Clarifying a Competitive Crystallization Mechanism of Upconversion Luminescent Oxyfluoride Glass Ceramics by Solid-State NMR Spectroscopy

Abstract

glass system by adjusting their compositions. The structural evolution at the atomic level driven by heat treatment and glass compositions was studied using the state-of-the-art magic-angle spinning nuclear magnetic resonance technique. From a comprehensive local structural study, we found that LaF 3 and NaLaF 4 crystals compete in crystallization in these glasses. The crystallization ability of NaLaF 4 increases with the increase of the content of Na + ions within the F-enriched phase, but for LaF 3 crystals, it is reverse. These two crystals can be selectively precipitated in the glasses by adjusting the content of these Na + ions within the F-enriched phase. When the crystallization ability of these two crystals becomes similar, none of them can be precipitated due to their mutual interference in crystallization. Intense single green upconversion luminescence occurs in glasses precipitating LaF 3 or NaLaF 4 crystals. The underlying relationship between compositions, structures, crystallization, and upconversion luminescence properties is unearthed based on the structural evolution, crystallization mechanism, and luminescence properties. This relationship will facilitate the compositional design of these kinds of glasses. It is inferred that it will be better to precipitate LaF 3 rather than NaLaF 4 crystals for achieving highly efficient upconversion luminescence.

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

“…This is because only at such a high BaF 2 content, the local concentration of F – ions around La 3+ and Ba 3+ is high enough to precipitate the Ba 2 LaF 7 crystal. The quantity of the F – ions attracted by the cations depends on the ability of the cations to attract F – . To estimate or characterize the abilities of the cations to attract F – , a parameter, R F–M , was defined, as follows: where M F denotes the total F content, f means the fraction (%) of the F–M species (M = La 3+ and Ba 2+ ) that were obtained via the deconvolution of the 19 F MAS spectra (Table S6).…”

“…The quantity of the F – ions attracted by the cations depends on the ability of the cations to attract F – . To estimate or characterize the abilities of the cations to attract F – , a parameter, R F–M , was defined, as follows: where M F denotes the total F content, f means the fraction (%) of the F–M species (M = La 3+ and Ba 2+ ) that were obtained via the deconvolution of the 19 F MAS spectra (Table S6). q denotes the electric charge of M , and [ M ] represents the molar ratios of La 3+ and Ba 2+ .…”

“…Consequently, the NaYF 4 crystals could be precipitated in Y 3+ -doped phosphate glasses, such as NaPO 3 –BaF 2 –YF 3. However, the La–F···Na species can be formed in silicate glasses, after which the NaLaF 4 crystals can be precipitated. Another novel phenomenon is that La 3+ and Ba 2+ could jointly attract F – .…”

“…The quantity of the F − ions attracted by the cations depends on the ability of the cations to attract F − . 62 To estimate or characterize the abilities of the cations to attract F − , a parameter, R F−M , was defined, as follows: 62…”

Local Structure Engineering in Lanthanide-Doped Nanocrystals for Tunable Upconversion Emissions

“…Taken together, these results indicated that the P 6 3 / m space group should be qualified to describe the structure of hexagonal NaYF 4 , which is congruent with the result of PDF analysis . Besides, SSNMR spectroscopy has also been used to unravel the proximity of oleic acid ligands to the surface of UCNs, the dynamic crystallization process of UCNs in oxyfluoride glasses, , and the random occupation of Na + and Ln 3+ ions in cubic NaLnF 4 UCNs . Interestingly, for hexagonal NaScF 4 :Yb,Er UCNs, the doped Yb 3+ /Er 3+ ions preferred to substitute Na + rather than Sc 3+ ions because of the similar ionic radius …”