Nature of the metal-nonmetal transition in metal-ammonia solutions. I. Solvated electrons at low metal concentrations

TL;DR

This paper develops a semi-analytical theory for metal-ammonia solutions, explaining how induced dipolar interactions lead to phase separation and metallization at low metal concentrations, aligning well with experimental data.

Contribution

It introduces a mean-spherical approximation-based model for the metal-nonmetal transition in metal-ammonia solutions, emphasizing the role of induced dipolar interactions.

Findings

Correlation of theoretical dielectric and optical properties with experimental data

Identification of induced dipolar interactions as key to phase separation and metallization

Prediction of critical concentrations consistent with phase diagrams

Abstract

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.