Conduction- and localized-electron spin resonance in the lithium–methylamine system: Inferences for the existence of the metallic compound tetramethylaminelithium(zero), Li(CH3NH2)4

Abstract

Electron spin resonance (ESR) studies of fluid and frozen solutions of lithium in anhydrous methylamine are reported. The composition and temperature dependence of the ESR properties (electron spin relaxation times, electronic ge factor) are indicative of metallic compound formation in the lithium–methylamine system at low temperatures. Marked changes in both the electron spin relaxation properties and the ESR line shapes in the temperature interval 156–158 K are consistent with those expected for a metallic system moving through the melting point. Electron spin relaxation characteristics of the corresponding metal–ammonia compounds Li(NH3)4 and Ca(NH3)6 have been adequately interpreted in terms of a nearly free-electron (NFE) picture for these low electron density materials. In contrast, electronic properties of the compound tetramethylaminelithium(zero), although nominally metallic (σ∼400 Ω−1 cm−1 for a 22 mole% metal fluid solution), cannot be described in the context of a NFE material. In particular, conduction electron spin relaxation properties reveal a breakdown in the Elliott treatment of spin relaxation in pure metals. We take this as evidence for incipient localization in the very strong scattering electronic regime. It is shown that an enhancement of the electron spin relaxation time relative to the predicted Elliott value in Li(CH3NH2)4 arises quite naturally if one considers the increasing importance of electron–electron and electron–phonon interaction in the approach to the metal nonmetal transition. This changeover in electron spin relaxation characteristics in lithium–methylamine solutions is correlated with the corresponding changes in the electronic conductivity. We suggest that the enhancement in the electron spin relaxation time can be used as an indicator, albeit qualitative, of the degree of localization of the electronic wave function in a nominally metallic system. This behavior contrasts markedly with that of nuclear spin relaxation in the lithium–methylamine system, where incipient localization gives rise to an enhancement in the nuclear relaxation rate relative to that predicted for a metallic system.