Low-field EPR study of the metal–non-metal transition in sodium–ammonia solutions

Abstract

Solutions of alkali metals in liquid ammonia show a metal–non-metal (MNM) transition at about 4 mol% metal (MPM). In this paper, we present the results of electron paramagnetic resonance (EPR) studies of sodium–ammonia solutions at 200 MHz as a function of temperature and metal concentration. It has been suggested that the transverse spin relaxation rate consists of two contributions: a hyperfine interaction with ¹⁴N nuclei, which is dominant at lower metal concentrations, and a spin–orbit interaction, dominant at higher metal concentrations. The cross-over occurs around 1.5 MPM, a little below the bulk MNM transition point of 4 MPM. From the integrated intensity data of EPR we have examined the spin-pairing equilibrium of excess electrons in the solutions.