Tuning of structure and enhancement of upconversion luminescence in NaLuF₄:Yb³⁺,Ho³⁺ crystals

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A series of NaLuF₄:Yb³⁺,Ho³⁺ nano/micro-crystals with different crystal structures were synthesized via a hydrothermal method using citric acid as a chelating agent. The influences of NaF content, Li⁺ doping, reaction temperature and reaction time on the crystal structure and shape of the as-synthesized NaLuF₄ crystals were systematically investigated. To the best of our knowledge, it is the first time to report Li⁺ doped α-NaLuF₄:Yb³⁺,Ho³⁺ nanocrystals and the phase transformation by introducing Li⁺ in NaLuF₄ crystals. As for Li⁺ doped α-NaLuF₄, UC luminescence intensities of green emission (538 nm) and red emission (644 nm) in α-NaLuF₄:Yb³⁺,Ho³⁺ nanocrystals with 20 mol% Li⁺ doping are enhanced by 20 and 3.5 times compared to their Li⁺-free counterpart. As for Li⁺ doped α/β-mixed NaLuF₄, with the increase of Li⁺ content, the phase transforms from the α/β-mixed phase to hexagonal then to cubic. UC emissions of 538 nm and 644 nm in NaLuF₄:Yb³⁺,Ho³⁺ crystals doped with 5 mol% Li⁺ are enhanced by 26.5 and 23 times, respectively. Besides, it is found that with the higher temperature and prolonged time, the morphology of NaLuF₄ changes from nanoparticles to microtubes, resulting in the dramatic increase of UC emission intensity. The effects of Li⁺ doping, reaction temperature and reaction time on the enhancement of UC emission intensity are discussed in detail. This study provides an effective and facile approach to obtain nano/micro-crystals with controllable structures and excellent optical properties.