Enhanced Red Emission of Na₅Lu₉F₃₂: Eu³⁺ Single Crystal by Introducing of GdF₃

The bulk Na5Lu9F32 single crystals with high quality serial Eu3+/Gd3+ co-doped were grown by an ameliorative Bridgman technique. The measurement of X-ray diffraction (XRD) and analysis of Rietveld refinement were implemented to validate the crystal phase. The Eu3+ doped Na5Lu9F32 single crystal emits intense red emission at 611 nm excited by 394 nm light. The intensity of 1.8 mol % Gd3+ doped sample is boosted by 3.56 times compared with the un-doped one. The doping of Gd3+ ions replace competitively Lu3+ lattice sites with Eu3+ ions. Thus, they cause the lattice distortion and reduction of symmetry, enabling Eu3+ ions to overcome their 4f forbidden transitions. The change of Eu3+ ion local environment induced by Gd3+ ion doping was explored by the Eu3+ emission spectra and the variations of ratios between red and orange emission intensities (R/O). The fluorescence lifetime indicates that the introduction of GdF3 decreases the probability of non-radiative transitions and increases the fluorescence lifetime. These results demonstrated that the Na5Lu9F32: Eu3+ single crystal doped with an appropriate concentration of GdF3 is a type of promising red emission single crystal for related optical devices of detector and laser due to its strong red emission, high transparency and high physico-chemical stability.