Nature of metal-nonmetal transition in metal-ammonia solutions. II. From uniform metallic state to inhomogeneous electronic microstructure

TL;DR

This paper models the metal-nonmetal transition in metal-ammonia solutions, revealing a thermally fluctuating mixed state and proposing a criterion for the transition based on compressibility maxima.

Contribution

It introduces a semi-analytical model to describe the inhomogeneous electronic microstructure and transition behavior in metal-ammonia solutions, connecting thermodynamic fluctuations with phase separation.

Findings

The metallic phase remains stable down to 5 MPM due to solvent effects.

The miscibility gap varies among alkali metals and aligns with experimental data.

A new electronic phase akin to microemulsion is proposed between spinodal and binodal lines.

Abstract

Applying semi-analytical models of nonideal plasma, we evaluate the behavior of the metallic phase in metal-ammonia solutions (MAS). This behavior is mainly controlled by the degenerate electron gas, which remains stable down to 5 MPM due to high solvent polarizability and strong dielectric screening of solvated ions. Comparing the behavior of the metallic state with those of localized solvated electrons, we have estimated the miscibility gap $\Delta n$ for various alkali metals and found $\Delta n$(Na)$> \Delta n($K$)$. It is rather narrow in Rb-NH$_3$ and does not occur in Cs-NH$_3$ solutions, which is in full agreement with the experiments. The case of Li is discussed separately. The difference calculated in the excess free energies of the metallic and nonmetallic phases is in the order of $k_BT$, yielding a thermally fluctuating mixed state at intermediate metal concentrations. It results in a continuous metal-nonmetal (MNM) transition above the consolute point $T_c$ and a phase separation below $T_c$. We propose a criterion for the MNM transition which may be attributed to the line of the maximum of compressibility above $T_c$. This line crosses the spinodal one at the critical temperature. Finally, we assert that a new electronic phase similar to microemulsion should also arise between the spinodal and the binodal lines.