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

Abstract

The pressure as a function of volume and temperature has been investigated for B2-type NaCl over the pressure range of 20–360GPa and at temperatures between 300 and 3000K. The simulations were performed using ab initio molecular dynamics method within the density-functional theory framework. A Vinet equation of state fitted to the 300K data yielded a bulk modulus of BTa=128.66GPa and a pressure derivative of BTa′=4.374 at standard state pressure of 30GPa. The thermal pressure contribution was determined to be of the form ΔPth=[αBT(Va)+(∂BT∕∂T)Vln(Va∕V)]ΔT. When αBT(Va) is assumed to be constant, the fit to the data yielded αBT(Va)=0.0033GPa∕K at standard volume, corresponding to the pressure of 30GPa. In contrast, the volume dependence of the thermal pressure was very small, and fitting yielded (∂BT∕∂T)V=0.00087.