Mr. Zhou Wei, a Ph.D. student in the research group led by Prof. Sun Hong in the Department of Physics at the Shanghai Jiao Tong University, recently published a paper in Physical Review Letters, one of the most prestigious journals in physics (“Soft Bond-Deformation Paths in Superhard γ-Boron”×Phys. Rev. Lett., vol.105, p.215503 (2010))
The paper studies the possible softening effect of covalent bonds of boron atoms in elemental boron materials using first-principles calculation methods. Because the light elements like boron, carbon and nitrogen atoms can form very strong and highly directional covalent bonds, these light elements are important in designing super-hard materials. However, boron atom is very special which has only three covalent electrons in its out-most orbital. Boron atoms can either form two-centered covalent bond between two boron atoms, or three-centered bond among three equilateral triangle boron atoms. Recent studies show that by mixing the boron two- and three-centered bonding structures, one can create more stable boron nano-structures, such as boron ?-sheet, boron fullerene B80, boron ?-nanotube, and born ?-nanoribbon. But no one has ever studied how boron three-centered bonds will affect the structural strengths and structural deformations in boron materials.
The computational simulation studies carried out by Mr. Zhou Wei in the Department of Physics discovered that during the structural deformation certain boron structures can complete their structural phase transformations by first transforming some of the two-centered boron covalent bonds into three-centered bonds, and then into new two-centered covalent bonds. Because the boron covalent bonds are not actually broken during these structural deformations, and because both the boron two- and three-centered bonds are low-energy configurations, so the structural deformations become easy to occur in these boron materials. The two-centered boron covalent bonds in these materials lost their original characters: the strong strength and high directionality, and become soft like metallic bonds with large plasticity. The study deepens our understanding on boron atomic bonds, and also introduced new ideas in the studies of structural strengths of other boron structures and its nano-structures. (contributed by Sun Hong’s research group).
