  Although most metal-insulator transitions in doped insulators are generally
viewed as Mott transitions, some systems seem to deviate from this scenario.
Alkali metal-ammonia solutions are a brilliant example of that. They reveal a
phase separation in the range of metal concentrations where a metal-insulator
transition occurs. Using a mean spherical approximation for quantum polarizable
fluids, we argue that the origin of the metal-insulator transition in such a
system is likely similar to that proposed by Herzfeld a long time ago, namely,
due to fluctuations of solvated electrons. We also show how the phase
separation may appear: the Herzfeld instability of the insulator occurs at a
concentration for which the metallic phase is also unstable. As a consequence,
the Mott transition cannot occur at low temperatures. The proposed scenario may
provide a new insight into the metal-insulator transition in condensed-matter
physics.
