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Subject: search.filters.subject.530Institute: search.filters.UBSInstitut.Institut für MaterialwissenschaftStart date: 2025End date: 2025

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    ItemOpen Access
    Ab initio machine-learning unveils strong anharmonicity in non-Arrhenius self-diffusion of tungsten
    (2025) Zhang, Xi; Divinski, Sergiy V.; Grabowski, Blazej
    The knowledge of diffusion mechanisms in materials is crucial for predicting their high-temperature performance and stability, yet accurately capturing the underlying physics like thermal effects remains challenging. In particular, the origin of the experimentally observed non-Arrhenius diffusion behavior has remained elusive, largely due to the lack of effective computational tools. Here we propose an efficient ab initio framework to compute the Gibbs energy of the transition state in vacancy-mediated diffusion including the relevant thermal excitations at the density-functional-theory level. With the aid of a bespoke machine-learning interatomic potential, the temperature-dependent vacancy formation and migration Gibbs energies of the prototype system body-centered cubic (BCC) tungsten are shown to be strongly affected by anharmonicity. This finding explains the physical origin of the experimentally observed non-Arrhenius behavior of tungsten self-diffusion. A remarkable agreement between the calculated and experimental temperature-dependent self-diffusivity and, in particular, its curvature is revealed. The proposed computational framework is robust and broadly applicable, as evidenced by first tests for a hexagonal close-packed (HCP) multicomponent high-entropy alloy. The successful applications underscore the attainability of an accurate ab initio diffusion database.
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    ItemOpen Access
    Synthesis and optical properties of single‐crystalline phosphors Gd3In2Ga3O12:RE3+ (RE = Nd3+ and Ho3+) grown via the optical float zone method
    (2025) Yilmaz, Hasan; Kinik, Gülsüm; Isobe, Masahiko; Puphal, Pascal; Suta, Markus; Clemens, Oliver
    The continuous development of innovative optical materials with lanthanoid ions as activators has emerged as a modern sector of materials chemistry. The experience with the fabrication of single crystals with the optical float zone has motivated one to investigate the luminescence of Nd3+ and Ho3+ ions in the garnets (Gd3−xREx)In2Ga3O12 (RE = Nd and Ho, x = 0; 0.15–0.30). Upon usage of an Ar/O2 (80:20 ratio) atmosphere and application of an auxiliary pressure (6 bar) to suppress In2O3 evaporation, single‐crystalline domain sizes in the order of ≈6 × 6 × 1 mm3 are obtained. Structural analysis confirms the formation of a cubic garnet phase with space group Ia3¯d, with the substituents incorporated in accordance with Vegard's law. Backscattered electron imaging and energy‐dispersive X‐ray spectroscopy are conducted, demonstrating a homogeneous elemental distribution within the crystals. Photoluminescence studies are carried out, revealing the characteristic narrow‐line 4fn → 4fn transitions of Nd3+ and Ho3+, with decay times in the submillisecond range, suggesting non‐negligible cross‐relaxation effects are present. Despite this, the large nearest‐neighbor Gd-Gd distance (3.88 Å) in Gd3In2Ga3O12 and the low phonon cutoff energy (≈700 cm-1) are found to limit cross‐relaxation pathways, preserving significant photoluminescence brightness. These results highlight the potential of Gd3In2Ga3O12:RE3+ single crystals as promising candidates for advanced optical applications.
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    ItemOpen Access
    Floating zone growth of large tetragonal Ruddlesden-Popper bilayer nickelate YySr3-yNi2-xAlxO7-δ single crystals
    (2025) Yilmaz, Hasan; Sosa-Lizama, Pablo; Knauft, Manuel; Küster, Kathrin; Starke, Ulrich; Isobe, Masahiko; Clemens, Oliver; Aken, Peter A. van ; Suyolcu, Y. E.; Puphal, Pascal
    The discovery of superconductivity under high pressure in Ruddlesden-Popper (RP) type phase bilayer La3Ni22.5+O7 and trilayer La4Ni32.66+O10 has initiated the frontier of nickelate-based superconductors. In this context, RP-type phases within the Sr-Ni-O system offer promising alternatives as they offer unconventional high oxidation states and Sr-T-O comprises the usual RP series. Here, the intrinsic stability of the undoped Sr-Ni-O framework is investigated using density functional theory (DFT). While Sr3Ni2O7 (SNO) is stable synthesis so far requires Al co-substition in Sr3Ni2-xAlxO7-δ (SNAO). Y-doping resulting in YySr3-yNi2-xAlxO7-δ (YSNAO) effectively mitigates the challenge posed by an insulating ground state. This modification yields a substantial reduction in resistivity, with the crystals exhibiting semiconducting behavior. To explore phase formation within the narrow compositional window of the Y-Sr-Ni-Al-O system, single crystals were grown using the optical floating zone (OFZ) technique under an oxygen partial pressure of approximately 10 bar. The optimized growth conditions for YSNAO enabled the production of large (6 × 5 x 3 mm3), high-quality crystals suitable for neutron scattering experiments. In the absence of Al, crystal growth yielded the n = 1 RP phase Sr1.66Y0.33NiO4-δ, for which single crystals were obtained. The structural, chemical, electrical, and magnetic properties of both the as-grown and topochemically reduced YSNAO compounds were comprehensively characterized through diffraction, spectroscopy, transport, and magnetization measurements.