A generic model for the ionic contribution to the equation of state

We have formulated a model that yields the thermodynamics for the ionic contribution to general materials from the low temperature solid region, through melting, to ideal gas. We use the Debye model for the solid. Melting is determined by the Lindemann formula with standard rules of thumb used to determine density and energy discontinuities. The model interpolates through the liquid regime to ideal gas assuming that the specific heat drops monotonically from about 3R at melting, to 9R/4 at five times melting, and continuing to 3R/2 at high temperatures. The area under the specific heat curve is constrained in the model to reproduce the correct high temperature entropy. Thus, for a compound, the extra contribution from the entropy of mixing forces into the model is, in a crude way, the extra specific heat due to dissociation.