Structural Evolution During Precipitation of Alkaline-Earth Fluoride Nanocrystals in Oxyfluoride Glasses: A Multinuclear Nuclear Magnetic Resonance Spectroscopic Study

摘要

The fluorine and aluminum coordination environments and their evolution during precipitation of CaF_2, SrF_2, and (Ca_xSr_(1-x))F_2 nanocrystals in oxyfluoride glasses and glass-ceramics are investigated using ~(19)F and ~(27)AI magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. These structural aspects of the parent glasses and of the resulting glass-ceramics are found to be strongly dependent on the nature of the modifier cation. In the calcium-modified glass the preexisting F-Ca(n) sites act as major precursors of F-Ca(4) sites in CaF_2 nanocrystals that are precipitated upon ceramming. In contrast, for the strontium-modified glass, besides F-Sr(n) sites a large fraction of the Al-F-Sr(n) sites in the parent glass are converted into F-Sr(4) sites in the glass-ceramic during precipitation of SrF_2 nanocrystals. In the case of a glass containing both calcium and strontium as modifying cations precipitation of (Ca_xSr_(1-x)F_2 solid solution nanocrystals is achieved upon ceramming. However, Ca~(2+) ions preferentially partition into the crystal phase resulting in a Ca:Sr atomic ratio that is significantly higher in the nanocrystal compared with that in the parent glass. These observations are consistent with the higher field strength of Ca~(2+) compared with Sr~(2+). Incorporation of Yb~(3+) ions into the lattices of these fluoride nanocrystals is also evidenced in the ~(19)F NMR spectra.