MSe4/2 tetrahedral sites are half-occupied, and different compositions display different ordering patterns: [MSe2]2– layers contain MSe4/2 tetrahedra that are exclusively edge-sharing (stripe-like), exclusively corner-sharing (checkerboard-like), or mixtures of both. This paper reports 60 new compositions in this family. We reveal that the transition-metal arrangement can be systematically controlled by either Ln or M doping, leading to an “infinitely adaptive” structural family. We show how this is achieved in La2O2Fe1–xZnxSe2, La2O2Zn1–xMnxSe2, La2O2Mn1–xCdxSe2, Ce2O2Fe1–xZnxSe2, Ce2O2Zn1–xMnxSe2, Ce2O2Mn1–xCdxSe2, La2–yCeyO2FeSe2, La2–yCeyO2ZnSe2, La2–yCeyO2MnSe2, and La2–yCeyO2CdSe2 solid solutions. © 2015 American Chemical Society
- ItemIon-transport phenomena and anomalous transformations in strontium uranium oxides.(International Union of Crystallography, 2017-12-01) Murphy, GL; Zhang, Z; Avdeev, M; Wang, CH; Beridze, G; Kowalski, PM; Gu, QF; Kimpton, JA; Johannessen, B; Kennedy, BJStructural-chemical elucidation of low dimensional ternary uranium oxide systems is considered an essential aspect of thenuclear fuel cycle since understanding of their physicochemical properties may guide the storage and disposal of spentnuclear fuel . The study of these systems allows for further exploration of the peculiar, exotic and poorly knownproperties of materials containing, or which can access, 5f electrons. SrUO₄ exemplifies this, a potential waste form resultingfrom reaction between spent UO₂+x fuel and the fission daughter Sr-90. We have found, through a combination of in situsynchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that during its first order rhombohedral-orthorhombic transition under oxidising conditions, the rhombohedral form of SrUO₄, α, undergoes a spontaneousreduction of the uranium valence state through oxygen vacancy formation . The process is synergetic, as the triality ofoxygen vacancy formation, subsequent ion diffusion and uranium reduction, seemingly reduces the activation energy barrierfor the transformation to the thermodynamically favoured stoichiometric orthorhombic form, β-SrUO₄. However formation ofthe orthorhombic form is only possible if a source of oxygen is present, without this, the oxygen deficient α-SrUO₄-xremains rhombohedral as shown by in situ neutron powder diffraction measurements. These experimental observations arefurther supported by ab initio DFT+U calculations using the self consistently calculated Hubbard U parameter values andbond valence sums calculations [2-3]. These methods indicate the affinity for α-SrUO₄-x to retain oxygen vacancies asopposed to β-SrUO₄, a consequence of the crystal lattice’s ability to stabilise the coordination environment of the Sr²⁺ cationvia the flexibility of uranium to undergo reduction through vacancy formation. CaUO4, isostructural to α-SrUO₄ , but unlike α-SrUO₄ does not have a stable orthorhombic polymorph as shown by both insitu synchrotron X-ray powder diffraction measurements and ab initio calculations. Introducing Sr ions into the CaUO₄ latticein the form of a solid solution, α-Sr₁-xCaxUO₄ (0 < x < 0.4), provides a means to atomically engineer the lattice to promoteoxygen vacancy formation, and presumably diffusion, at high temperatures. When CaUO₄ or α-SrUO₄ is treated underhighly reducing conditions, both materials undergo unusual reconstructive phase transformations at high temperatures to amonoclinic structure. These phase transformations are reversible, and cooling the sample yields the correspondingrhombohedral structure again. It is remarkable that the ordered monoclinic structure is favoured at high temperatures andthe disordered rhombohedral structure at low temperatures. This investigation in SrUO₄ highlights the rich and remarkablestructural chemistry and crystallography that may be found within poorly understood actinide systems whilst demonstratingthe successful marriage of experimental and theoretical approaches towards elucidating their chemical and physicalphenomena. © International Union of Crystallography
