Transport processes in heavy metal fluoride glasses |
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Institution: | 1. Institut für Nichtmetallische Werkstoffe, Technische Universität Clausthal, Zehntnerstraße 2a, 38678 Clausthal-Zellerfeld, Germany;2. Fachbereich Chemie, Physikalische Chemie, Philipps-Universität Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany |
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Abstract: | 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. |
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