Phase Change of NaYF4:Er Crystals in Oxyfluoride Phosphate Upconversion Luminescent Glass Ceramics: An Advanced Solid-State NMR Study.

A series of glasses with composition 60NaPO3-(40-x)CdF2-xYF3-yEr2O3 were synthesized via melt-quenching methods and subsequently heat-treated to obtain upconversion luminescent glass ceramics containing NaYF4:Er crystals. Hexagonal and/or cubic NaYF4 crystals were controlled to be bred in the glasses by changing the glass composition. The structure evolution driven by crystallization was characterized using advanced solid-state nuclear magnetic resonance (SSNMR) techniques. The SSNMR results reveal that the Y/Na ratio determines the crystalline phases of NaYF4 precipitated in this glass system. Y3+ attracts extra F- ions from P5+ and Cd2+ during crystallization because of its stronger ability to attract F- ions, leading to most Y3+ ions being crystallized into the NaYF4 crystals. The paramagnetic broadening effect of the Er3+ ions on NMR signals as well as the upconversion luminescence results indicate that, before crystallization, most Er3+ ions are surrounded by oxygen within the glasses; however, after crystallization, almost all of them enter the NaYF4 crystals. On the basis of this local structure investigation, a composition design strategy is developed to obtain highly efficient upconversion luminescent glass ceramics.