By using an improved Bridgman method, 0.3 mol% Tm³⁺/0.6 mol% Tb³⁺/y mol% Eu³⁺ (y = 0, 0.4, 0.6, 0.8) doped Na₅Y₉F₃₂ single crystals were prepared. The x-ray diffraction, excitation spectra, emission spectra and fluorescence decay curves were used to explore the crystal structure and optical performance of the obtained samples. When excited by 362 nm light, the cool white emission was realized by Na₅Y₉F₃₂ single crystal triply-doped with 0.3 mol% Tm³⁺/0.6 mol% Tb³⁺/0.8 mol% Eu³⁺, in which the Commission Internationale de l’Eclairage (CIE) chromaticity coordinate was (0.2995, 0.3298) and the correlated color temperature (CCT) was 6586 K. The integrated normalized emission intensity of the tri-doped single crystal at 448 K could keep 62% of that at 298 K. The internal quantum yield (QY) was calculated to be ∼ 15.16% by integrating spheres. These results suggested that the single crystals tri-doped with Tm³⁺, Tb³⁺ and Eu³⁺ ions have a promising potential application for white light-emitting diodes (w-LEDs).

Na₅Y₉F₃₂ single crystals doped with ∼ 0.8-mol% Ho³⁺, ∼ 1-mol% Tm³⁺, and various Er³⁺ ion concentrations were prepared by a modified Bridgman method. The effects of Er³⁺ ion concentration on 2.0-μm emission excited by an 800-nm laser diode were investigated with the help of their spectroscopic properties. The intensity of 2.0-μm emission reached to maximum when the Er³⁺ ion concentration was ∼ 1 mol%. The energy transfer mechanisms between Er³⁺, Ho³⁺, and Tm³⁺ ions were identified from the change of the absorption spectra, the emission spectra, and the measured decay curves. The maximum 2.0-μm emission cross section of the Er³⁺/Ho³⁺/Tm³⁺ tri-doped Na₅Y₉F₃₂ single crystal reached 5.26 × 10⁻²¹ cm². The gain cross section spectra were calculated according to the absorption and emission cross section spectra. The cross section for ∼ 2.0-μm emission became a positive gain once the inversion level of population was reached 30%. The energy transfer efficiency was further increased by 11.81% through the incorporation of Er³⁺ ion into Ho³⁺/Tm³⁺ system estimated from the measured lifetimes of Ho³⁺/Tm³⁺- and Er³⁺/Ho³⁺/Tm³⁺-doped Na₅Y₉F₃₂ single crystals. The present results illustrated that the Er³⁺/Ho³⁺/Tm³⁺ tri-doped Na₅Y₉F₃₂ single crystals can be used as promising candidate for 2.0-μm laser.