Universität Stuttgart

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Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftInstitute: search.filters.UBSInstitut.Institut für Anorganische Chemie

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    Li5Sn, the most lithium-rich binary stannide : a combined experimental and computational study
    (2022) Stelzer, Robert U.; Ikeda, Yuji; Srinivasan, Prashanth; Lehmann, Tanja S.; Grabowski, Blazej; Niewa, Rainer
    From reaction of excess lithium with tin, we isolate well-crystallized Li5Sn and solve the crystal structure from single-crystal X-ray diffraction data. The orthorhombic structure (space group Cmcm) features the same coordination polyhedra around tin and lithium as previously predicted by electronic structure calculations for this composition, however differently arranged. An extensive ab initio analysis, including thermodynamic integration using Langevin dynamics in combination with a machine-learning potential (moment tensor potential), is conducted to understand the thermodynamic stability of this Cmcm Li5Sn structure observed in our experiments. Among the 108 Li5Sn structures systematically derived using the structure enumeration algorithm, including the experimental Cmcm structure and those obtained in previous ab initio studies, another new structure with the space group Immm is found to be energetically most stable at 0 K. This computationally discovered Immm structure is also found to be thermodynamically more stable than the Cmcm structure at finite temperatures, indicating that the Cmcm Li5Sn structure observed in our experiments is favored likely due to kinetic reasons rather than thermodynamics.
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    Crystal structure and phase stability of Co2N: a combined first-principles and experimental study
    (2021) Ikeda, Yuji; Lehmann, Tanja S.; Widenmeyer, Marc; Coduri, Mauro; Grabowski, Blazej; Niewa, Rainer
    The crystal structure and phase stability of Co2N are revisited based on experiments and first-principles calculations. Powder X-ray diffraction (PXRD) measurements and Rietveld refinements clearly confirm that the stable crystal structure of Co2N is an isotype of η-Fe2C and Co2C with the space group Pnnm rather than the closely related ζ-Fe2N with the space group Pbcn. The refined lattice parameters of Co2N in the Pnnm structure are a = 4.6108(1) Å, b = 4.3498(1) Å, c = 2.85592(7) Å, obtained from X-ray diffraction using synchrotron radiation. Furthermore, differential scanning calorimetry (DSC) with subsequent diffraction experiments reveal an endothermal transition to an ε-type order at 398 °C followed by an exothermal decomposition at 446 °C. First-principles density-functional-theory (DFT) calculations including the Hubbard U correction (DFT+U) demonstrate that it is essential for transition metal nitrides to consider strong electron correlation to predict the correct experimental structure and magnetic state. In particular, an effective value of Ueff = 2.75 eV can be utilized to obtain an antiferromagnetic Pnnm phase of Co2N in agreement with experiments.
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    Revealing an intercalation nature of high‐capacity conversion cathode materials for fluoride‐ion batteries by operando studies
    (2025) Chen, Hong; Schoch, Roland; Chotard, Jean‐Noel; Thiebes, Yannick M.; Wissel, Kerstin; Niewa, Rainer; Bauer, Matthias; Clemens, Oliver
    To improve the performance of high‐energy‐density electrode materials for all‐solid‐state fluoride‐ion batteries (ASSFIBs), it is important to understand the structure and phase evolution during operation, which is closely correlated to capacity fading. In this study, an operando cell is designed compatible with laboratory X‐ray diffraction (XRD) to monitor real‐time structural changes of bismuth trifluoride (BiF3) cathodes and degradation of the ionic conductor BaSnF4 under negative potentials at 100 °C. Supported by ex‐situ XRD, our results reveal a multi‐step defluorination of BiF3: from orthorhombic (o‐BiF3) to cubic (c‐BiF3), then to distorted orthorhombic (o’‐BiF3), and finally to metallic bismuth (Bi), indicating partial intercalation‐type character. Formation of bismuth oxidefluoride (BiOF) beyond 200 mAh g-1 is attributed to oxygen impurities introduced via solid‐state synthesis. operando X‐ray absorption spectroscopy (XAS) confirms a continuous reduction of Bi3+ to Bi0 with intermediate phases. Rietveld refinement quantifies the phase fractions and structural transitions, enabling a model for BiF3 defluorination. Comparison of operando XRD and XAS reveals that BaSnF4 contributes transport of both fluoride and oxygen impurities, leading to BiOF formation. BaSnF4 also exhibits a broad stability window, with degradation occurring below -200 mV, rather than the expected -50 mV vs. Sn/SnF2.