A combined DFT and NPD approach to determine the structure and composition of the ε-phase of tungsten boride

The e-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 e-phase. Here, we identify the stable crystal structure and stoichiometry range of epsilon tungsten borides through a combination of ab initio simulations and neutron diffraction of isotopically enriched samples. We considered the ability to accommodate hypostoichiometry in six published structures of the e phase. Chemical disorder was modelled using configurational ensembles to account for entropy of non-stoichiometry. We show that two W2B4-x structures (with x=similar to 0.25 0.5), with space group symmetry P6(3)/mmc and P6(3)/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 B-11-enriched arcmelted and crushed samples. Rietveld refinement using the neutron data shows the epsilon-phase to be better described as W2B3.60( 2) (P6(3)/mcm), in keeping with density functional theory (DFT) calculations. Linear change in DFT-derived lattice parameters of the candidates for the epsilon-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.