Browsing by Author "Morgenroth, W"

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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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    On the temperature dependence of H-Uiso in the riding hydrogen model
    (Acta Crystallographica, 2014-07) Lübben, J; Volkmann, C; Grabowsky, S; Edwards, AJ; Morgenroth, W; Fabbiani, FPA; Sheldrick, GM; Dittrich, B
    The temperature dependence of H-Uiso in N-acetyl-L-4-hydroxyproline monohydrate is investigated. Imposing a constant temperature-independent multiplier of 1.2 or 1.5 for the riding hydrogen model is found to be inaccurate, and severely underestimates H-Uiso below 100 K. Neutron diffraction data at temperatures of 9, 150, 200 and 250 K provide benchmark results for this study. X-ray diffraction data to high resolution, collected at temperatures of 9, 30, 50, 75, 100, 150, 200 and 250 K (synchrotron and home source), reproduce neutron results only when evaluated by aspherical-atom refinement models, since these take into account bonding and lone-pair electron density; both invariom and Hirshfeld-atom refinement models enable a more precise determination of the magnitude of H-atom displacements than independent-atom model refinements. Experimental efforts are complemented by computing displacement parameters following the TLS+ONIOM approach. A satisfactory agreement between all approaches is found. © 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