Temperature-induced structural changes in fluorozirconate glasses and liquids

The atomic structure and its temperature dependence in fluorozirconate glasses and supercooled liquids have been studied with high-resolution and high-temperature ¹⁹F and ²³Na nuclear-magnetic-resonance (NMR) spectroscopy. The ¹⁹F NMR spectra in these glasses show the presence of multiple F environments. Temperature dependence of the ¹⁹F magic-angle-spinning NMR spectra indicates a progressive change in the average F coordination environment in the glass structure, besides motional narrowing due to substantial mobility of F^- ions. The observed change in the average ¹⁹F NMR chemical shift is consistent with progressive breaking of the Zr-F-Zr linkages in the glass structure with increasing temperature. The onset of such a change in F speciation is observed at temperatures well below Tg. This result is evidence of changes in the average equilibrium structure in an inorganic glass-forming liquid at T<Tg, albeit on a local scale. The ²³Na NMR spectra indicate that the cations in these glasses become significantly mobile only at temperatures T>~Tg, which allows for the onset of global structural relaxation and viscous flow.