Effect of Structural Perfection of Crystalline β-NaYF4:Yb,Er Phosphor Powders on the Efficiency of Their Upconversion Luminescence.

Sodium yttrium fluoride codoped with ytterbium and erbium has been prepared by reacting sodium fluoride with rare-earth nitrates in molten sodium nitrate, using a sevenfold excess of the fluorinating agent. The synthesis temperature was 320 to 450°C and the synthesis time was varied from 15 to 500 min. After washing with double-distilled water, the synthesized powders were characterized by X-ray diffraction (to determine the crystallite size and lattice strain), scanning electron microscopy, upconversion luminescence measurements at an excitation wavelength of 974 nm, and the energy yield of their luminescence was determined. The synthesis products had the form of single-phase hexagonal β-NaYF₄ samples. We observed the formation of distorted hexagonal prisms through nanoparticle aggregation. The luminescence energy yield was found to be a nonlinear function of the lattice strain e₀ in the β-NaYF₄ crystals, rising sharply with decreasing e₀. Reducing the defect density in powder particles is an effective strategy for improving the efficiency of their upconversion luminescence. [ABSTRACT FROM AUTHOR]