Browsing by Author "Brown, AJ"
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- ItemCompeting magnetic interactions and the role of unpaired 4f llectrons in oxygen-deficient perovskites Ba3RFe2O7.5 (R = Y, Dy)(American Chemical Society (ACS), 2023-05-01) Brown, AJ; Avdeev, M; Manjón-Sanz, A; Brand, HEA; Ling, CDOxygen-deficient perovskite compounds with the general formula Ba3RFe2O7.5 present a good opportunity to study competing magnetic interactions between Fe3+ 3d cations with and without the involvement of unpaired 4f electrons on R3+ cations. From analysis of neutron powder diffraction data, complemented by ab initio density functional theory calculations, we determined the magnetic ground states when R3+ = Y3+ (non-magnetic) and Dy3+ (4f9). They both adopt complex long-range ordered antiferromagnetic structures below TN = 6.6 and 14.5 K, respectively, with the same magnetic space group Ca2/c (BNS #15.91). However, the dominant influence of f-electron magnetism is clear in temperature dependence and differences between the size of the ordered moments on the two crystallographically independent Fe sites, one of which is enhanced by R-O-Fe superexchange in the Dy compound, while the other is frustrated by it. The Dy compound also shows evidence for temperature- and field-dependent transitions with hysteresis, indicating a field-induced ferromagnetic component below TN. © 2024 American Chemical Society
- ItemDistinct intercalation and conduction behaviors within an isostructural series Ba5R2Al2SnO1(American Chemical Society, 2024-07-30) Brown, AJ; Wagstaff, OJ; Evans, IR; Evans, JSO; Mole, RA; Wykes, JL; Avdeev, M; Ling, CDThe series Ba5R2Al2SnO13 (R = In, Y, Er, Ho, Tb) has been synthesized and structurally characterized by X-ray and neutron powder diffraction. All members have oxygen-deficient 10-layer hexagonal (10H) perovskite-type structures at high temperature and gain mass on cooling equivalent to ∼0.5 oxygen atoms per formula unit, observed by both thermogravimetric analysis and the occupancy of a vacant site in the oxygen substructure refined against neutron powder diffraction data. The origin of this mass gain varies with R: for R = In, Y, Er, and Ho, it is due to water uptake via a hydroxylation mechanism to form Ba5R2Al2SnO13.xH2O (x ≤ 0.5), with OH– ions occupying the vacant site and the other proton forming a second OH– in the oxygen substructure; while for R = Tb, it due to the oxidation of Tb3+ to Tb4+, with O2– ions occupying the vacant site. These chemico-structural differences are consistent with the measured conductivity behavior of the samples, whereby Ba5Er2Al2SnO13 is a proton conductor in air at moderate temperatures (∼10–4 S cm–1 at 500 °C) while Ba5Tb2Al2SnO13 is a mixed oxide ionic and electronic conductor. These differences were further confirmed by X-ray absorption spectroscopy and corroborated by quasielastic neutron scattering. © 2024 American Chemical Society.
- ItemDominant Kitaev interactions in the honeycomb materials Na3Co2SbO6 and Na2Co2TeO6(American Physical Society (APS), 2022-07-01) Sanders, AL; Mole, RA; Liu, J; Brown, AJ; Yu, DH; Ling, CD; Rachel, SCobaltates with 3d based layered honeycomb structure were recently proposed as Kitaev magnets and putative candidates to host the long-sought Kitaev spin liquid. Here we present inelastic neutron scattering results down to 50 mK for powder samples of Na3Co2SbO6 and Na2Co2TeO6, with high resolution in regions of low momentum and energy transfers. We compare the experimental data below the antiferromagnetic zigzag ordering temperature with dynamical structure factors obtained within spin wave theory. We search the wide parameter range of a K-J1-Γ-Γ′-J3 spin 1/2 model and identify the best fits to constant momentum cuts of the inelastic neutron data. The powder average limits our ability to uniquely select a best-fit model, but we find that the experimental data is matched equally well by two classes of parameters: one with a dominant K<0, |K/J1|∼5...25, and another with K>0, |K/J1|∼1. We show that these classes are equivalent under the exact self-duality transformation identified by Chaloupka and Khalliulin [Phys. Rev. B 92, 024413 (2015)10.1103/PhysRevB.92.024413]. This model symmetry unifies a number of previous parameter estimates. Though the two cases are indistinguishable by our experiment, there is evidence in favor of the K<0 case. A purely isotropic Heisenberg model is incompatible with our results. ©2024 American Physical Society. All rights reserved.
