The effect of temperature on the structure of LiAlSiO4 and NaAlSiO4 liquids has been studied by 17O 3QMAS NMR spectroscopy using glass samples prepared with different fictive temperatures. The abundance of Al-O-Al and Si-O-Si bridging O linkages increases with increasing fictive temperature, indicating that the reaction 2 Al-O-Si ↔ Al-O-Al + Si-O-Si shifts to the right with increasing temperature in the liquid. The observed temperature dependence of Al-O-Al species abundance allows us to estimate the ΔH of this reaction to be 28.8 ± 11.3 kJ/mol for NaAlSiO4 glasses and 31.7 ± 13.3 kJ/mol for LiAlSiO4 glasses. Extrapolating our results to higher temperature, we estimate that 13.18% of the bridging O atoms occur as Al-O-Al in NaAlSiO4 and 15.25% in LiAlSiO4 at ~2000 K, as compared to 25% Al-O-Al predicted by a statistically random bridging O distribution in compositions with an Al/Si ratio of 1. Using the experimental data to estimate the contribution to configurational heat capacity from Al/Si disordering in NaAlSiO4 liquid, we find that redistribution of bridging O species with increasing temperature produces a negative dCP/dT above the glass transition and therefore cannot account for the observed positive dCP/dT. We discuss a generalized mechanism for structural rearrangement characterized by a small initial species concentration at Tg and a large ΔH, which produces a positive dCP/dT.
© 2015 by Walter de Gruyter Berlin/Boston