Browsing by Author "Injac, S"

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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 structures and phase transition behaviour in the 5d transition metal oxides AReO4 (A = Ag, Na, K, Rb, Cs and Tl)
    (Royal Society of Chemistry, 2019-11-11) Chay, C; Avdeev, M; Brand, HEA; Injac, S; Whittle, TA; Kennedy, BJ
    The structures of the six perrhenates (AReO4 A = Ag, Na, K, Rb, Cs and Tl) at room temperature have been established using powder neutron diffraction methods. These demonstrate the rigid nature of the ReO4 tetrahedra, with the Re–O distances decreasing very slightly and the O–Re–O bond angles approaching the regular tetrahedron value of 109.5° as the size of the A-type cation increases. Variable temperature synchrotron X-ray diffraction measurements show that RbReO4 undergoes a I41/a to I41/amd transition near 650 K that is associated with a change in the orientation of the ReO4− tetrahedra about the scheelite b-axis associated with a Γ3+ mode. CsReO4 has an orthorhombic pseudo scheelite structure at room temperature with rotation of the ReO4 tetrahedra about the c-axis described by mode M4+ and this undergoes a first order orthorhombic to tetragonal (Pnma to I41/a) transition near 450 K with a transition to the I41/amd structure occurring above this. TlReO4 is a rare example of a crystalline material displaying a re-entrant phase transition; 141/a to P21/c to 141/a. The monoclinic structure can be described as a scheelite superstructure that contains an ordering of tetrahedral rotations around the c-axis and along the b-axis with the irrep Γ3+ and M4+ both present. This behaviour is different to that described recently for the analogous Tc oxide TlTcO4, which highlights the differences in the chemistry of these two systems. © Royal Society of Chemistry 2019
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    Magnetic and structural studies of Sc containing ruthenate double perovskites A2ScRuO6 (A = Ba, Sr)
    (American Chemical Society, 2017-07-10) Kayser, P; Injac, S; Ranjbar, B; Kennedy, BJ; Avdeev, M; Yamaura, K
    Ruthenium-containing double perovskites A2ScRuO6 have been synthesized as polycrystalline powders and structurally characterized using a combination of synchrotron X-ray and neutron powder diffraction methods. When A = Ba, a hexagonal 6L perovskite structure is obtained if the synthesis is conducted at ambient pressure and a rock-salt ordered cubic structure is obtained if the sample is quenched from high pressures. The Sr oxide Sr2ScRuO6 is obtained with a rock-salt ordered corner sharing topology. Heat capacity and bulk magnetic susceptibility measurements show that the three oxides are antiferromagnets. Cubic Ba2ScRuO6 undergoes a metal–insulator transition near 270 K and hexagonal Ba2ScRuO6 is a semiconductor with an activation energy of 0.207 eV. The magnetic structures of the two rock-salt ordered double perovskites were established using powder neutron diffraction and are described by k = (0,0,1) and k = (0,0,0) for the Ba and Sr oxides, respectively, corresponding to type I antiferromagnetic structures, with ferromagnetic layers stacked antiferromagnetically. The ambient-pressure hexagonal polymorph of Ba2ScRuO6 has partial Sc-Ru ordering at both the face-sharing B2O9 dimer and corner-sharing BO6 sites. The magnetic structure is described by k = (1/2,0,0) with the basis vector belonging to the irreducible representation Γ3. © 2017 American Chemical Society
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    Structural and magnetic properties of the osmium double perovskites Ba2–xSrxYOsO6
    (American Chemical Society, 2017-05-17) Kayser, P; Injac, S; Kennedy, BJ; Vogt, T; Avdeev, M; Maynard-Casely, HE; Zhang, Z
    The crystal and magnetic structures of double perovskites of the type Ba2–xSrxYOsO6 were studied by synchrotron X-ray and neutron powder diffraction methods, bulk magnetic susceptibility measurements, and X-ray absorption spectroscopy. The structures were refined using combined neutron and synchrotron data sets based on an ordered array of corner-sharing YO6 and OsO6 octahedra, with the Ba/Sr cations being completely disordered. The structure evolves from cubic to monoclinic Fm3̅m (x ≈ 0.6) → I4/m (x ≈ 1.0) → I2/m (x ≈ 1.6) → P21/n as the Sr content is increased, due to the introduction of cooperative tilting of the octahedra. Bulk magnetic susceptibility measurements demonstrate the oxides are all anti-ferromagnets. The decrease in symmetry results in a nonlinear increase in the Neel temperature. Low-temperature neutron diffraction measurements of selected examples show these to be type-I anti-ferromagnets. X-ray absorption spectra collected at the Os L3- and L2-edges confirm the Os is pentavalent in all cases, and there is no detectable change in the covalency of the Os cation as the A-cation changes. Analysis of the L3/L2 branching ratio shows that the spin–orbit coupling is constant and insignificant across the series. © 2017 American Chemical Society
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    Structural 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, BJ
    Oxides 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
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    Structural and magnetic studies of KOsO4, a 5d1 quantum magnet oxide
