A combined DFT and NPD approach to determine the structure and composition of the ε-phase of tungsten boride
Loading...
Date
2023-10-15
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier
Abstract
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
Description
Keywords
Tungsten, Borides, Reactors, Crystal structure, Stoichiometry, Neutron diffraction, Density functional method, Phase diagrams, X-ray diffraction
Citation
Setayandeh, S. S., Stansby, J. H., Obbard, E. G., Brand, M. I., Miskovic, D. M., Laws, K. J., Peterson, V. K., Astbury, J. O., Wilson, C. L., Irukuvarghula, S., & Burr, P. A. (2023). A combined DFT and NPD approach to determine the structure and composition of the ε-phase of tungsten boride. Acta Materialia, 259, 119282. doi:10.1016/j.actamat.2023.119282