Mechanism Insight into H ₂ O–Driven Phase Transition from NaYF ₄ to YOF for Tailoring Upconversion Luminescence

For over a decade, the thermal instability and luminescence evolution of NaYF 4:Yb 3+,Er 3+ during annealing have been widely attributed to oxygen (O 2 ), yet this long-standing assumption has never been rigorously validated. Here we overturn this view by unambiguously demonstrating that water (H 2 O), not O 2, is the decisive agent driving the NaYF 4 → YOF phase transition below the β-to-α phase transition temperature (692 °C). H 2 O is the key factor triggering the reaction pathway: NaYF 4 + H 2 O → YOF + NaF + 2HF. Above this temperature, O 2 contributes to YOF formation, but with lower efficiency and kinetics than those of H 2 O. Based on this mechanism, we developed a novel postannealing strategy using humid atmospheres to achieve quantitative, stepwise controlled phase conversion from NaYF 4 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 H 2 O as a previously overlooked thermodynamic factor in fluoride-to-oxyfluoride conversion, with broader implications for the processing of fluoride-based optical materials.