Barium diffusion in mixed cation glasses.

Diffusion of barium (Ba) in mixed cation glasses of the composition xNa2O x (3 - x)BaO.4SiO2 with x = 0.0, 0.1, 0.3 and 1.0 and 0.4K2O.2.6BaO.4SiO2 was investigated by means of the radiotracer diffusion technique below the respective glass transition temperatures. In accord with our previous results of calcium (Ca) diffusion in soda-lime silicate glasses (F. Natrup et al., Phys. Chem. Chem. Phys., 2005, 7, 2279), the mobility of alkaline-earth ions increases with the alkali content in all analyzed glass systems with no decrease in the diffusion activation enthalpy, but a raise in the pre-exponential factor. A distinct dependency of the activation enthalpy of alkaline-earth ions on the type and content of the alkali ions in the glass is observed. The results provide evidence for elastic and electrostatic contributions to cation diffusion in glasses and support the formation of dissimilar cation pairs, that were derived from nuclear magnetic resonance (NMR) investigations of soda-lime silicate glasses and glasses containing sodium and barium. Finally, a striking correlation between the pre-exponential factor of alkaline-earth ion diffusion in soda-lime and potassium barium glasses is found, the origin of which remains unsolved.

Cation diffusion and ionic conductivity in soda-lime silicate glasses.

We have studied the mobilities of calcium and sodium ions in silicate glasses of compositions xNa2O (3 - x)CaO x 4SiO2 with x = 0.0, 0.1, 0.3, 1.0 and 3.0 by means of radiotracer diffusion, electrical conductivity measurements, and dynamic mechanical thermal analyses. In glasses containing sodium oxide, the Na+ ions are much more mobile than the Ca2+ ions, and are, therefore, governing the electrical conductivity. In the pure calcium silicate glass, the activation energy of Ca2+ diffusion is higher than the activation energy of the electrical conductivity. This provides strong evidence that the electrical conductivity of this glass is not determined by the migration of Ca2+ ions, but by impurity charge carriers, which are most likely Na+ ions. We sketch the composition-dependent mobilities of Na+ and Ca2+ ions in soda-lime silicate glasses with variable Na2O and CaO content. Our results indicate that the coordination environment of Ca2+ ions remains unchanged when CaO is replaced by Na2O which is consistent with recent results of molecular dynamic simulations. Moreover, our results confirm the formation of dissimilar Na-Ca pairs which lead to a non-random mixing of the cations in the glass. The formation of such pairs was recently deduced from nuclear magnetic resonance spectra of soda-lime silicate glasses.