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.

From Phase Separation to Nanocrystallization in Fluorosilicate Glasses: Structural Design of Highly Luminescent Glass-Ceramics

“…The observed coordinations of fluorine around barium, and the formation of the Al–F–Ba and Al–F–Al linkages, as well as six-coordinated Al 3+ , from nuclear magnetic resonance (NMR) spectra (Figure ) are typical features of fluoride separated phases that are devitrified to BaF 2 or BaAlF 5 crystals in the oxyfluoride glasses. − In these glasses, the coordination and linkage ranges are n ≤ 4 for F–Ba­( n ) and n ≤ 3 for Al–F–Ba­( n ), where n stands for coordination number (CN). In crystals, n has a fixed value, but in glasses n is usually a range due to the amorphous nature.…”

Transparent oxyfluoride glass ceramics

“…Zheng Xue [62] used 19 F and 27 Al NMR techniques to reveal that CaF 2 or SrF 2 modifiers determined structural features of 50SiO 2 -20Al 2 O 3 -30CaF 2 (G-Ca) or 50SiO 2 -10Al 2 O 3 -20ZnF 2 -20SrF 2 or 50SiO 2 -10Al2O 3 -20ZnF 2 -16SrF 2 -4CaF 2 initial glass and prepared glass ceramics. If Ca cations were used as modifiers, then F-Ca(n) sites in glass were the precursors of FCa(4) sites in the nanocrystals of formed glass ceramics.…”

Ce3+‐ and Dy3+‐Doped Oxyfluoride Borosilicate Glasses with Near White Luminescence

“…Thus, the enlarged width implies a decreasing number of nonbridging oxygen existing in the glass network structure. The structural transitions of some oxyfluoride glasses during quenching and heat‐treatment process were investigated in recent years . Although the FT‐IR results presented here provide the preliminary information about the structural units of the as‐prepared glasses, some other advanced techniques (e.g., NMR) need to be used to further examine the detailed structural evolutions of the materials and the related work will be performed soon.…”