ELECTRON LOCALIZATION AND THE NON-METAL-METAL TRANSITION IN ALKALI-ALKALI-HALIDE SOLUTIONS

We evaluate and extend an earlier proposal for a microscopic theory of the non-metal-to-metal (NM-M) transition which occurs on dissolving a n alkali metal in its molten halide. The transition is viewed as invol ving a balance between the free-energy gain from the binding of valenc e electrons into localization centres and the excess free energy of th e ionic assembly screened by the electrons. Using parameters estimated for solutions of potassium in potassium chloride and assuming that th e elementary process of electronic trapping is the formation of F-cent re-like clusters, Thomas-Fermi screening by metallic electrons is show n to lead to a Very sharp NM-M transition at a concentration in the ra nge of 25-30% added metal. Thermally activated hopping of the localize d electrons and the evolution of the localization centres with composi tion 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 progre ssive break-up of the localization dusters, accelerating into a NM-M t ransition in the same concentration range. This simplified theoretical scenario is consistent with the available experimental evidence.