Peculiarities of the Synthesis of LiRF₄ Nanoparticles (R = Er–Lu) by High-Temperature Coprecipitation and Their Photoluminescent Properties

LiRF₄ (R = Y, Yb, Lu) nanoparticles (NPs), co-doped by Yb³⁺/Er³⁺ and Yb³⁺/Tm³⁺ ions, have been synthesized by high-temperature coprecipitation method. The influence of the molar ratio of precursors and the cation composition of matrices on the sizes and morphology of particles is investigated. The method of heterogeneous crystallization of these compounds using LiYF₄ nanoseeds is optimized, which opens opportunities to controlled synthesis of LiRF₄ NPs with required characteristics. Among the objects studied, LiYF₄@LiYbF₄:Tm³⁺@LiYF₄ NPs demonstrate the most intense anti-Stokes photoluminescence in the UV (λ = 362 nm) and blue (λ = 450 nm) ranges, which exceeds the corresponding characteristics for β-NaYF₄:Yb³⁺/Tm³⁺@NaYF₄ particles. LiYF₄@LiLuF₄:Yb³⁺/Er³⁺@LiYF₄ NPs are the most efficient IR converters in the (λ = 1530 nm) among the investigated isostructural matrices; their spectral-luminescence characteristics are close to those of the β-NaYF₄:Yb³⁺/Er³⁺@NaYF₄ compound with the equivalent degree of codoping. The results obtained give possibilities to consider LiYF₄@LiYbF₄:Tm³⁺@LiYF₄ and LiYF₄@LiLuF₄:Yb³⁺/Er³⁺@LiYF₄ NPs as a real alternative to the most widely used phosphors based on hexa-gonal β-NaYF₄ host for photonics and biotechnology applications.