Gas-Fluid and Fluid-Solid Phase Instability for Restricted Primitive Model

By considering the fluctuation of grand potential Ω around equilibrium with respect to small one-particle density fluctuations δρα( r-vector ), the phase instability of restricted primitive model (RPM) of ionic systems is investigated. We use the integral equation theory to calculate the direct correlation functions in the reference hypernetted chain approximation and obtain the spinodal line of RPM. Our analysis explicitly indicates that the gas-fluid phase instability is induced by k = 0 fluctuation mode, while the fluid-solid phase instability is related to k ≠ 0 fluctuation modes. The spinodal line is qualitatively consistent with the result of computer simulations by others. (condensed matter: structural, mechanical, and thermal properties)