Electron localization and the nonmetal-metal transition in alkali-alkali halide solutions

We evaluate and extend an earlier proposal for a microscopic theory of the nonmetal-to-metal (NM-M) transition which occurs on dissolving an alkali metal in its molten halide. The transition is viewed as involving a balance between the free energy gain from the binding of valence electrons into localization centres and the excess free energy of the ionic assembly screened by the electrons. Using parameters estimated for solutions of potassium in potassium chloride and assuming that the elementary process of electronic trapping is the formation of F-centre-like clusters, Thomas-Fermi screening by metallic electrons is shown to lead to a very sharp NM-M transition at a concentration in the range of 25-30% added metal. Thermally activated hopping of the localized electrons and the evolution of the localization centres with composition are next crudely taken into account by allowing for an additional contribution to the inverse screening length, which is estimated from the electronic localization length. This is shown to lead to a progressive break-up of the localization clusters, accelerating into a NM-M transition in the same concentration range. This simplified theoretical scenario is consistent with the available experimental evidence. (author). 18 refs, 5 figs