- ItemKondo behavior, ferromagnetic correlations, and crystal fields in the heavy-fermion compounds Ce3X (X = In, Sn).(American Physical Society, 2010-06-25) Wang, CH; Lawrence, JM; Christianson, AD; Goremychkin, EA; Fanelli, VR; Gofryk, K; Bauer, ED; Ronning, F; Thompson, JD; de Souza, NR; Kolesnikov, AI; Littrell, KCWe report measurements of inelastic neutron scattering, magnetic susceptibility, magnetization, and the magnetic field dependence of the specific heat for the heavy Fermion compounds Ce3In and Ce3Sn. The neutron scattering results show that the excited crystal field levels have energies E1=13.2 meV, E2=44.8 meV for Ce3In and E1=18.5 meV, E2=36.1 meV for Ce3Sn. The Kondo temperature deduced from the quasielastic linewidth is 17 K for Ce3In and 40 K for Ce3Sn. The low-temperature behavior of the specific heat, magnetization, and susceptibility cannot be well described by J=1/2 Kondo physics alone, but require calculations that include contributions from the Kondo effect, broadened crystal fields, and ferromagnetic correlations, all of which are known to be important in these compounds. We find that in Ce3In the ferromagnetic fluctuation makes a 10%–15% contribution to the ground state doublet entropy and magnetization. The large specific heat coefficient γ in this heavy fermion system thus arises more from the ferromagnetic correlations than from the Kondo behavior. © 2010, American Physical Society
- ItemMultiple competing magnetic interactions in Na4Ni7(PO4)6(American Chemical Society, 2019-07-22) Xia, Q; Wang, CH; Schmid, S; Avdeev, M; Ling, CDThe low-temperature magnetic behavior and ground state of the candidate sodium-ion battery cathode compound Na4Ni7(PO4)6 have been investigated by physical property measurements and neutron powder diffraction. On cooling, Na4Ni7(PO4)6 undergoes three successive long-range spin ordering transitions to Phase I (below TN = 17 K), Phase II (below TN′ = 9.1 K), and Phase III (below TN″ = 4.6 K) with ordering vectors [0, 1, 1/2], [0, 2/3, 1/2], and [∼0.076, 2/3, 1/2], respectively. All three magnetic phases can be described in terms of ferromagnetic Ni2+ stripes with antiferromagnetic interactions between them. The moment amplitude of all stripes is the same in Phase I but varies in Phase II, while Phase III is an incommensurate variation on Phase II. Phases I and II both feature a crystallographically unique Ni site with no ordered magnetic moment due to geometric frustration; the resolution of which may be the driving force behind the final transition to Phase III. Even among transition-metal phosphates, which typically show complex spin ordering due to competition between superexchange and super-superexchange (through PO4 linkers), Na4Ni7(PO4)6 has one of the richest magnetic phase diagrams explored so far. © 2019 American Chemical Society
- ItemRevisiting the cubic crystal structures of Sr4Nb2O9 and Sr5Nb2O10(Elsevier, 2021-11-01) Li, JY; Wang, CH; Wang, XM; Avdeev, M; Ling, CD; Kennedy, BJWe have synthesized polycrystalline and single crystal samples of Sr4Nb2O9 and Sr5Nb2O10 and revisited the crystal structure of the high-temperature cubic phase. By careful analysis of single-crystal X-ray diffraction (SXRD), powder synchrotron X-ray diffraction (Syn-PXRD) and powder neutron diffraction (PND) data, we arrive at a structure model in space group F4¯3m (#216), a subgroup of the reported Fm3¯m (#225) model. The F4¯3m model gives a better fit to the diffraction data, especially the PND data. We observed an interstitial oxide ion (O3) on the 48h site near the typical perovskite 24e site (O1), which gives a Td Nb–O symmetry rather than an Oh one as found in the Fm3¯m model. The temperature-dependent conductivities of Sr4Nb2O9 and Sr5Nb2O10 in dried O2 were studied using impedance spectroscopy. The activation energies of Sr4Nb2O9 and Sr5Nb2O10 were estimated to be 1.18(1) eV and 1.17(4) eV, respectively. This disordered crystallographic arrangement of the O1 and O3 anions is likely a key structural factor behind oxide ionic migration in Sr4Nb2O9 and Sr5Nb2O10. © 2021 Elsevier Inc.
