Peculiar structure and tensile strength of WB₄: Nonstoichiometric origin

Tungsten tetraboride (WB₄) is experimentally considered as potentially superhard material and is therefore expected to have highly structural stability and enhanced resistance against plastic deformation and failure. The examinations of bonddeformation mechanism suggest a significantly soft bond-deformation pattern induced by ionic W-B bonding for nominal WB ₄ in experiments, largely responsible for the limitation of its strength and structural integrity. Computations on the structures and mechanical properties for WB₄ show a novel thermodynamically favored MoB ₄-type phase with excellent mechanical properties and remarkable incompress-ibility along c direction. The illustrations of nonstoichiometry and x-ray diffraction spectra rationalize the experimental observation of nominal composition WB₄ as defective tungsten borides (W_1-xB ₃ (x<0.25) or WB_4-x (x>0.25)). The results provide new insight into the real structural and mechanical properties of tungsten borides.