Self- and foreign alkaline-earth diffusion in mixed cation silicate glasses

Self-diffusion of alkaline-earth ions in various mixed cation silicate glasses of the composition A2O·2MO·4SiO2 was investigated by radiotracer diffusion measurements below the respective glass transition temperatures. Including previous measurements, we have analyzed the diffusivity of divalent ions in mixed cation systems with A = Li, Na, K, Cs and M = Ca, Sr, Ba. The activation enthalpy depends strongly on the type of cations, revealing an increasing diffusivity for glasses with cations of similar size. The behavior is consistent with mechanical loss spectroscopy measurements performed on the same glasses. In addition foreign atom diffusion of calcium and barium in strontium containing glasses were performed. In the case the ionic radius of the alkali ions exceeds that of the alkaline-earth ions the diffusion activation enthalpy of the foreign ion equals the activation enthalpy of self-diffusion. The results can be described on the basis of a theoretical model proposed by Kirchheim [R. Kirchheim, J. Non-Cryst. Solids 328 (2003) 157] that relates the activation enthalpy to electrostatic and elastic contributions.