Browsing by Author "Macquart, RB"

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    Circularly polarized soft x-ray diffraction study of helical magnetism in hexaferrite
    (American Physical Society, 2010-03-01) Mulders, AM; Lawrence, SM; Princep, AJ; Staub, U; Bodenthin, Y; García-Fernández, M; Garganourakis, M; Hester, JR; Macquart, RB; Ling, CD
    Magnetic spiral structures can exhibit ferroelectric moments as recently demonstrated in various multiferroic materials. In such cases the helicity of the magnetic spiral is directly correlated with the direction of the ferroelectric moment and measurement of the helicity of magnetic structures is of current interest. Soft x-ray resonant diffraction is particularly advantageous because it combines element selectivity with a large magnetic cross-section. We calculate the polarization dependence of the resonant magnetic x-ray cross-section (electric dipole transition) for the basal plane magnetic spiral in hexaferrite Ba0.8Sr1.2Zn2Fe12O22 and deduce its domain population using circular polarized incident radiation. We demonstrate there is a direct correlation between the diffracted radiation and the helicity of the magnetic spiral. © 2010, American Physical Society
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    Compression mechanism of HoBaCo4O7, a compound with oxygen absorption/desorption capabilities
    (Deutsches Elektronen-Synchrotron, 2007) Juarez-Arellano, EA; Avdeev, M; Macquart, RB; Friedrich, A; Morgenroth, W; Wiehl, L; Winkler, B
    Recently, a new family of isostructural cobaltates (MBaCo4O7, M = In, Y, Ln) has been synthesized [1]. These cobaltates belong to a new class of geometrically frustrated magnets which not only exhibit interesting magnetic-, electronic-, thermoelectric- and electrochemical-properties; but they also have a remarkable low-temperature oxygen absorption/desorption capability which makes them suitable as oxygen sensors, oxygen permeation membranes and solid oxide fuel cells (SOFCs) [2-3]. For example, YBaCo4O7+δ reversibly absorb and desorb oxygen up to δ ≈ 1.5 in a narrow temperature range, 470-673 K [2]. Hence, an amount of oxygen that corresponds to ~20% of the total oxygen content is readily loaded or removed being triggered by just a tiny change in temperature or atmosphere. This oxygen capability substantially exceeds in the overall magnitude and in the response sensitivity to those achieved with, for example, SrFeO3 (perovskite structure) and YBa2Cu3O7 (perovskite-like structure) [3]. It is well known that the oxygen diffusion properties of perovskite-like compounds are affected not only by the temperature and surrounding oxygen partial pressure but also by their crystal structures. Therefore, a different crystal structure will result in different oxygen diffusion properties. There is currently a discussion about whether the MBaCo4O7 crystallizes in the space group P63mc or in the trigonal subgroup P31c at room temperature; or whether MBaCo4O7 undergoes temperature-induced structural phase transitions at low temperature or not. Nothing is known about the influence of pressure on MBaCo4O7 compounds, but the apparent thermal instability suggests that these compounds will undergo structural phase transitions at elevated pressure. © 2021 HASYLAB
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    Diffuse scattering and QENS study of copper chalcogenides
    (Australian Institute of Physics, 2009-02-05) Danilkin, SA; Avdeev, M; Studer, AJ; Ling, CD; Macquart, RB; Russina, M; Izaola, Z
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    Neutron diffraction study of diffuse scattering in Cu2−δSe superionic compounds
    (Elsevier, 2011-05-05) Danilkin, SA; Avdeev, M; Sakuma, T; Macquart, RB; Ling, CD
    Crystal structure and short-range order in Cu2-delta Se compounds were studied in superionic and non-superionic phases using high-resolution neutron diffractometer Echidna at ANSTO. In diffraction patterns of beta-Cu-1.98 Se (ordered phase at ambient T), both Bragg peaks and diffuse background change sharply through the beta -> alpha structural phase transition at T = 414K during heating. In case of alpha-Cu1.75Se (disordered superionic phase at ambient T) the changes are monotonic, showing gradual shifts of Bragg peaks and increased intensity of the diffuse background as a function of temperature. On cooling, both compounds undergo a beta -> beta ' transformation. Diffuse scattering in the alpha-phase shows an oscillating dependence on wavevector, with broad peaks centred at Q similar to 3, 5.5 and 8 angstrom(-1). The measurements taken in energy dispersive mode show that the oscillating diffuse background arises from correlated thermal displacements of the ions. Diffuse scattering is higher for compositions close to stoichiometry and increases with temperature. Theoretical calculations show that the increase in diffuse intensity both with temperature and Cu content is related to correlated thermal vibrations of Se and Cu atoms, with Se-Cu(8c, 32f) and Cu(8c)-Cu(8c) correlations being the most important. (C) 2011 Elsevier B.V.
