Using first-principles calculations, we show that, in spite of its relatively low shear modulus, boron suboxide (B${}_6$O) is superhard because of its high shear strength of ⩾38 GPa which originates from three-dimensional covalently bonded network of B12 icosahedral units connected by boron and oxygen atoms. We further demonstrate that the high shear resistance of ${\mathrm{B}}_6$O is related to strong B–B covalent bonds that connect the ${\mathrm{B}}_{12}$ units. These results challenge the concept of design intrinsically superhard materials based on high elastic moduli only.

• Received 16 January 2011

DOI:https://doi.org/10.1103/PhysRevB.83.092101