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 W 2 B, WB, WB 2 and WB 3 , the new stable compound W 8 B 7 and two nearly stable compounds, W 2 B 3 and WB 4 , 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, hP 6-WB 2 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). hP 16-WB 3 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 oC 8-WB shows the highest bulk modulus of about 350 GPa. The new stable compound W 8 B 7 crystallizes in the monoclinic mP 15 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 G t have been derived for both oC 8-WB and hP 16-WB 3 . 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 hP 20-WB 4 is in fact the defect-containing hP 16-WB 3 .