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    Neutron diffraction study of phase transitions in perovskite-type strontium molybdate SrMoO3
    (Elsevier, 2010-01) Macquart, RB; Kennedy, BJ; Avdeev, M
    The structure of a polycrystalline sample of SrMoO3 has been investigated using powder neutron diffraction from 5 to 300 K, to reveal two structural phase transitions, the first from the cubic structure with a=3.97629(3) Å to a tetragonal structure in I4/mcm near 266 K and the second to an orthorhombic Imma phase below 125 K. The average Mo–O distance is essentially independent of temperature. The temperature dependence of the octahedral tilting appears typical of a tricritical phase transition. © 2009, Elsevier Ltd.
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    Neutron scattering study of short-range correlations and ionic diffusion in copper selenide
    (Springer, 2011-02-01) Danilkin, SA; Avdeev, M; Sakuma, T; Macquart, RB; Ling, CD; Russina, M; Izaola, Z
    The paper reports the results of a neutron scattering study of Cu(2-delta)Se superionic compounds. The crystallographic model fitted to the diffraction data shows the occupation of 8c and 32f sites by Cu atoms. Observed diffuse background is related to correlated thermal vibrations of Se and Cu atoms, with Sea dagger"Cu (8c,32f) and Cu (8c)a dagger"Cu (8c) correlations being most important. The quasi-elastic neutron experiments show the decrease of the self-diffusion coefficient with the deviation from the stoichiometry due to the longer residence time of Cu ions between diffusion hops. Combination of neutron diffraction, diffuse scattering and quasi-elastic scattering experimental data suggests that the Cu atoms diffuse between the nearest 8c sites through the 32f sites. © 2011, Springer.
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    Primitive tetragonal intermediate in the orthorhombic-cubic phase transition of perovskite-type strontium niobate Sr0.92NbO3
    (Elsevier, 2010-10) Macquart, RB; Kennedy, BJ; Avdeev, M
    The Sr deficient perovskite Sr0.92NbO3 was synthesized from Sr5Nb4O15 and Nb and its crystal structure was determined using powder neutron diffraction. At room temperature the structure is orthorhombic in space group Pnma with both in-phase and out-of-phase tilting of the NbO6 octahedra. High temperature measurements have shown that the oxide undergoes a sequence of phase transitions with increasing temperature: Pnma→P4/mbm→Pm3¯m. The intermediate tetragonal phase has only in-phase tilts of the NbO6 octahedra, rather than the out-of-phase tilts present in the more commonly observed I4/mcm structure, due to initial softening at the M point rather than R point. The tetragonal phase exists only over a very narrow temperature range. The importance of M–M and M–O bonding in controlling the transition temperatures in SrMO3 perovskites is discussed. © 2010, Elsevier Ltd.
