Computational materials discovery: the case of the W-B system

By means of variable-compositional evolutionary algorithms, in combination with first-principles calculations, the compositions, structures and mechanical properties of the W-B system have been theoretically investigated. As well as confirming the experimental observations (including their crystal structures) for the four known compounds W2B, WB, WB2 and WB3, the new stable compoundW(8)B(7) and two nearly stable compounds, W2B3 and WB4, have also been predicted in the ground state. The elastic properties and estimated Vickers hardnesses of all these borides have been systematically derived. The results show that, among these borides, hP6-WB2 exhibits the largest ultra-incompressibility along the c axis, with the highest C-33 value (953 GPa, comparable with that of the most incompressible diamond). hP16-WB3 exhibits the highest hardness of 36.9 GPa, in good agreement with the experimentally measured data from 28.1 to 43.3 GPa, close to the superhard threshold, and oC8-WB shows the highest bulk modulus of about 350 GPa. The new stable compound W8B7 crystallizes in the monoclinic mP15 phase, with infinite zigzag B chains running parallel to the W-atom layers, resulting in a relatively high estimated hardness of 19.6 GPa. The anisotropic Young's modulus E and torsion shear modulus Gt have been derived for both oC8-WB and hP16-WB3. The current state of research and the historic inconsistency of the W-B system are briefly summarized, in particular clarifying the fact that the previous experimentally attributed hP20-WB4 is in fact the defect-containing hP16-WB3.