THEORETICAL-MODELS FOR THE LIQUID-VAPOR AND METAL-NONMETAL TRANSITIONS OF ALKALI FLUIDS

Theoretical models for the liquid-vapor and metal-nonmetal transitions of alkali fluids are investigated. Plasma models are considered first but shown to be inadequate, apparently due to their inability to allo w a microscopically consistent treatment of coexisting localization an d delocalization of the valence electrons in the materials. An alterna te approach is then studied in which each statistical configuration of the material is treated as inhomogeneous, with the energy of each ion being determined by its local environment. Nonadditive interactions, due to valence electron delocalization, are a crucial feature of the m odel. This alternate approach is implemented within a lattice-gas appr oximation which takes into account the observed mode of expansion in t he materials of interest (a change in the average coordination rather than a change in the average nearest-neighbor distance) and which is a ble to treat the equilibrium density fluctuations. We have carried out grand canonical Monte Carlo simulations, for this model, which allow a unified, self-consistent, study of the structural, thermodynamic, an d electronic properties of alkali fluids. Applications to Cs, Rb, K, a nd Na yield results in good agreement with experimental observations.