Study On The Preparation And Luminescence Properties Of Er³⁺/Yb³⁺ Doped Titanate Phosphors

Rare earth ion-doped upconversion luminescence materials are widely used in display materials,medical diagnosis,photocatalysis,solar cells,temperature probes,and anti-counterfeiting technologies due to their sharp spectral lines,tunable color and no background fluorescence.It is well known that the up-conversion（UC）luminescence properties are closely related to the local crystal field enviro nment where doped ions reside.Thus,exploring the influence of local crystal field enviro nment on UC luminescence performance plays a key role in optimizing the application of UC luminescence.Titanate is an oxide with stable physicochemical properties and low phonon energy,which can provide a suitable crystal field enviro nment for doping ions and is a good upconversion luminescent material.In this paper,rare earth ion Er³⁺/Yb³⁺doped titanate UC luminescence materials were prepared,and their morphology,structure and upconversion luminescence properties were studied.The effects of phonon energy,band gap,local structural symmetry and interatomic distance on upconversion luminescence properties are also investigated.The main research work and results of this paper are as follows:（1）Er³⁺/Yb³⁺double-doped co-doped titanate(La₂Ti₂O₇,La₂TiO₅,La₄Ti₃O₁₂,La₄Ti₉O₂₄,Gd₂Ti₂O₇,and Y₂Ti₂O₇)with different crystal field enviro nments were prepared by a high-temperature solid-phase method.The results of the first-principles calculations revealed that the RE ions preferentially occupy the La1 position in La₂TiO₅ and La₄Ti₃O₁₂,but reside in the La3 position in La₄Ti₉O₂₄.The DFT results also revealed that La₂TiO₅ and La₂Ti₂O₇ are both indirect semiconductors with the bandgaps of 3.38and 2.77 e V,respectively.Similarly,La₂Ti₂O₇ and La₄Ti₉O₂₄ are also indirect semiconductors with the bandgaps of 3.38 and 2.77 e V,respectively.The results of XRD structure and Rietveld refinement by GSAS show that La₂TiO₅ and La₄Ti₉O₂₄belongs to orthorhombin-phased,La₄Ti₃O₁₂、Gd₂Ti₂O₇ and Y₂Ti₂O₇ belongs to hexagonal-phased,and La₂Ti₂O₇ belongs to monoclinic-phased,respectively.XPS spectrum showed that the valence states of each element in the six titanate are the same.TEM reveal that the morphology of prepared phosphors are inrregular.Er³⁺/Yb³⁺co-doped six tiaanate phosphors exhibit superior UC emissions and bright emissions of green and red.Power pump dependency,fluorescence lifetime,and variation of fluorescence intensity ratios are used to demonstrate the UC mechanisms in the Er³⁺/Yb³⁺co-doped six tiaanate phosphor.（2）Based on the different luminescence properties of the six phosphors,the effects of interatomic distance and local structural symmetry on the upconversion luminescence properties were investigated.Since a shorter distance gives rise to a more significant quenching effect,La₂Ti₂O₇:Er³⁺/Yb³⁺has shown the highest luminescence intensity.Furthermore,La₂Ti₂O₇ has the lowest local symmetry and the low local symmetry will increase the possibility of f-f transition through the electron coupling effect and finally improve the luminescent performance.Thus,the La₂Ti₂O₇:Er³⁺/Yb³⁺show the best luminescent performance.Thus,to illustrate this issue,we established a mathematical model that can satisfactorily describe the variation trend of the red/green integrated intensity ratio in terms of the product of local crystal field intensity and interatomic distance.