Herzfeld instability versus Mott transition in metal–ammonia solutions

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 this. 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 to be 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 temperature. The proposed scenario may provide a new insight into the metal–insulator transition in condensed-matter physics. To cite this article: G.N. Chuev, P. Quémerais, C. R. Physique 8 (2007).