Purification of NaYF₄‑Based Upconversion Phosphors

Applications of upconversion phosphors have grown extensively in number during the past decade. Hexagonal sodium yttrium fluoride (β-NaYF₄) is known to be one of the best host lattices for upconversion materials. We developed a novel technique for transforming cubic sodium yttrium fluoride (α-NaYF₄) phosphors into the hexagonal modification and remove oxygen impurities that hinder the upconversion luminescence. We transformed cubic α-NaYF₄ nanoparticles from flame-spray synthesis with a particle size less than 50 nm into more efficient β-NaYF₄ phosphors. The application of SnF₂ and ZnF₂ as oxygen scavengers allowed the formation of the pure hexagonal phase and improved the upconversion luminescence intensity. The developed process utilizes no free HF gas in the production and does not contaminate the upconversion phosphors with scavenger material. The treatment increases the particle size to between approximately 500 nm and 1 μm. Upconversion luminescence spectra revealed the characteristic blue Tm³⁺ and green Er³⁺ emissions of β-NaYF₄: Yb,Tm and Yb,Er, respectively