- ItemStructural and magnetic studies of ABO4-type ruthenium and osmium oxides(American Chemical Society, 2020-02-14) Injac, S; Yuen, AKL; Avdeev, M; Wang, CH; Turner, P; Brand, HEA; Kennedy, BJOxides of the form ABO4 with A = K, Rb, Cs and B = Ru and Os have been synthesized and characterized by diffraction and magnetic techniques. For A = K the oxides adopted the tetragonal (I41/a) scheelite structure. RbOsO4, which crystallizes as a scheelite at room temperature, underwent a continuous phase transition to I41/amd near 550 K. RbRuO4 and CsOsO4 were found to crystallize in the orthorhombic (Pnma) pseudoscheelite structure, and both displayed discontinuous phase transitions to I41/a at high temperatures. CsOsO4 was determined to undergo a phase transition to a P21/c structure below 140 K. CsRuO4 crystallizes with a baryte-type structure at room temperature. Upon heating CsRuO4 a first order phase transition to the scheelite structure in I41/a is observed at 400 K. A continuous phase transition is observed to P212121 below 140 K. DC magnetic susceptibility data is consistent with long-range antiferromagnetic ordering at low temperatures for all compounds except for CsOsO4, which is paramagnetic to 2 K. The effective magnetic moments are in agreement with the spin only values for an S = 1/2 quantum magnet. Effective magnetic moments calculated for Os compounds were lower than their Ru counterparts, reflective of an enhanced spin orbit coupling effect. A magnetic structure is proposed for RbRuO4 consisting of predominately antiferromagnetic (AFM) ordering along the 001 direction, with canting of spins in the 100 plane. A small ordered magnetic moment of 0.77 μB was determined. © 2020 American Chemical Society
- ItemStructure evolution of Na2O2 from room temperature to 500 °C(American Chemical Society, 2020-09-21) Wang, CH; Gui, DY; Xia, Q; Avdeev, M; Ling, CD; Kennedy, BJNa2O2 is one of the possible discharge products from sodium–air batteries. Here, we report the evolution of the structure of Na2O2 from room temperature to 500 °C using variable-temperature neutron and synchrotron X-ray powder diffraction. A phase transition from α-Na2O2 to β-Na2O2 is observed in the neutron diffraction measurements above 400 °C, and the crystal structure of β-Na2O2 is determined from neutron diffraction data at 500 °C. α-Na2O2 adapts a hexagonal P62m (no. 189) structure, and β-Na2O2 adapts a tetragonal I41/acd (no. 142) structure. The thermal expansion coefficients of α-Na2O2 are a = 2.98(1) × 10–5 K–1, c = 2.89(1) × 10–5 K–1, and V = 8.96(1) × 10–5 K–1 up to 400 °C, and a ∼10% volume expansion occurs during the phase transition from α-Na2O2 to β-Na2O2 due to the realignment/rotation of O22– groups. Both phases are electronic insulators according to DFT calculations with band gaps (both indirect) of 1.75 eV (α-Na2O2) and 2.56 eV (β-Na2O2). An impedance analysis from room temperature to 400 °C revealed a significant enhancement of the conductivity at T ≥ 275 °C. α-Na2O2 shows a higher conductivity (∼10 times at T ≤ 275 °C and ∼3 times at T > 275 °C) in O2 compared to in Ar. We confirmed, by dielectric analysis, that this enhanced conductivity is dominated by ionic conduction. © 2020 American Chemical Society
- ItemUnexpected crystallographic phase transformation in nonstoichiometric SrUO4–x: Reversible oxygen defect ordering and symmetry lowering with increasing temperature(American Chemical Society, 2018-05-01) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Kimpton, JA; Avdeev, M; Kowalski, PM; Kennedy, BJIn situ synchrotron powder X-ray diffraction measurements have demonstrated that SrUO4 undergoes a reversible phase transformation under reducing conditions at high temperatures, associated with the ordering of oxygen defects resulting in a lowering of crystallographic symmetry. When substoichiometric rhombohedral α-SrUO4–x, in space group R3̅m with disordered in-plane oxygen defects, is heated above 200 °C in a hydrogen atmosphere it undergoes a first order phase transformation to a (disordered) triclinic polymorph, δ-SrUO4–x, in space group P1̅. Continued heating to above 450 °C results in the appearance of superlattice reflections, due to oxygen-vacancy ordering forming an ordered structure δ-SrUO4–x. Cooling δ-SrUO4–x toward room temperature results in the reformation of the rhombohedral phase α-SrUO4–x with disordered defects, confirming the reversibility of the transformation. This suggests that the transformation, resulting from oxygen vacancy ordering, is not a consequence of sample reduction or decomposition, but rather represents a change in the energetics of the system. A strong reducing atmosphere is required to generate a critical amount of oxygen defects in α-SrUO4–x to enable the transformation to δ-SrUO4–x but once formed the transformation between these two phases can be induced by thermal cycling. The structure of δ-SrUO4–x at 1000 °C was determined using symmetry representation analysis, with the additional reflections indexed to a commensurate distortion vector k = ⟨1/4 1/4 3/4⟩. The ordered 2D layered triclinic structure of δ-SrUO4–x can be considered a structural distortion of the disordered 2D layered rhombohedral α-SrUO4–x structure through the preferential rearrangement of the in-plane oxygen vacancies. Ab initio calculations using density functional theory with self-consistently derived Hubbard U parameter support the assigned ordered defect superstructure model. Entropy changes associated with the temperature dependent short-range ordering of the reduced U species are believed to be important and these are discussed with respect to the results of the ab initio calculations. © 2018 American Chemical Society