    (Royal Society of Chemistry, 2019-03-12) Injac, S; Yuen, AKL; Avdeev, M; Orlandi, F; Kennedy, BJ
    The quantum magnet KOsO4 has been characterized by a combination of X-ray and neutron diffraction techniques. The tetrahedrally coordinated Os7+ 5d1S = 1/2 cations were determined to order antiferromagnetically along the c axis below 35 K. A miniscule ordered magnetic moment of 0.46(18) μB was determined per Os7+ cation. © Royal Society of Chemistry 2021
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    Structural and magnetic studies of the ruthenium perovskites Ba2-xSrxHoRuO6
    (Elsevier, 2018-07-01) Injac, S; Kayser, P; Avdeev, M; Kennedy, BJ
    The series of ruthenate double perovskites Ba2-xSrxHoRuO6 (0 ≤ x ≤ 2) have been synthesized using solid state methods. The crystal structures of the series have been determined by high resolution synchrotron X-ray diffraction and show the sequence of structures to be Fm3̅m (a0a0a0) (0 ≤ x ≤ 0.6) → I4/m (a0a0c-) (x = 0.8) → I2/m (a0b-b-) (1.0 ≤ x ≤ 1.2) → P21/n (a-a-c+) (1.4 ≤ x ≤ 2.0). A similar progression of structures is observed in the sample BaSrHoRuO6 with increasing temperature. Magnetic characterisation of these materials was undertaken utilising variable temperature bulk magnetic susceptibility, isothermal magnetisation and low temperature neutron powder diffraction measurements. All members of the series order antiferromagnetically with the Ru and Ho sublattices ordering at different temperatures between 50 and 16 K. The presence of the magnetic Ho3+ at the perovskite B site stabilises the antiferromagnetic (AFM) ordering of the Ru sublattice. The addition of Sr for Ba beyond x = 1.2 weakens the AFM superexchange interactions through induced structural distortions, resulting in a canted AFM ground state. © 2018 Elsevier Inc.
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    Structural studies of the high temperature phases of AgTaO3
    (Elsevier, 2018-02-01) Farid, U; Khan, HU; Avdeev, M; Injac, S; Kennedy, BJ
    Abstract: The temperature dependence of the structure of a polycrystalline sample of AgTaO3 has been determined using in-situ Synchrotron X-ray powder diffraction methods. This work finds no evidence for the presence of a monoclinic phase, rather three phase transitions have been identified, namely R3c↔390°CCmcm↔465°CP4/mbm↔580°CPm3̅m. The rhombohedral phase was further studied at room temperature by neutron powder diffraction. The co-existence of the rhombohedral and orthorhombic phases around 380–400 ° C indicates that the transition between these is first order, and gives rise to unusual peak shapes in the diffraction patterns. © 2017 Elsevier Inc.
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    Structures and phase transitions in pertechnetates
    (American Chemical Society, 2019-07-07) Kennedy, BJ; Injac, S; Thorogood, GJ; Brand, HEA; Poineau, F
    The temperature dependence of the structures of four pertechnetates (ATcO4A = Ag, Tl, Rb, Cs) from 90 K to their melting points is described. Synchrotron X-ray diffraction measurements show that RbTcO4 undergoes a I41/a to I41/amd transition near 530 K that is associated with a change in the orientation of the TcO4– tetrahedra about the scheelite b axis. AgTcO4 also exhibits a tetragonal scheelite type structure, and this is retained between 90 and 750 K, above which it melted. CsTcO4 has an orthorhombic pseudo-scheelite structure at room temperature and this undergoes a first-order orthorhombic to tetragonal transformation (Pnma to I41/a) near 430 K. TlTcO4 is isostructural with CsTcO4 at 90 K, but the orthorhombic to tetragonal transformation proceeds via an intermediate orthorhombic phase. The different behavior found here and described previously for the analogous Re oxide TlReO4 highlights the differences in the chemistry of these two systems. © 2019 American Chemical Society
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    Studies of the 4d and 5d 6H perovskites Ba3BM2O9, B = Ti, Zn, Y; M = Ru, Os, and cubic BaB1/3Ru2/3O3 polymorphs stabilised under high pressure
    (Royal Society of Chemistry, 2020-08-05) Injac, S; Solana-Madruga, E; Avdeev, M; Brand, HEA; Attfield, JP; Kennedy, BJ
    The synthesis, structures and magnetism of six mixed 3d–5d oxides Ba3BM2O9 (B = Ti, Y, Zn; M = Ru, Os) are described. When prepared at ambient pressure the six oxides display a 6H type perovskite structure comprised of corner sharing BO6 and face sharing M2O9 motifs. Synchrotron X-ray diffraction reveals a small monoclinic distortion in Ba3ZnRu2O9; the remaining oxides exhibit a hexagonal structure. The magnetic properties are dominated by the M–M interactions across the shared face. Only in the mixed valent (M4+/M5+) Y oxides is evidence of long-range magnetic order found. Application of high pressure/high temperature synthetic methods for the Ru containing oxides changes the structure to the archetypical cubic Pm[3 with combining macron]m perovskite structure, where the B and Ru cations are disordered on the corner sharing BO6 octahedral sites. The magnetic properties of the cubic oxides are dominated by short range antiferromagnetic interactions, the chemical disorder inhibiting long range ordering. © The Royal Society of Chemistry 2020