Browsing by Author "Maljuk, A"

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    A (3 + 3)-dimensional “hypercubic” oxide-ionic conductor: type ii bi2o3–nb2o5
    (ACS Publications, 2013-04-09) Ling, CD; Schmid, S; Blanchard, PER; Petříček, V; McIntyre, GJ; Sharma, N; Maljuk, A; Yaremchenko, AA; Kharton, VV; Gutmann, MJ; Withers, RL
    The high-temperature cubic form of bismuth oxide, δ-Bi2O3, is the best intermediate-temperature oxide-ionic conductor known. The most elegant way of stabilizing δ-Bi2O3 to room temperature, while preserving a large part of its conductivity, is by doping with higher valent transition metals to create wide solid-solutions fields with exceedingly rare and complex (3 + 3)-dimensional incommensurately modulated ?hypercubic? structures. These materials remain poorly understood because no such structure has ever been quantitatively solved and refined, due to both the complexity of the problem and a lack of adequate experimental data. We have addressed this by growing a large (centimeter scale) crystal using a novel refluxing floating-zone method, collecting high-quality single-crystal neutron diffraction data, and treating its structure together with X-ray diffraction data within the superspace symmetry formalism. The structure can be understood as an ?inflated? pyrochlore, in which corner-connected NbO6 octahedral chains move smoothly apart to accommodate the solid solution. While some oxide vacancies are ordered into these chains, the rest are distributed throughout a continuous three-dimensional network of wide δ-Bi2O3-like channels, explaining the high oxide-ionic conductivity compared to commensurately modulated phases in the same pseudobinary system. © 2013, American Chemical Society.
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    Competing exchange interactions on the verge of a metal-insulator transition in the two-dimensional spiral magnet Sr3Fe2O7
    (Americal Physical Society, 2014-10-03) Kim, JH; Jain, A; Reehuis, M; Khaliullin, G; Peets, DC; Ulrich, C; Park, JT; Faulhaber, E; Hoser, A; Walker, HC; Adroja, DT; Walters, AC; Inosov, DS; Maljuk, A; Keimer, B
    We report a neutron scattering study of the magnetic order and dynamics of the bilayer perovskite Sr 3 Fe 2 O 7 , which exhibits a temperature-driven metal-insulator transition at 340 K. We show that the Fe 4+ moments adopt incommensurate spiral order below T N =115  K and provide a comprehensive description of the corresponding spin-wave excitations. The observed magnetic order and excitation spectra can be well understood in terms of an effective spin Hamiltonian with interactions ranging up to third-nearest-neighbor pairs. The results indicate that the helical magnetism in Sr 3 Fe 2 O 7 results from competition between ferromagnetic double-exchange and antiferromagnetic superexchange interactions whose strengths become comparable near the metal-insulator transition. They thus confirm a decades-old theoretical prediction and provide a firm experimental basis for models of magnetic correlations in strongly correlated metals. © 2014, American Physical Society.
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    Growth of LiCoO2 single crystals by the TSFZ method
    (American Chemical Society, 2018-11-08) Nakamura, S; Maljuk, A; Maruyama, Y; Nagao, M; Watauchi, S; Hayashi, T; Anzai, Y; Furukawa, Y; Ling, CD; Deng, G; Avdeev, M; Büchner, B; Tanaka, I
    We have grown LiCoO2 single crystals by the traveling solvent floating zone (TSFZ) growth with Li-rich solvent, having observed the incongruent melting behavior of LiCoO2 between 1100 and 1300 °C. The optimum growth conditions in terms of atmosphere and solvent composition were determined to be Ar flow and an atomic ratio Li/Co 85:15, respectively. The crystals grown using a conventional-mirror-type furnace contained periodic inclusions of a Co–O phase due to the influence of Co–O phase segregation on the stability of the molten zone during growth. By using a tilted-mirror FZ furnace, inclusion-free LiCoO2 crystals of about 5 mm in diameter and 70 mm long were obtained at a tilting angle θ = 10°. The grown crystals were confirmed to be single-domain by neutron Laue diffraction. © 2018 American Chemical Society
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    Magnetic phase diagram of Sr3Fe2O7-delta
    (American Physical Society, 2013-06-10) Peets, DC; Kim, JH; Dosanjh, P; Reehuis, M; Maljuk, A; Aliouane, N; Ulrich, C; Keimer, B
    Magnetometry, electrical transport, and neutron scattering measurements were performed on single crystals of the Fe4+-containing perovskite-related phase Sr3Fe2O7−δ as a function of oxygen content. Although both the crystal structure and electron configuration of this compound are closely similar to those of well-studied ruthenates and manganates, it exhibits very different physical properties. The fully oxygenated compound (δ=0) exhibits a charge-disproportionation transition at TD=340 K, and an antiferromagnetic transition at TN=115 K. For temperatures T≤TD, the material is a small-gap insulator; the antiferromagnetic order is incommensurate, which implies competing exchange interactions between the Fe4+ moments. The fully deoxygenated compound (δ=1) is highly insulating, and its Fe3+ moments exhibit commensurate antiferromagnetic order below TN∼600 K. Compounds with intermediate δ exhibit different order with lower TN, likely as a consequence of frustrated exchange interactions between Fe3+ and Fe4+ sublattices. A previous proposal that the magnetic transition temperature reaches zero is not supported. © 2013, American Physical Society.
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    Neutron diffraction study of spin and charge ordering in SrFeO3-delta
    (American Physical Society, 2012-05-22) Reehuis, M; Ulrich, C; Maljuk, A; Niedermayer, C; Ouladdiaf, B; Hoser, A; Hofmann, T; Keimer, B
    We report a comprehensive neutron diffraction study of the crystal structure and magnetic order in a series of single-crystal and powder samples of SrFeO3-delta in the vacancy range 0 <= delta <= 0.23. The data provide detailed insights into the interplay between the oxygen vacancy order and the magnetic structure of this system. In particular, a crystallographic analysis of data on Sr8Fe8O23 revealed a structural transition between the high-temperature tetragonal and a low-temperature monoclinic phase with a critical temperature T = 75 K, which originates from charge ordering on the Fe sublattice and is associated with a metal-insulator transition. Our experiments also revealed a total of seven different magnetic structures of SrFeO3-delta in this range of delta, only two of which namely an incommensurate helix state in SrFeO3 and a commensurate, collinear antiferromagnetic state in Sr4Fe4O11) had been identified previously. We present a detailed refinement of some of the magnetic ordering patterns and discuss the relationship between the magnetotransport properties of SrFeO3-delta samples and their phase composition and magnetic microstructure. © 2012, American Physical Society.
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    Selective 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, CD
    The 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.