A Combined Dtf and NPD Approach to Determine the Structure and Composition of the ε-Phase of Tungsten Boride

Abstract

Tungsten borides have recently been identified as promising candidate materials for shielding application in spherical tokamak fusion reactors due to their outstanding properties. Of these, the ε-phase, conventionally labelled as W2B5, is of particular interest given its high boron content. However, surprising lack of agreement on the structure and even composition of the ε-phase has hindered further research on these materials. 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. 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. The simulations also reveal that the material can accommodate a range of stoichiometric variations with relatively small stored energy, which is a desirable feature for neutron shielding application.

Keywords: ε tungsten boride, DFT simulations, Neutron diffraction