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    Quasi-elastic neutron scattering study of diffusion in Cu-Se superionic conductor
    (Malaysian Nuclear Agency, 2009-06-29) Danilkin, SA; Avdeev, M; Ling, CD; Macquart, RB; Russina, M; Izaola, Z
    Copper selenide is a mixed ionic-electronic conductor and received attention from the technological and physical point of view in particular due to a high ionic conductivity. According to [1, 2] only a fraction of Cu atoms takes part in the ionic transport in Cu{sub 2-x}Se compounds: the number of mobile atoms is about 1/3 - 1/8 of the total cation concentration in stoichiometric Cu{sub 2}Se and decreases with x causing the ionic conductivity to drop. This conclusion was based on the assumption that Cu mobility does not depend on composition. Therefore the QENS study is of interest because the width and intensity of quasi-elastic peak associated with Cu diffusion are directly related to Cu coefficient of self-diffusion and the number of mobile ions, respectively. This paper presents results of QENS measurements performed on Cu{sub 1.77}Se, Cu{sub 1.90}Se and Cu{sub 1.98}Se compounds at 313 and 430 K. We found that in Cu{sub 1.98}Se the quasielastic component is not observed in ordered non-superionic {alpha}-phase at T = 313 K, however it is clearly seen in superionic {beta}-phase at T=430 K. By contrast the Cu{sub 1.77}Se compound which is superionic at ambient temperature has relatively small quasielastic component showing little difference between 313 and 430 K. The analysis shows that fraction of Cu atoms which takes part in the ionic transport indeed decreases with x in general agreement with papers [1, 2], but not vanishes at x = 0.23. [1] R.A. Yakshibaev et al., Sov. Phys. Solid State, 26 2189 (1984) [2] M.A. Korzhuev, Phys. Solid State 40 217 (1998)
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    Re-investigation of the structure and crystal chemistry of the Bi2O3-W2O6 'type (Ib)' solid solution using single-crystal neutron and synchrotron x-ray diffraction
    (International Union of Crystallography, 2010-04) Sharma, N; Macquart, RB; Avdeev, M; Christensen, M; McIntyre, GJ; Chen, YS; Ling, CD
    Single crystals of composition Bi35.66W4.34O66.51 (or Bi8.2WO15.3, bismuth tungsten oxide), within the type (Ib) solid-solution region of the Bi2O3-WO3 system, were synthesized using the floating-zone furnace method. Synchrotron X-ray and neutron single-crystal diffraction data were used to confirm the previously tentative assignment of the room-temperature space group as I41. Fourier analysis of the combined X-ray and neutron datasets was used to elucidate and refine fully the cation and anion arrays for the first time. The mixed cation site M1 is shown to be coordinated by eight O atoms in an irregular cube when M = Bi, and by six O atoms in an octahedron when M = W. The resulting disorder in the average structure around M1 is discussed in the context of experimentally observed oxide-ion conductivity. © 2010, International Union of Crystallography
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    Single-crystal structure of HoBaCo4O7 at ambient conditions, at low temperature, and at high pressure
    (American Physical Society, 2009-02-18) Juarez-Arellano, EA; Friedrich, A; Wilson, DJ; Wiehl, L; Morgenroth, W; Winkler, B; Avdeev, M; Macquart, RB; Ling, CD
    We show that the correct space group of HoBaCo4O7 at ambient conditions is P63mc and that no temperature-induced or pressure-induced structural phase transition occurs down to 100 K or up to 9 GPa. The ompressibility of HoBaCo4O7 is mainly determined by a combination of bond compression and changes in the three-membered and six-membered rings of the kagomé layers. HoBaCo4O7 is more compressible than structurally related compounds due to the comparatively compressible Co-O bonds. The structural analysis allows us to propose an atomistic model for the extremely high oxygen incorporation capability of HoBaCo4O7. ©2009 American Physical Society
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    Structure and crystal chemistry of fluorite-related Bi38Mo7O78 from single crystal x-ray diffraction and ab initio calculations
    (Elsevier, 2009-06) Sharma, N; Macquart, RB; Christensen, M; Avdeev, M; Chen, YS; Ling, CD
    The floating-zone furnace method was used to synthesize single crystals of the fluorite-related delta-Bi2O3-type phase Bi(3)gMo(7)O(78) for the first time. Single crystal synchrotron X-ray diffraction data, in conjunction with ab initio (density functional theory) calculations, were used to solve, optimize, and refine the 5 x 3 x 3 commensurate superstructure of fluorite-type delta-Bi2O3 in Pbcn (a = 28.7058(11) angstrom, b = 16.8493(7) angstrom and c = 16.9376(6) angstrom, Z = 4, R-F= 11.26%, wR(I) = 21.67%). The structure contains stepped channels of Mo6+ in tetrahedral environments along the b axis and chains of Mo6+ in octahedral environments along the ac plane. The role of the stepped channels in oxide ion conduction is discussed. The simultaneous presence of both tetrahedral and octahedral coordination environments for Mo6+, something not previously observed in Mo6+-doped delta-Bi2O3-type phases, is supported by charge balance considerations in addition to the results of crystallographic and ab initio analysis. © 2009, Elsevier Ltd.
