Transport processes in heavy metal fluoride glasses

Na self-diffusion, Li self-diffusion, Na+-Li+ ion exchange, electrical conductivity, and mechanical relaxation have been studied below Tg on glasses of the system ZrF4-BaF2-LaF3-AF (A=Na, Li), with A=10, 20, 30 mol%. Compared to the transport mechanism in alkali-containing silicate glasses, the mechanisms in these non-oxide glasses are anomalous. Thus the self-diffusion coefficient of Na decreases with increasing NaF content, whereas that of Li increases with increasing LiF content. Both the electrical conductivity and the Na+-Li+ ion exchange reach a minimum at ≈ 20 mol% LiF, and the mechanical relaxation shows one peak for the 20 and 30 mol% LiF-glasses and two peaks for the glass with 10 mol% LiF, evidencing both a contribution of F− and Li+ ions to the transport. Moreover, the presence of the three partially interacting mobile species F−, Na+, Li+ obviously leads to an anionic-cationic mixed ion effect. Applying the Nernst-Einstein equation to the Li+ transport in LiF-containing glasses shows that its mechanism is dissimilar to that in oxide glasses. Calculated short jump distances possibly can be interpreted as an Li+ movement via energetically suitable sites near F− ions. Likewise the Nernst-Planck model, successfully applied to the ionic transport in mixed alkali silicate glasses, obviously does also not hold for the present heavy metal fluoride glasses.