It was suggested that the spectrum of ion site energies in glasses exhibited a so-called band-gap; thus establishing an analogy between ion-conducting glasses and intrinsic semiconductors. This implied that ion conduction took place via vacancies and interstitial ions.

Is there a ‘Native’ Band Gap in Ion-Conducting Glasses? J.C.Dyre: Journal of Non-Crystalline Solids, 2003, 324[1-2], 192-5