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    Structures, phase transitions, hydration, and ionic conductivity of Ba4Nb2O9
    (American Chemical Society, 2009-08-25) Ling, CD; Avdeev, M; Kutteh, R; Kharton, VV; Yaremchenko, AA; Fialkova, S; Sharma, N; Macquart, RB; Hoelzel, M; Gutmann, MJ
    Ba4Nb2O9 is shown to have two basic polymorphs: a high-temperature γ phase, which represents an entirely new structure typed and a low-temperature (x phase, which has the rare Sr4Ru2O9 structure type. The phases are separated by a reconstructive phase transition at similar to 1370 K, the kinetics of which are sufficiently slow that the γ phase can easily be quenched to room temperature. Below similar to 950 K, both (α and γ phases absorb significant amounts of water. In the case of the γ phase, protons from absorbed water occupy ordered positions in the structure, giving rise to a stoichiometric phase γ-III-Ba4Nb2O9.1/3H(2)O at room temperature. γ-III-Ba4Nb2O9-1/3H(2)O partially dehydrates, at similar to 760 K to give another stoichiometric phase γ-II-Ba4Nb2O9.1/3H(2)O, which completely dehydrates at similar to 950 K to γ-I- Ba4Nb2O9. The hydrated γ phases exhibit faster protonic and oxide ionic transport than the hydrated (x phases because of the presence in the γ phases of 2D layers containing Nb5+ cations with unusually low oxygen coordination numbers (4 or 5) separated by discrete OH groups. Hydration appears to play an important role in stabilizing the γ phases at low temperatures, with the γ -> α transition oil reheating a quenched sample occurring at higher temperatures in humid atmospheres. © 2009, American Chemical Society
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    Structures, phase transitions, hydration, and ionic conductivity of Ba4Ta2O9
    (American Chemical Society, 2010-01-26) Ling, CD; Avdeev, M; Kharton, VV; Yaremchenko, AA; Macquart, RB; Hoelzel, M
    Low-temperature α-Ba4Ta2O9 is isostructural with α-Ba4Nb2O9 (Sr4Ru2O9 type), and it undergoes a reconstructive phase transition at approximately the same temperature (1400 K) to a γ form that can easily be quenched to room temperature. However, the γ forms of the two compounds are completely different. Whereas γ-Ba4Nb2O9 represents a unique structure type, γ-Ba4Ta2O9 adopts a more conventional 6H-perovskite type. The α→γ transition is virtually irreversible in the tantalate, unlike the niobate, which can be converted back to the α form by annealing slightly below the transition temperature. Quenched γ-Ba4Ta2O9 is highly strained due to the extreme size mismatch between Ba2+ (1.35 Å) and Ta5+ (0.64 Å) cations in perovskite B-sites, and undergoes a series of symmetry-lowering distortions from P63/mmc→P63/m→P21/c; the second of these transitions has not previously been observed in a 6H perovskite. Below 950 K, both α-Ba4Ta2O9 and γ-Ba4Ta2O9 hydrate to a greater extent than the corresponding phases of Ba4Nb2O9. Both hydrated forms show significant mixed protonic and oxide ionic conductivity, and electronic conductivity upon dehydration. © 2010, American Chemical Society
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    YCa3(VO)3(BO3)4: a kagom compound based on vanadium(III) with a highly frustrated ground state
    (American Chemical Society, 2011-01-31) Miiller, W; Christensen, M; Khan, A; Sharma, N; Macquart, RB; Avdeev, M; McIntyre, GJ; Piltz, RO; Ling, CD
    A new S = 1 kagomé compound based on vanadium(III) is reported. The structure was refined simultaneously against single-crystal neutron and X-ray diffraction data, as a gaudefroyite-type with a new supercell (a′ = 2a + b, b′ = −a − 2b, c′ = c) driven by the ordering of columns of isolated triangular BO3(3)− ions. Low-temperature neutron powder diffraction and magnetic (dc and ac susceptibility) data rule out the presence of long-range magnetic order above at least 1.5 K, but specific heat data suggest that the ground state involves short-range magnetic order, which is frustrated by the coexistence/competition of FM and AFM correlations, together with the characteristic geometric frustration of the kagomé lattice. Magnetic susceptibility data rule out a spin-glass state, pointing to an exotic ground state comparable to the spin-ice or spin-liquid states. This makes YCa3(VO)3(BO3)4 one of the most highly frustrated experimental realizations of the kagomé lattice yet discovered. © 2011, American Chemical Society