- ItemExpanded chemistry and mixed ionic-electronic conductivity in vanadium-substituted variants of γ-Ba4Nb2O9(International Union of Crystallography, 2021-08-14) Brown, AJ; Schwaighofer, B; Avdeev, M; Johannessen, B; Evans, IR; Ling, CDTwo new compositional series with the previously unique γ-Ba4Nb2O9 type structure, γ-Ba4VxTa2-xO9 and γ-Ba4VxNb2-xO9 (x = 0-2/3), have been synthesised via solid-state methods. Undoped Ba4Ta2O9 forms a 6H-perovskite type phase, but with sufficient V doping the γ-type phase is thermodynamically preferred and possibly more stable than γ-Ba4Nb2O9, forming at a 200 °C lower synthesis temperature. This is explained by the fact that Nb5+ ions in γ-Ba4Nb2O9 simultaneously occupy 4-, 5- and 6-coordinate sites in the oxide sublattice, which is less stable than allowing smaller V5+ to occupy the former and larger Ta5+ to occupy the latter. We characterised the structures of the new phases using a combination of X-ray and neutron powder diffraction. All compositions hydrate rapidly and extensively (up to 1/3 H2O per formula unit) under ambient conditions, like the parent γ-Ba4Nb2O9 phase, and show moderate but improved mixed-ionic electronic conduction. At lower temperatures the ionic conduction is predominately protonic, while at higher temperatures it is dominated by oxide and electron-hole conduction.
- ItemExpanded chemistry and proton conductivity in vanadium-substituted variants of γ-Ba4Nb2O9(American Chemical Society, 2021-09-09) Brown, AJ; Schwaighofer, B; Avdeev, M; Johannessen, B; Evans, IR; Ling, CDWe have substantially expanded the chemical phase space of the hitherto unique γ-Ba4Nb2O9 type structure by designing and synthesizing stoichiometric ordered analogues γ-Ba4V1/3Ta5/3O9 and γ-Ba4V1/3Nb5/3O9 and exploring the solid-solution series γ-Ba4VxTa2–xO9 and γ-Ba4VxNb2–xO9. Undoped Ba4Ta2O9 forms a 6H-perovskite type phase, but with sufficient V doping the γ-type phase is thermodynamically preferred and possibly more stable than γ-Ba4Nb2O9, forming at a 200 °C lower synthesis temperature. This is explained by the fact that Nb5+ ions in γ-Ba4Nb2O9 simultaneously occupy 4-, 5-, and 6-coordinate sites in the oxide sublattice, which is less stable than allowing smaller V5+ to occupy the former two and larger Ta5+ to occupy the latter. The x = 1/3 phase γ-Ba4V1/3Ta5/3O9 shows greatly improved ionic conduction compared to the x = 0 phase 6H-Ba4Ta2O9. We characterized the structures of the new phases using a combination of X-ray and neutron powder diffraction. All compositions hydrate rapidly and extensively (up to 1/3 H2O per formula unit) in ambient conditions, like the parent γ-Ba4Nb2O9 phase. At lower temperatures, the ionic conduction is predominately protonic, while at higher temperatures it is likely other charge carriers make increasing contributions.© 2021 American Chemical Society
