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

Abstract

The fluorine and aluminum coordination environments and their evolution during precipitation of CaF2, SrF2, and (CaxSr1−x)F2 nanocrystals in oxyfluoride glasses and glass–ceramics are investigated using 19F and 27Al 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 CaF2 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 SrF2 nanocrystals. In the case of a glass containing both calcium and strontium as modifying cations precipitation of (CaxSr1−x)F2 solid solution nanocrystals is achieved upon ceramming. However, Ca2+ 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 Ca2+ compared with Sr2+. Incorporation of Yb3+ ions into the lattices of these fluoride nanocrystals is also evidenced in the 19F NMR spectra.