A MODIFIED VAN-DER-WAALS MODEL FOR THE COEXISTENCE CURVE OF EXPANDED METALS

Recent measurements for expanded Hg, Cs and Rb have shown that the liq uid-vapour coexistence curves for these metals do not obey the 'law of rectilinear diameters'. It is shown here that a two-state Van der Waa ls equation with a density-dependent cohesive energy will lead to comp ositional fluctuations that result in a breakdown of the 'law'. Calcul ations predict that Hg and the alkali metals will exhibit similar beha viour near the critical point. However, in the case of Hg the 6p-6s ba nd-gap closure, which occurs at higher than critical densities, leads to an anomalous behaviour not observed in the alkali metals. The influ ence of this transition on the rectilinear behaviour is treated by int roducing a further modification of the two-state model.