Barium diffusion in mixed cation glasses

Diffusion of barium (Ba) in mixed cation glasses of the composition xNa2O·(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.