  Using a theory of polarizable fluids, we extend a variational treatment of an
excess electron to the many-electron case corresponding to finite metal
concentrations in metal-ammonia solutions (MAS). We evaluate dielectric,
optical, and thermodynamical properties of MAS at low metal concentrations. Our
semi-analytical calculations based on a mean-spherical approximation correlate
well with the experimental data on the concentration and the temperature
dependencies of the dielectric constant and the optical absorption spectrum.
The properties are found to be mainly determined by the induced dipolar
interactions between localized solvated electrons, which result in the two main
effects: the dispersion attractions between the electrons and a sharp increase
in the static dielectric constant of the solution. The first effect provides a
classical phase separation for the light alkali metal solutes (Li, Na, K) below
a critical temperature. The second effect leads to a dielectric instability,
i.e., polarization catastrophe, which is the onset of metallization. The locus
of the calculated critical concentrations is in a good agreement with the
experimental phase diagram of Na-NH3 solutions. The proposed mechanism of the
metal-nonmetal transition is quite general and may occur in systems involving
self-trapped quantum quasiparticles.
