Shear stresses in shock-compressed diamond from density functional theory

We report density functional theory (DFT) results for the shear stresses of uniaxially compressed diamond under conditions corresponding to strong shock wave compression. A nonmonotonic dependence of shear stresses on uniaxial strain was discovered in all three low-index crystallographic directions: $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$. For $⟨100⟩$ compression the shear stress even becomes negative in the region near the minimum of the shear stress-strain curve. The DFT results suggest that anomalous elastic regime observed in recent molecular dynamics shock simulations is a real phenomenon caused by a significant delay or even freezing of the plastic response.

Figure 1

(Color online) Snapshots of the shock front in the anomalous elastic region for a $⟨110⟩$ shock wave (top) resulting in a compression ratio of 0.795, and a $⟨111⟩$ shock wave (bottom) resulting in a compression ratio of 0.715. Atoms are colored according to potential energy [blue (dark gray) corresponds to low, and yellow (light gray) to high energy]. For clarity only the top few layers are shown and small cutouts of the full MD simulations are depicted.

Figure 2

Unit cells used in calculating the effects of uniaxial compression along the $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$ directions.

Figure 3

(Color online) Binding energy (top), longitudinal stress (middle), and shear stress (bottom) for uniaxial compression along $⟨100⟩$, $⟨110⟩$, and $⟨111⟩$ directions.