From the definition of isothermal bulk modulus and an assumption of its relationship with the pressure, an isothermal three-parameter equation of state has been proposed, which is found to be applicable at high pressure. The proposed equation of state has been used to compute the pressure-volume behavior of NaCl crystal in the pressure range 0~30 GPa and CsCl crystal in the pressure range 0~40 GPa. The isothermal bulk modulus in the pressure range 0~30 GPa of NaCl crystal were also calculated. The results are in very good agreement with the experimental data. The pressure dependence of the isothermal bulk modulus B_T and its first- and second-order pressure derivative are discussed, and the bulk modulus increases continuously with the increasing pressure and has a finite value as p→∞.

isothermal equation of state  /  isothermal bulk modulus  /  NaCl crystal  /  CsCl crystal  /  high pressure