A MOLECULAR-DYNAMICS STUDY OF THE MIXED-ALKALI EFFECT IN SILICATE-GLASSES

Mixed alkali silicate glasses have been studied in detail by the molecular dynamics method in order to understand the microscopic origin of the mixed alkali effect (MAE). Subtle structural variations in the local environment of alkali ions are observed as a function of the inter-alkali ratio. The MAE has been observed in the variation of self-diffusion coefficients and thus in the electrical conductivity. Cage vibration frequencies of the alkali ions remain unaffected in the mixed alkali regime. Van Hove correlation functions have been employed to study the nature of transport of the alkali ions. Results indicate that alkali ions preferentially jump only to like-ion sites which have been rationalized in terms of their site energies. The MAE has been attributed to blockage of preferred migration paths in interalkali compositions.