Diffusion across the glass transition in silicate melts: Systematic correlations, new experimental data for Sr and Ba in calcium-aluminosilicate glasses and general mechanisms of ionic transport

Viscosity and diffusivity of silicate melts and glasses are related to each other through relaxation timescales. The systematic is explored based on published data. Diffusion coefficients for Sr and Ba were measured in calcium aluminosilicate glasses at conditions near the glass/supercooled liquid boundary in temperature - time space making use of thin film technology and Rutherford Backscattering Spectroscopy (RBS) to measure concentration profiles on nanoscales. These data extend the range of published diffusion coefficients and combined with the systematic noted above allow the nature of change of diffusion coefficients across the glass transition region to be studied. Activation energies for diffusion in the glassy state (~ 360 kJ/mol) are higher than in the molten liquid (~ 213 kJ/mol). A defect based model of glass transition derived by Ojovan and coworkers, where attainment of a percolation threshold of configuron-type defects accounts for the glass - liquid transition, can explain the observed diffusion behaviour. Data treatment using this model yields a defect formation enthalpy of ~ 146 kJ/mol and a migration enthalpy of ~ 213 kJ/mol. The results of this study provide generalized expressions for the prediction of diffusion coefficients of cations in silicate melts for any composition at any temperature.