Metal‐Nonmetal Transition in Metal‐Ammonia Solutions

Abstract

The properties of metal‐ammonia solutions in the concentration range of the metal‐nonmetal transition between 0.1 and 10 mol percent metal (MPM) are discussed. The high‐frequency electrical properties show a number of characteristic features in this intermediate concentration range, such as additional dielectric relaxations at about 0.1 MPM and 0.7 MPM for LiNH3 and NaNH3, respectively, and a resonance‐like behavior at about 1.0 MPM and 1.5 MPM for LiNH3 and NaNH3, respectively. For both LiNH3 and NaNH3 the real part of the high‐frequency dielectric function changes to large negative values around 2.5 MPM, indicating a transition to metallic behavior at high frequencies. The high‐frequency properties, which are also discussed in terms of electron conductivity, are explained quantitatively as a function of concentration, temperature, and frequency using a model of polaron hopping. The metal‐nonmetal transition is related to structural changes in the solutions which lead to phase separations in LiNH3 and NaNH3 at lower temperatures. The structural properties of the solutions are investigated using sound velocity and ultrasonic attenuation data. Concentration and temperature dependence of the ultrasonic attenuation in LiNH3 and NaNH3 can be described quantitatively in terms of structural fluctuation.

NMR study of lithium in liquid ammonia and methylamine

“…Metallization at high frequencies has also been inferred around 2.5 MPM in Li-NH3 and Na-NH3 from measurements of the microwave dielectric constant. 33 In Li-CH3NH2, on the other hand, a decrease of Tle and an increase of the dc conductivity occur approximately at the same composition, ~15 MPM,24 reflecting the absence of a significant concentration fluctuation around the transition region. 29 Thus, enhancement of the relaxation rates in M-NH3 is expected to occur in solutions with lower dc conductivities compared with more homogeneous solutions, in agreement with the experimental results shown in Figure 10.…”

Multielement NMR Studies of the Liquid–Liquid Phase Separation and the Metal-to-Nonmetal Transition in Fluid Lithium– and Sodium–Ammonia Solutions

“…The region 1–5 MPM, where the electrical conductivity shows a large temperature dependence, is characterized by the emergence of quantum mechanical tunnelling in the electron transfer processes, which leads to a significant increase in electrical conductivity. ,,,,− These solutions are unique in that the excess or solvated electron is not bound in a chemical sense to a particular solvent molecules (i.e., NH 3 ) in either the donor or the acceptor states. ,, We now outline models for these (various) electronic conduction processes as the system transforms into the liquid metallic state, taking account of the NMR (and other) data and these unique properties of the excess or solvated electrons.…”

“…As Mott noted, , one may therefore have two or three of these electronic conduction mechanisms operating in parallel for metal–ammonia solutions: the drift/tunnelling of the isolated solvated electron (eq ) and the hopping/tunnelling process from doubly charged e 2 to singly charged, e 1 , centers (eqs and ).…”