In this paper we report the observation of delocalized excess electron states in frozen solutions of lithium, potassium, rubidium, and cesium in HMPA. The delocalized states are identified as mobile electrons residing in a metallic impurity band within the host (HMPA) bandgap. In potassium, rubidium, and cesium solutions both delocalized (metallic) and localized (insulating) states coexist within the same sample. It is proposed that spin exchange between localized and delocalized electrons in these amorphous materials is very inefficient. The implications of these observations to an inhomogeneous model for metal solutions are discussed. A transition from delocalized to localized states was monitored in frozen lithium-HMPA solutions at metal concentrations of the order of 5 × 10-3 M. A simple criterion is proposed which relates the observed magnetic properties of localized Wannier-type impurity states to the critical density (nc) of free carriers at the metal-insulator (M-I) transition. Critical densities obtained from this relation are in good agreement with both experimental values and theoretical estimates (Hubbard model) for nc in doped group 4 semiconductors and in frozen lithium-HMPA solutions. It is proposed that the M-I transition in a homogeneous fluid metal-HMPA solution would proceed via a Mott transition, although the presence of microscopic inhomogeneities in the solution will almost certainly preclude its direct experimental verification at the predicted metal concentration.
