Ab initio study of the elastic properties of sodium chloride at high pressure

Abstract

The equation of state and elastic properties for B1- and B2-NaCl up to 160 GPa have been studied by using the density functional simulation within the generalized gradient approximation (GGA). The calculated lattice constants of NaCl agree well with experimental values in a precision of 0.1% over the pressure range studied. It is found that the cell volume decreases 5.5% at the phase transition point. All three independent elastic stiffness coefficients, c11, c12 and c44 for B1‐ and B2-NaCl are evaluated by a calculated stress tensor which was generated by forcing small strain to the optimized unit cell. The calculated zero-pressure elastic moduli, wave velocities, and their initial pressure dependences of B1-NaCl are in excellent agreement with experiments. Systematic investigation on the elasticity of NaCl has been done through four parameters, the Zener anisotropy ratio (AZ), the acoustic anisotropy factor (Aa), the Cauchy deviation (δ), and the normalized elastic constants (cij′). With the pressure, the Zener anisotropy ratio AZ decreases in the B1-phase, but increases in the B2-phase and reaches 1 at about 174 GPa, it suggests that NaCl would become elastic isotropic at this pressure range. The acoustic anisotropy factor Aa shows the similar pressure behavior as AZ. The Cauchy deviation (δ)) increases with pressures, it demonstrates that in the interatomic interaction, the many-body contribution becomes more important at higher pressures. A discussion on the normalized elastic constants is also presented.