Transparent Aluminosilicate Oxyfluoride Glass Ceramics Containing Upconversion Luminescent CaF2 Nanocrystals: Glass-to-Crystal Structural Evolution Studied by the Advanced Solid-State NMR Spectroscopy

Oxyfluoride glasses in the systems (55 – x)SiO2–(15 + x – z)Al2O3–zLa2O3–10CaO–20CaF2–yErF3 (x = 0, 5, 10, 15; y = 0.5, 1, 1.5; z = 0, 2, 5 mol %) were prepared by conventional melt-quenching method and afterward heated to synthesize transparent glass ceramics containing CaF2 nanocrystals. The local structures of glasses and glass ceramics were characterized by advanced one- and two-dimensional solid-state nuclear magnetic resonance (SSNMR) techniques. La3+ ions were investigated as the diamagnetic mimics for paramagnetic rare-earth ions (REIs) in local structure. The bond connectivities between F– ions and positive ions were probed by 19F magic angle spinning (MAS) NMR spectra and 19F {27Al} double-resonance techniques. F– ions have bond connectivities with Ca2+, La3+, and Al3+ ions, respectively. Ca2+ ions have comparable competition ability in capturing F– ions with La3+ ions, but stronger than Al3+ ions. Multiple F–Al linkages exist in the glasses. 19F static spin echo decay results indicate that F– ions aggregate around Ca2+ ions forming F–Ca linkages-enriched microregions, which form the precursors of CaF2 nanocrystals. However, both La3+ and Al3+ ions can disperse the distribution of F– ions around Ca2+ ions and consequently suppress the crystallization of CaF2. 139La wideband uniform-rate smooth truncation QCPMG (WURST-QCPMG) spectra and the paramagnetic effects of Er3+ on the NMR spectra were measured to reveal the distribution of REIs. The average local environments of most REIs have no significant change during the crystallization of CaF2. Only partial REIs have entered CaF2 nanocrystals after the crystallization, which results in the intensive Er3+ upconversion luminescence. The glass structural evolution during the crystallization is investigated by the evolution of the 19F, 27Al, 29Si, and 139La NMR spectra with the heat treatment conditions. On the basis of the comprehensive glass structure study at the atomic level, the microcrystallization mechanisms of the glasses are revealed.