Fluorophosphate Upconversion-Luminescent Glass-Ceramics Containing Ba2LaF7:Er3+ Nanocrystals: An Advanced Solid-State Nuclear Magnetic Resonance Study

Fluorophosphate glass-ceramics containing Ba2LaF7:Er3+ nanocrystal upconversion-luminescent materials were prepared via a melt-quenching method, followed by heat treatment. The glass structure evolution, which was induced by the change in composition, as well as heat treatment, was characterized via solid-state nuclear magnetic resonance (SSNMR) spectroscopy. Multiple QnmLa phosphorous species (n and m represent the numbers of P–O–P and P–O–La bonds, respectively) were proved via multiple 31P magic-angle spinning (MAS). Three fluorine species [P–F···Na, Ba–F···Na, and (Ba, La)–F···Na] were resolved using 19F MAS spectra. Unlike Y3+, La3+ cannot attract F to form an La–F···Na linkage in phosphate glasses. Furthermore, dissimilar to the competition behavior between the positive ions to attract F–, the 19F NMR results demonstrates that Ba2+ and La3+ combined preferentially to attract F–, thus inducing the preferential formation of the (Ba, La)–F···Na species, followed by the ultimate precipitation of the Ba2LaF7 crystal after the heat treatment. The paramagnetic effects of Er3+ on 19F and 31P indicates that there are still many Er3+ ions in the glass phase after crystallization, even though the Er3+ ions dominated the Ba2LaF7 crystal. The evolution of the 31P spectrum via crystallization indicates that the precipitation of Ba2LaF7 facilitates the polymerization of the phosphorus glass network. Finally, a structural evolution model is developed.