Mechanism Insight into H2O-Driven Phase Transition from NaYF4 to YOF for Tailoring Upconversion Luminescence

For over a decade, the thermal instability and luminescence evolution of NaYF4:Yb3+,Er3+ during annealing have been widely attributed to oxygen (O2), yet this long-standing assumption has never been rigorously validated. Here we overturn this view by unambiguously demonstrating that water (H2O), not O2, is the decisive agent driving the NaYF4 → YOF phase transition below the β-to-α phase transition temperature (692 °C). H2O is the key factor triggering the reaction pathway: NaYF4 + H2O → YOF + NaF + 2HF. Above this temperature, O2 contributes to YOF formation, but with lower efficiency and kinetics than those of H2O. Based on this mechanism, we developed a novel postannealing strategy using humid atmospheres to achieve quantitative, stepwise controlled phase conversion from NaYF4 to YOF. This enables continuous and uniform tuning of upconversion emission color from green to yellow, orange, and red under fixed excitation power, without significant loss in overall luminescence intensity. This work not only corrects a persistent misconception in fluoride upconversion materials but also establishes H2O as a previously overlooked thermodynamic factor in fluoride-to-oxyfluoride conversion, with broader implications for the processing of fluoride-based optical materials.