- ItemSelective interstitial hydration explains anomalous structural distortions and ionic conductivity in 6H-Ba4Ta2O9·1/2H2O(American Chemical Society, 2023-04-11) Marlton, FP; Brown, AJ; Sale, M; Maljuk, A; Büchner, B; Lewis, W; Luck, I; Wood, ML; Mole, RA; Ling, CDThe mixed ionic-electronic conductor 6H-Ba4Ta2O9 undergoes an unconventional symmetry-lowering lattice distortion when cooled below 1100 K in the presence of atmospheric water. This temperature corresponds to the onset of hydration, which reaches a maximum value for 6H-Ba4Ta2O9·1/2H2O below ∼500 K. We use a combination of diffraction, ab initio calculations, and spectroscopy to show that both processes are intimately linked. The presence of very large Ba2+ cations in octahedral interstitial sites (B sites of its hexagonal perovskite-type structure) forces the adjacent vacant octahedral interstitial sites also to expand, making room for them to incorporate hydration species with a total stoichiometric H2O in constrained and highly acidic environments, where they show structural and dynamic characteristics intermediate between those of covalent water molecules and discrete protons and hydroxide ions. This in turn destabilizes the structure so that it distorts on cooling in a way that cannot be explained by conventional symmetry-lowering mechanisms. The resulting synergistic hydration-distortion mechanism is, to the best of our knowledge, unique to close-packed ionic compounds. © 2023 American Chemical Society.
- ItemSynthesis and crystal structures of two polymorphs of Li4–2xMg1+ xTeO6(Elsevier, 2020-07-01) Brown, AJ; Liu, JT; Marlton, FP; Avdeev, M; Kennedy, BJ; Ling, CDTwo polymorphs of lithium magnesium tellurate Li4–2xMg1+xTeO6 have been prepared by solid-state reactions and their crystal structures characterised by powder X-ray and neutron diffraction. For x ≈ 0, a monoclinic C2/m phase is obtained, structurally similar to other O3 type honeycomb layered tellurate and antimonate compounds. The basic structure consists of [Mg2TeO6]3− honeycomb layers alternating with Li layers, with some anti-site disorder of Li and Mg between layers, analogous to the structure of Li4ZnTeO6. For 0 < x < ~0.5 (specifically, x = 0.33) an orthorhombic Fddd phase is obtained, with a rock-salt superstructure containing disordered Li/Mg cation sites surrounding ordered TeO6 octahedra, analogous to the structure of Li3Co2TaO6.© 2020 Elsevier Inc.
- ItemSynthesis-controlled polymorphism and magnetic and electrochemical properties of Li3Co2SbO6(American Chemical Society, 2019-10-04) Brown, AJ; Xia, Q; Avdeev, M; Kennedy, BJ; Ling, CDLi3Co2SbO6 is found to adopt two highly distinct structural forms: a pseudohexagonal (monoclinic C2/m) layered O3-LiCoO2 type phase with “honeycomb” 2:1 ordering of Co and Sb, and an orthorhombic Fddd phase, isostructural with Li3Co2TaO6 but with the addition of significant Li/Co ordering. Pure samples of both phases can be obtained by conventional solid-state synthesis via a precursor route using Li3SbO4 and CoO, by controlling particle size, initial lithium excess, and reaction time. Both phases show relatively poor performance as lithium-ion battery cathode materials in their as-made states, but complex and interesting low-temperature magnetic properties. The honeycomb phase is the first of its type to show A-type antiferromagnetic order (ferromagnetic planes, antiferromagnetically coupled) below TN = 14 K. Isothermal magnetization and in-field neutron diffraction below TN show clear evidence for a metamagnetic transition at H ≈ 0.7 T to three-dimensional ferromagnetic order. The orthorhombic phase orders antiferromagnetically below TN = 112 K and then undergoes two more transitions at 80 and 60 K. Neutron diffraction data show that the ground state is incommensurate. © 2019 American Chemical Society