Herzfeld instability versus Mott transition in metal-ammonia solutions

TL;DR

This paper investigates the metal-insulator transition in alkali metal-ammonia solutions, proposing that Herzfeld instability, rather than Mott transition, explains the phase separation and transition behavior observed.

Contribution

It introduces a mean spherical approximation model for quantum polarizable fluids to explain the Herzfeld instability as the cause of metal-insulator transitions in these solutions.

Findings

Herzfeld instability occurs at a concentration where the metallic phase is also unstable.

Phase separation results from the simultaneous instability of insulator and metal phases.

Mott transition is unlikely at low temperatures in these systems.

Abstract

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.