Simultaneous realization of structure manipulation and emission enhancement in NaLuF₄ upconversion crystals

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Gd³⁺ doped NaLuF₄:Yb³⁺,Er³⁺,Tm³⁺ nano/micro-crystals with the enhanced upconversion (UC) emission were successfully prepared through a hydrothermal method with the assistance of citric acid. As for Gd³⁺ doped α-NaLuF₄, before the phase transformation (when Gd³⁺ content ≤40 mol%), UC luminescence intensities in the red region from α-NaLuF₄:Yb³⁺,Er³⁺,Tm³⁺ nanocrystals doped with 40 mol% Gd³⁺ are enhanced by 13 times compared to the Gd³⁺-free sample. As for Gd³⁺ doped α/β-mixed NaLuF₄, with the increase of Gd³⁺ content, the crystal structure transforms from the α/β-mixture to a pure hexagonal phase without a long reaction time or a high reaction temperature. Red UC emissions from NaLuF₄:Yb³⁺,Er³⁺,Tm³⁺ crystals with 60 mol% Gd³⁺ doping are enhanced by 9 times. The mechanisms of the enhanced UC emission and phase transformation by introducing Gd³⁺ are discussed in detail. Furthermore, in order to obtain various structures of NaLuF₄ nano/micro-crystals before Gd³⁺ doping and demonstrate the advantages of Gd³⁺ doping, the effects of NaF content, reaction time and temperature on the crystal structure of Gd³⁺-absent NaLuF₄ crystals are systematically studied. The results suggest that the intense red UC luminescence from Gd³⁺ doped NaLuF₄:Yb³⁺,Er³⁺,Tm³⁺ nano/micro-crystals may have potential applications in biological imaging and optoelectronic devices.