Modified lattice-gas model for the gas-liquid-solid phase diagram

Crystalline order parameters related to the localization of the particles within the cells are introduced into the usual lattice-gas model. The coupling of these order parameters to the usual liquid-gas transition is shown to produce, in the simplest approximation, phase diagrams of qualitatively correct shapes. The Goldstone modes of the solid are retained in this picture. The Landau theory of melting is reviewed and shown to always lead to a first-order solid-fluid transition. The question of the possibility of the transition becoming second order due to fluctuations is discussed qualitatively. This possibility is shown to depend on the relative sizes of the first-order transition and the critical region of the fluctuations.