THE PHASE-DIAGRAM AND FLUID-STRUCTURE FROM A SIMPLE-MODEL OF THE ALKALIS

We present a lattice-gas model with attractive interactions arising fr om valence electron delocalization from single-site orbitals. Such eff ects cannot be described by a pair-interaction potential and produce a strongly asymmetric phase diagram for a nonmetallic molecular vapour coexisting with a liquid metal. Although the model is extremely simpli fied, it reproduces features of the alkali fluids which contrast with those of simple fluids. A unified treatment of the ionic and electroni c structures allows obtaining the electrical conductivity at chosen th ermodynamic conditions. We have found that the metal-nonmetal transiti on in this model is driven by the break in the percolation of the ioni c structure, which takes place in the vapour phase close to the critic al temperature. rather than by electronic localization induced by othe r effects of disorder.