First-principles calculations on mechanical and elastic properties of 2H- and 3R-WS2 under pressure

The structure, mechanical stability and elastic properties of 2H- and 3R-WS2 under pressure have been investigated using first-principles calculations based on density functional theory (DFT). The equilibrium lattice parameters of 2H- and 3R-WS2 at 0 GPa are consistent with experimental and other theoretical values. 2H-WS2 is more stable than 3R-WS2 when pressure is less than 5.8 GPa whereas 3R-WS2 is more stable than 2H-WS2 when pressure is higher than 5.8 GPa. According to the mechanical stability criteria, both 2H- and 3R-WS2 exhibit mechanical stability under the pressure range from 0 to 20 GPa. With the increasing pressure, the elastic moduli (E, B, G), sound velocities (vs, vp, vm) and Debye temperatures of 2H- and 3R-WS2 increase monotonously whereas volume and specific heat decrease. Large elastic anisotropies in compressibility and in shear were demonstrated for 2H- and 3R-WS2 at 0 GPa. As the pressure increases, anisotropies in compressibility and in shear become weak for 2H- and 3R-WS2. Moreover, 2H- and 3R-WS2 under pressure have higher hardness and better ductility than those at 0 GPa.