Structural and Luminescent Properties of EuIII-doped β-NaGdF₄ Core@Shell Nanoparticles Synthesized by Thermal Decomposition

Herein, β-NaGd_0.95Eu_0.05F₄@NaGdF₄ core@shell nanoparticles were synthesized by thermal decomposition of lanthanide acetate precursors^[2] to investigate the influence of the synthetic method on their structural and luminescence properties. Transmission electron microscopy reveals highly crystalline, spherical nanoparticles, with the average diameter increasing from 13±2 nm to 26±3 nm after shell growth, confirming successful coating. Photoluminescence properties were investigated by excitation and emission spectroscopy. The excitation spectra monitored at 612 nm show characteristic Eu^III f-f transitions as well as a Gd^III -related band at 271 nm (⁸S_7/2→⁶I_J), indicating Gd^III→Eu^III energy transfer. Under 271 nm excitation, the typical Eu^{III 5}D₀→⁷F_J (J = 0–4) emission lines are observed. Notably, the comparable intensity of the ⁵D₀→⁷F₂ and ⁵D₀→⁷F₁ bands suggests Eu^III sites with relatively high local symmetry. In addition, a broad emission band centered at 491 nm is attributed to defect-related states in the NaGdF₄ lattice.