Ultraviolet upconversion emissions of Gd³⁺ in β-NaLuF ₄:Yb³⁺,Tm³⁺,Gd³⁺ nanocrystals

β-NaLuF₄:20%Yb,0.5%Tm,x%Gd (x = 10, 20, 30) nanocrystals were synthesized by high temperature thermal decomposition method. X-ray diffraction (XRD) show the as-prepared samples are characteristic of a pure β-NaLuF₄. Transmission electron microscope (TEM) investigations indicate the nanocrystals have particle size of -85 nm with good monodispersity and well-defined crystallographic facets. Particularly, under 980 nm excitation, upconversion (UC) emissions in the UV range of 270-320 nm were observed in these nanocrystals, which further prove that β-NaLuF₄ is an excellent host for building UV compact solid-state lasers or fiber lasers.