Morphology evolution and pure red upconversion mechanism of β-NaLuF₄ crystals

View Article Page [PDF]

A series of β-NaLuF₄ crystals were synthesized via a hydrothermal method. Hexagonal phase microdisks, microprisms, and microtubes were achieved by simply changing the amount of citric acid in the initial reaction solution. Pure red upconversion (UC) luminescence can be observed in β-NaLuF₄:Yb³⁺, Tm³⁺, Er³⁺ and Li⁺ doped β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺. Based on the rate equations, we report the theoretical model about the pure red UC mechanism in Yb³⁺/Tm³⁺/Er³⁺ doped system. It is proposed that the pure red UC luminescence is mainly ascribed to the energy transfer UC from Tm³⁺:³F₄ → ³H₆ to Er³⁺:⁴I_11/2 → ⁴F_9/2 and the cross-relaxation (CR) effect [Er³⁺:⁴S_3/2 + ⁴I_15/2 → ⁴I_9/2 + ⁴I_13/2] rather than the long-accepted mechanism [CR process among Er³⁺:⁴F_7/2 + ⁴I_11/2 → ⁴F_9/2 + ⁴F_9/2]. In addition, compared to the Li⁺-free counterpart, the pure red UC luminescence in β-NaLuF₄:20% Yb³⁺, 1% Tm³⁺, 20% Er³⁺ with 15 mol% Li⁺ doping is enhanced by 13.7 times. This study provides a general and effective approach to obtain intense pure red UC luminescence, which can be applied to other synthetic strategies.