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

Abstract

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, WB2and WB3, the new stable compound W8B7and two nearly stable compounds, W2B3and 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-WB2exhibits the largest ultra-incompressibility along thecaxis, with the highestC33value (953 GPa, comparable with that of the most incompressible diamond).hP16-WB3exhibits 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, andoC8-WB shows the highest bulk modulus of about 350 GPa. The new stable compound W8B7crystallizes in the monoclinicmP15 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 modulusEand torsion shear modulusGthave been derived for bothoC8-WB andhP16-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 attributedhP20-WB4is in fact the defect-containinghP16-WB3.