Gd³⁺-Doping Effect on Upconversion Emission of NaYF₄: Yb³⁺, Er³⁺/Tm³⁺ Microparticles

β-NaYF₄ microcrystals co-doped with Yb³⁺, Er³⁺/Tm³⁺, and Gd ions were synthesized via a hydrothermal method using rare-earth chlorides as the precursors. The SEM and XRD data show that the doped β-NaYF₄ form uniform hexagonal prisms with an approximate size of 600-800 nm. The partial substitution of Y by Gd results in size reduction of microcrystals. Upconversion luminescence spectra of microcrystals upon 980 nm excitation contain characteristic intra-configurational ff bands of Er³⁺/Tm³⁺ ions. An addition of Gd³⁺ ions leads to a significant enhancement of upconversion luminescence intensity with maxima at 5 mol % of dopant.