RELAXATIONS AND LOCAL MOTIONS IN GLASSY CATIONIC CONDUCTORS

Superionic glasses are a very interesting class of materials because of their high ionic mobility. A high degree of decoupling of the mobile ionic species from the host matrix characterizes these systems below the glass-transition temperature T(g). The migration of cations has a strong influence on the mechanical spectra in the high temperature region (T > 77 K), giving rise to broad relaxational losses and dispersions over the entire frequency range. A study of these features supplies useful information on the microscopic characteristics of the ionic motion and shows a decreasing non-exponentiality of the relaxation with increasing cation concentration. In the low temperature region (T < 77 K), acoustic anomalies are revealed that are due to thermally activated processes and to tunneling effects, arising from some kind of local motion in the glassy structure. All the peculiarities are discussed in order to underline the questions still unanswered and the theoretical approaches, available for their explanation, are examined.