- ItemThe unusual structural chemistry of uranium: controlling phase transformations in ternary uranium oxides(Society of Crystallographers in Australia and New Zealand, 2017-12-03) Murphy, GL; Wang, CH; Beridze, G; Zhang, Z; Avdeev, M; Kowalski, PM; Brand, HEA; Johannessen, B; Kennedy, BJStructural investigations of ternary uranium oxides are pertinent to the management of spent nuclear fuel since such environmentally hazardous and potentially dangerous secondary phases may form during the process and/or storage of the material . They further allow for the exploration of the peculiar, exotic and poorly known properties of materials containing, or which can access 5f electrons. The rhombohedral oxides AUO4 for A = α-Sr or Ca in space group 𝑅𝑅3�𝑚𝑚 exemplifies this. We have found through a combination of in situ synchrotron X-ray powder diffraction and X-ray absorption spectroscopy, that α-SrUO4 undergoes a fascinating first-order phase transformation under oxidising conditions . This involves the synergetic loss of lattice oxygen resulting in oxygen vacancy defect formation and reduction of the uranium cations, which seemingly reduces the activation energy barrier for transformation to its orthorhombic form, β-SrUO4. Under similar conditions, CaUO4 does not display any transformative behaviour, however, defects can be engineered through the substitution of Ca2+ for Sr2+ in the solid solution α-SrxCa1-xUO4 when heated to high temperature under oxidising conditions. The introduction of Sr2+ cations in α-SrxCa1-xUO4 was found to decrease the temperature at which oxygen vacancy defects form. This phenomenon was rationalised as a consequence of the introduction of Sr2+ cations leading to lattice expansion, which causes the proximity of defects to increase. This subsequently reduces free energy increasing defect-defect interactions, allowing defects to form at lower temperature. Remarkably, when heated under reducing conditions, the disordered oxygen defect containing rhombohedral α-SrUO4-x structure undergoes a reversible first-order phase transformation that involves the ordering of the oxygen defects resulting in lowering of the crystallographic symmetry to triclinic in space group P 1� denoted δ-SrUO4-x. This remarkable transformation, which implies entropy is being decreased as temperature increases, could be replicated in CaUO4 and also in α-Sr0.4Ca0.6UO4 where the transformation temperature is reduced by increasing the Sr2+ content, consistent with the effects of reducing defect-defect interactions. The S-XRD data shows the structure of δ-CaUO4-x to be incommensurate whereas δ-SrUO4-x is commensurate. This implies a miscibility gap may exist between the isostructural CaUO4 and α-SrUO4 related to shortrange order. This investigation demonstrates the rich and fascinating crystal chemistry present in uranium oxides, which, in some cases, may have profound societal importance if it can either be safely used or if associated properties can be replicated into non-actinide materials.
- ItemYCa3 (CrO) 3 (BO3) 4: a Cr3+ kagomé lattice compound showing no magnetic order down to 2 K(American Chemical Society, 2016-07-13) Wang, CH; Avdeev, M; Kennedy, BJ; Küpers, M; Ling, CDWe report a new gaudefroyite-type compound YCa3(CrO)3(BO3)4, in which Cr3+ ions (3d3, S = 3/2) form an undistorted kagomé lattice. Using a flux agent, the synthesis was significantly accelerated with the typical calcining time reduced from more than 2 weeks to 2 d. The structure of YCa3(CrO)3(BO3)4 was determined by combined Rietveld refinements against X-ray and neutron diffraction data. Symmetry distortion refinement starting from a disordered YCa3(MnO)3(BO3)4 model was applied to avoid overparameterization. There are two ordering models, namely, K2–1 and K2–2, with the space groups P63 (No. 173) and P3̅ (No. 147), respectively, that differ in the [BO3] ordering between different channels (in-phase or out-of-phase). Both models give similarly good fits to the diffraction data. YCa3(CrO)3(BO3)4 is an insulator with the major band gap at Eg = 1.65 eV and a second transition at 1.78 eV. Magnetically, YCa3(CrO)3(BO3)4 is dominated by anti-ferromagnetic exchange along edge-sharing CrO6 octahedral chains perpendicular to the kagomé planes, with Θ ≈ −120 K and μeff ≈ 3.92 μB. The compound shows no spin ordering or freezing down to at least 2 K. © 2016 American Chemical Society