Dielectric anomaly at the metal-insulator transition as seen by electron spin resonance 2. Application to lithium-ammonia solutions

Abstract

A formalism appropriate to the analysis of the electron spin resonance (ESR) lineshape in poor conducting media has been derived by us previously. It has been shown in particular that it should be possible to measure the dielectric constant from ESR in doped semiconducting media. This formalism is applied to the study of the non-metal to metal transition in lithium-ammonia solutions. Agreement between actual and theoretical lineshapes has been found. From the results given in the present paper, solutions of lithium in ammonia at 240 K may be separated into three concentration ranges. Firstly, an ‘insulating’ range below 1.5 mole per cent of metal (MPM) with evidence of a dielectric anomaly. Secondly, a metallic range above 3 MPM; even when the conductivity is relatively low, the screening is of Thomas-Fermi type and the dielectric effect rapidly becomes negligible. Finally, in the intermediate range, a dramatic decrease of the dielectric constant towards large negative values is observed; this region is characterized by important thermal effects on most of the physical properties.