We report surprisingly soft covalent bond-deformation paths in recently discovered superhard $\gamma$-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 $\gamma$-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 $\gamma$-boron, which expands our knowledge about structural transformation in strong covalent solids.

• Received 16 May 2010

DOI:https://doi.org/10.1103/PhysRevLett.105.215503