Soft bond-deformation paths in superhard γ-boron.

We report surprisingly soft covalent bond-deformation paths in recently discovered superhard γ-boron. First-principles calculations reveal an intriguing mechanism for bond transformation mediated by three-center bonding, which reduces considerably the rigidity and directionality of the two-center covalent bonds in γ-boron. It leads to much lower strength and large plastic deformation along the selected paths. These results establish a new type of bond-deformation pattern in γ-boron, which expands our knowledge about structural transformation in strong covalent solids.