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Mechanism Insight into H2O-Driven Phase Transition from NaYF4 to YOF for Tailoring Upconversion Luminescence
Jingyufeng Lei1, Shaoyang He1, Jing Peng1
1School of Materials Science and Engineering, Beihang University, Beijing100191, P. R. China.
Inorganic Chemistry
|July 16, 2026
Summary
Water (H2O), not oxygen (O2), drives the NaYF4 to YOF phase transition in fluoride upconversion materials. This discovery enables controlled tuning of luminescence color by using humid atmospheres during annealing.
Area of Science:
- Materials Science
- Nanotechnology
- Optical Engineering
Background:
- The thermal instability and luminescence evolution of sodium yttrium fluoride (NaYF4) doped with ytterbium (Yb3+) and erbium (Er3+) during annealing have been attributed to oxygen (O2).
- This long-standing assumption regarding the role of O2 in NaYF4:Yb3+,Er3+ has lacked rigorous validation.
Purpose of the Study:
- To rigorously validate the role of oxygen (O2) versus water (H2O) in the thermal degradation and phase transition of NaYF4:Yb3+,Er3+.
- To establish the primary agent responsible for the NaYF4 to yttrium oxyfluoride (YOF) phase transition and develop a new annealing strategy based on this understanding.
Main Methods:
- Investigated the phase transition of NaYF4:Yb3+,Er3+ under varying annealing atmospheres (O2 and H2O).
- Analyzed the reaction pathway and kinetics of NaYF4 conversion to YOF at different temperatures.
- Developed and applied a novel postannealing strategy using controlled humid atmospheres.
Main Results:
- Demonstrated that water (H2O), not O2, is the primary agent driving the NaYF4 → YOF phase transition below 692 °C via the reaction NaYF4 + H2O → YOF + NaF + 2HF.
- Showed that O2 contributes to YOF formation above 692 °C but with lower efficiency than H2O.
- Achieved quantitative, stepwise phase conversion from NaYF4 to YOF using humid atmospheres, enabling continuous tuning of upconversion emission color (green to red) without significant luminescence loss.
Conclusions:
- Corrected the persistent misconception attributing NaYF4 degradation solely to O2, identifying H2O as the key factor in fluoride-to-oxyfluoride conversion.
- Established H2O as a critical, previously overlooked thermodynamic factor in processing fluoride-based optical materials.
- The developed humid atmosphere annealing strategy offers precise control over phase composition and luminescence properties of upconversion materials.
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