Ab initio study on the lattice instability of silicon and aluminum under [0 0 1] tension

Two different single crystals, Si with the diamond structure and Al with face-centered-cubic, are subjected to [0 0 1] tension in ab initio molecular dynamics (static) simulations based on Bachelet–Hammann–Schlüter (BHS) pseudopotential. Not only the ideal tensile strength under isotropic Poisson contraction, but also the crystal stability and bifurcation to anisotropic contraction are discussed in terms of the elastic stiffness matrix and the change in the charge density. The ideal tensile strengths are overestimated to as high as ε=0.18, σ=18.7 GPa for Si and ε=0.25, σ=23.5 GPa for Al, respectively. These values are inconsistent with the experimentally observed characteristics such as the hardness of Al being lower than that of Si. The elastic stiffness matrix reveals that the crystals become unstable at far lower strain and stress, ε=0.094, σ=10.7 GPa for Si and ε=0.055, σ=5.65 GPa for Al, and bifurcate to the lower energy pass of the anisotropic contraction. The change in the electronic structure suggests that nucleation/passage of a partial dislocation would take place in the bifurcated anisotropic contraction. Thus the instability point indicates the onset of the nonelastic deformation and is much more important than the ideal tensile strength.