Browsing by Author "Astbury, JO"

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    A combined DFT and NPD approach to determine the structure and composition of the ε-phase of tungsten boride
    (Elsevier, 2023-10-15) Setayandeh, SS; Stansby, JH; Obbard, EG; Brand, MI; Miskovic, DM; Laws, KJ; Peterson, VK; Astbury, JO; Wilson, CL; Irukuvarghula, S; Burr, PA
    The ε-phase of tungsten boride, conventionally labelled as W2B5, has been identified as a promising candidate for shielding application in spherical tokamak fusion reactors. However, further research has been hindered by a lack of agreement on the structure and even composition of the ε-phase. Here, we identify the stable crystal structure and stoichiometry range of ε tungsten borides through a combination of ab initio simulations and neutron diffraction of isotopically enriched samples. We considered the ability to accommodate hypo-stoichiometry in six published structures of the ε phase. Chemical disorder was modelled using configurational ensembles to account for entropy of non-stoichiometry. We show that two W2B4-x structures (with x=∼0.25 − 0.5), with space group symmetry P63/mmc and P63/mcm, appear to be thermodynamically stable. These candidate compounds have 6.2 − 7.8 at.% less B than the W2B5 composition reported in exiting phase diagrams. We confirm these findings by means of neutron powder diffraction, performed on 11B-enriched arc-melted and crushed samples. Rietveld refinement using the neutron data shows the ε-phase to be better described as W2B3.60(2) (P63/mcm), in keeping with density functional theory (DFT) calculations. Linear change in DFT-derived lattice parameters of the candidates for the ε-phase proposes a simple model to assess the tungsten boride composition by measuring the lattice parameter (e.g. by X-ray diffraction. The simulations also reveal that the material can accommodate a range of stoichiometric variations (via B vacancies) with relatively small stored energy, which is a desirable feature for neutron shielding application. © 2023 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. Open Access CC-NC-ND