1) Could you explain the significance of your article to the non-specialist?

 Soda-lime silicate glasses are widely used as window glasses and optical glasses. Beside these commercial applications this glassy material is also important for geoscientists since alkaline-earth rich silicate glasses are constituents of the earth's crust.

Our study of the mobility of the alkaline-earth element calcium in soda-lime silicate glasses shows that the motion of the calcium ion is strongly facilitated by the more mobile sodium ions. The sodium ions occupy the sites vacated by the calcium ions and behave as catalysts for calcium diffusion, that is, the mobility of the calcium ions increases with the sodium concentration. The impact of the alkali ion on the mobility of alkaline-earth ions, which only affects the probability of the backward jump of the alkaline-earth ion into its previous position and not the energy barrier of the jump, is a very peculiar phenomenon which has not been observed in glassy materials so far. This phenomenon is considered to be not restricted to soda-lime silicate glasses but could also hold for other glassy systems.   

2)  What has motivated you to conduct this work?

Conductivity measurements of Schwartz and Mackenzie (M. Schwartz and J.D. Mackenzie, J. Am. Ceram. Soc., 1966, 49, 582) and Malki et al. (M. Malki, M. Micoulaut, F. Chaimbault, Y. Vaills, and P. Simon, Europhysics Letters, 2003, 64, 661) suggest an activation energy in the range from 1.35 eV to 1.45 eV for calcium ion diffusion in xCaO·(1 ? x)4SiO₂ glasses with x varying between 0.4 and 0.55. In contrast radiotracer diffusion studies and dynamic mechanical loss spectroscopy reveal activation energies of calcium ion diffusion of about 2 eV and above (B. Roling and M.D. Ingram, Solid State Ionics, 1998, 105, 47; G.H. Frischat, and H.J. Oel, Glastechnische Berichte/ Zeitschrift für Glaskunde, 1966, 39, 524). These results suggest that calcium ions are more mobile in pure calcium silicate glasses than in soda-lime-silicate glasses containing small amounts of sodium. In order to clarify this issue we investigated in detail the mobility of the cations in soda-lime silicate glasses of various compositions by means of radiotracer, electrical conductivity, and mechanical loss measurements. 

3) Where do you see this work developing in the future?

Understanding the mechanisms of ionic conductivity in glassy systems is still a challenging task due to the complexity of disordered materials. During the last decades much progress has been achieved to characterize the dynamic of ions in glasses. This can be attributed not only to the increased computer capacity, which enables more sophisticated Monte Carloand Molecular Dynamic simulations, but also to the development of experimental analyses techniques. To gain a comprehensive picture on the dynamic of ions several different techniques should be applied to the same glass.

Beside mobile ions, which are generally considered as network modifiers, it is a challenge to study the dynamic of the network formers such as silicon and oxygen in silicate glasses. The availability of isotopically enriched silicon and oxygen makes it possible to study the diffusion of the network components in silicate glasses. In particular, the impact of the network modifiers on the dynamic of the network formers can now be investigated.  

4) Are there any particular challenges facing future research in this area?

To determine the factors which control the glass transition temperature of disordered materials still remains a great challenge. Information about the dynamics of both the network modifiers and formers can help to provide additional insight into the glass properties.

Additionally, there is a need to characterize the mechanisms of ionic conductivity in nanostructured glasses.   Traditionally, macroscopic techniques are used to characterise the ion mobility in glasses. However these techniques average generally over the ion dynamics in different phases and at interfaces leading to a loss of information about the microscopic and nanoscopic mechanisms of ion transport. Therefore, experimental techniques which provide spatially resolved dynamic information are highly desirable. Recently, we demonstrated that the conductivity of a glass can be measured on a nanoscopic scale by means of electrostatic force spectroscopy (Appl. Phys. Lett. 2004, 85, 2053 and Phys. Chem. Chem. Phys. 2005, 7, 1472).