Cation dynamics and premelting in lithium metasilicate (Li2SiO3) and sodium metasilicate (Na2SiO3): A high-temperature NMR study

Premelting effects are common in silicates and have been mostly observed as significant heat capacity anomalies beginning 100 to 200 degrees C below c ongruent melting points. To assess the role of cation dynamics in this phen omenon, we collected nuclear magnetic resonance (NMR) spectra and relaxatio n time data to within 20 to 50 degrees C of the melting points of sodium me tasilicate (Na2SiO2), which displays a large premelting effect, and isostru ctural lithium metasilicate (Li2SiO3), which displays little premelting. Fr om Li-7 NMR, Li+ site hopping is clearly observed in Li2SiO3 by a partial a veraging of the Li-7 quadrupolar peak shape, requiring exchange among a few , ordered orientations of LiO4 tetrahedra. From Na-23 NMR, Na+ site hopping in Na2SiO3 appears to involve a more liquid-like behavior, implying exchan ge among many sites with different orientations in a disordered fashion. Fo r this phase, Si-29 spectra indicate that in an oxidizing environment, no l iquid phase is present at 20 degrees C below the melting point, well within the calorimetric premelting regime. However, partial averaging of the low- temperature, biaxial chemical shift powder pattern (typical of Si sites in chain silicates) occurs, suggesting some kind of extensive, librational mot ion of SiO4 tetrahedra that is possibly linked to rapid Na+ diffusion near the melting point. In contrast to the simple Li+ diffusion in Li2SiO3, this process may require considerable non-vibrational energy and may thus be re lated to the heat capacity anomaly just prior to melting.