ON THE POSSIBILITY OF AN INSULATOR-METAL TRANSITION IN ALKALI METAL-DOPED ZEOLITES

The authors briefly review the geometric and electronic structure of new solids formed by 'dissolving' alkali metals in zeolites. Here the incoming alkali atoms readily give up their ns valence electron to the solid in return for a framework coordination site for the resulting cation. The authors argue that the metal doping of zeolites is reminiscent of the situation in metal-ammonia solutions and doped semiconductors (e.g. P in Si). The possibility exists that, at some critical stage of alkali metal loading, a 'matrix-bound' Insulator-to-Metal Transition occurs within the zeolite host. ESR, NMR, and magnetic susceptibility measurements provide important information on the electronic properties of these remarkable materials.