The nature of glassforming liquids, the origin of superionics and 'tight' vs. 'loose' glassy conductors

As background for the phenomenology of fast ion conducting glassy materials, we review key features of the ionic liquid states from which they form. Current issues concerning the behavior of molecular liquids, including the relation of crossovers in dynamic behavior during cooling to the onset of clustering and the splitting-off of secondary relaxations, are considered. The decoupling of conducting modes from viscous modes involved in the generation of ion conducting glasses on cooling below T_g, is apparently quite distinct from the clustering involved in the α-β bifurcation, and tends to be most pronounced in the 'stronger' liquids. Using ion dynamics simulations, we trace its origin to the existence of high anharmonicity in the vibrational modes of the mobile species. This is most probable when the mobile ion is small but uncompetitive in the determination of local and intermediate range glass structure. Glasses with such species should share a number of characteristics. The classification of 'loose' (vs. 'tight') ionic conductors is introduced to separate the behavior of such systems from that of 'tight' ionic conductors which should be insulators below T_g. The absence of conductivity dispersion should be a feature of the latter when the liquids are 'strong' as opposed to 'fragile'. The possible relation of 'looseness' to the presence or otherwise of the 'universal' background loss (ε″ ∼ f) is discussed.