Ab initio molecular dynamics simulations for thermal equation of state of B2-type NaCl

The pressure as a function of volume and temperature has been investigated for B2-type NaCl over the pressure range of 20-360 GPa and at temperatures between 300 and 3000 K. The simulations were performed using ab initio molecular dynamics method within the density-functional theory framework. A Vinet equation of state fitted to the 300 K data yielded a bulk modulus of B(Ta)=128.66 GPa and a pressure derivative of B(Ta)'=4.374 at standard state pressure of 30 GPa. The thermal pressure contribution was determined to be of the form Delta P(th)=[alpha B(T)(V(a))+(partial derivative B(T)/partial derivative T)(V) ln(V(a)/V)]Delta T. When alpha B(T)(V(a)) is assumed to be constant, the fit to the data yielded alpha B(T)(V(a))=0.0033 GPa/K at standard volume, corresponding to the pressure of 30 GPa. In contrast, the volume dependence of the thermal pressure was very small, and fitting yielded (partial derivative B(T)/partial derivative T)(V)=0.000 87.