The Synthesis And Properties Of Up-conversion Luminescent Materials With Cryolite Structure

Lanthanide-based up-conversion(UC)luminescent materials have attracted worldwide attention due to their unique electronic structure,good chemical and optical properties,as well as wide applications in the area of photovoltaic cells,biomedicine,anti-counterfeiting,solid-state lighting and display technology.The performance of UC luminescent materials largely depends on the host material.Because of its low phonon energy,good optical transparency and stable structure,fluoride is regarded as the most ideal UC luminescent materials matrix.Therefore,a variety of cryolite(Na3Al F6)structured luminescent materials and co-doped with rare earth elements have been prepared in this work.The effects of sample composition and crystal form on the luminescence properties were studied by using advanced analysis and characterization methods combined with structural refinement.This thesis can provide theoretical basis and guidance for in-depth understanding of the luminescence mechanism of fluoride-based UC luminescent materials,the development of new UC luminescent material systems,and performance optimization and application of UC luminescent materials.The main results are as follows:1.Seven cryolite-structure compounds were prepared.Among them,cubic K3Ga F6,K3ScF6,K3In F6 and monoclinic K3Lu F6,K3YF6 have been prepared by high-temperature solid-phase synthesis method.It was found that the atomic radius of the trivalent metallic element has an important influence on the crystal form and purity of K3Ln F6(Ln,trivalent metal ion).The pure phases of cryolite-structure K2Na ScF6,K3ScF6,(NH4)2Na ScF6 and K3Ga F6 were prepared by liquid-phase synthesis method through adjusting the type of raw material and dosage,reaction p H value and solvent.2.Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺co-doped cubic K₃ScF₆ and monoclinic K3Lu F6 UC luminescent materials with cryolite structure were prepared by solid-phase methods.The optimal doping concentrations of Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺in the cubic K3ScF6 are 0.05/0.15,0.1/0.20 and 0.05/0.25 mol.Under excitation at 980 nm,K₃ScF₆ doped with Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺at the optimal doping concentration emitted yellow-green,near-infrared and green light,respectively;The optimal doping concentration of Er³⁺/Yb³⁺,Tm³⁺/Yb³⁺and Ho³⁺/Yb³⁺in monoclinic K3Lu F6 are 0.05/0.10,0.005/0.25 and 0.005/0.20 mol,which emitting red,blue and green light,respectively.3.K₃Sc_1-xLu_xF₆:5%Er³⁺,15%Yb³⁺(0?x?0.8)were prepared under the same condition by solid-phase synthesis method.The crystal form of K3Sc1-x Lux F6:5%Er³⁺,15%Yb³⁺(0?x?0.8)changed from cubic to monoclinic with increasing the substitution of Sc³⁺for Lu³⁺.The influence of the crystal form on the properties of cryolite structure UC luminescent materials was studied.The luminescent intensity of monoclinic phase is much higher than that of the cubic phase for K₃Sc_1-xLu_xF₆:5%Er³⁺,15%Yb³⁺(0?x?0.8).The reason may be that some f-f parity forbidden transitions of Er³⁺are allowed because of the reduction of structural symmetry and the mixing of 4f-5d orbitals induced by Lu³⁺through lattice valence band energy level.4.For the first time,cubic UC luminescent material K₂Na ScF₆:Er³⁺,Yb³⁺and K₂Na Ga F₆:Er³⁺,Yb³⁺were prepared by hydrothermal method and high temperature thermal decomposition method respectively.Coated with Si O2,core shell structure UC luminescent material K₂Na ScF₆:Er³⁺,Yb³⁺@SiO₂ and K₂Na Ga F₆:Er³⁺,Yb³⁺@SiO₂ were prepared and heated.Result showed that Si O2 coating weakens the luminescent intensity of K₂Na ScF₆:Er³⁺,Yb³⁺and K₂Na Ga F₆:Er³⁺,Yb³⁺,while heat treatment can increase the luminescent intensity of K₂Na ScF₆:Er³⁺,Yb³⁺@SiO₂ by 20 times and that of K₂Na Ga F₆:Er³⁺,Yb³⁺@SiO₂ by 2.5 times compared with the unheated sample.