A DFT study to determine the structure and composition of ε-W2B5−x

• Published in: Journal of alloys and compounds Vol. 911; p. 164962
• Main Authors: Setayandeh, S.S., Obbard, E.G., Stansby, J., Frost, D., Astbury, Jack O., Wilson, C.L., Burr, P.A.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 05.08.2022; Elsevier BV
• Subjects: Ab initio calculations; Composition; Convexity; Crystal structure; Diffraction patterns; Energy of formation; Epsilon phase (crystals); Free energy; Heat of formation; Neutron diffraction; Stability criteria; Stoichiometry; Tungsten boride; X-ray diffraction; X-ray diffraction; Tungsten boride; Ab initio calculations; Neutron diffraction
• ISSN: 0925-8388; 1873-4669
• DOI: 10.1016/j.jallcom.2022.164962

There is no consensus in the literature on the structure and composition of the ε phase of the W-B system, variously reported as WB2 and W2B5. We used ab initio calculations at two levels of theory to identify the stable crystal structure and stoichiometry of this phase. Among the sixteen structures investigated in the composition range of 67–71.4 at% B (WB2–W2B5), nine exhibited unfeasibly high formation energies; the remaining seven were dynamically stable (did not exhibit any soft phonon modes), and satisfied mechanical stability criteria. When including the thermal vibrational contribution to the free energy, all structures with the W2B5 composition lay above the convex hull, suggesting that this composition is metastable, while those with WB2 composition lay on the convex hull or within DFT precison of the convex hull. We found that four of the candidate structures exhibit negative vacancy formation energy, suggesting that the structures are unstable, or that they are naturally hypo-stoichiometric. Combining these results with a comparison of simulated and experimental x-ray and neutron diffraction patterns, we concluded that the ε phase is most likely a hypo-stochiometric W2B5−x compound with space group P63/mmc. •DFT calculations applied to identify the stable crystal structure and stoichiometry of the ε phase of tungsten borides.•Seven structures were found to be promising candidates based on a convex hull analysis of the free energy of formation.•All candidates were dynamically and mechanically stable.•All candidates were predicted to be stiff, brittle and display significant elastic anisotropy.•DFT results suggested a hypo-stochiometric W2B5-x compound with space group P63/mmc for the ε phase.