Insight In The Influence Of Polymorphic Modulation Of K₃Lu（PO₄）₂ On The Photoluminescence Properties Of Rare Earth Ions

Due to rare earth luminescent ions possess unique properties,such as colorful and controllable emission,up/down-conversion luminescence,high quantum efficiency,as well as rich transition energy levels,their doping materials have attracted wide attention in the field of optoelectronics.The crystal-field splittings of rare earth luminescent ions is closely related to the local coordination environment,so the alteration of the local coordination structure is an important strategy to modulate the optical properties of rare earth luminescent ions.The structural phase transition of materials can cause significant changes in the local coordination space of luminescent sites,and the modulation effect on the photoluminescence properties of rare earth luminescent ions has been proved in the fields of white LED,anti-counterfeiting,optical sensing,etc.K₃Lu（PO₄）₂is an excellent phosphate-based phase change material with three different crystal structures at different temperatures.At present,the modulation mechanism of rare earth luminescent ions induced by different K₃Lu（PO₄）₂crystals is not clear.Pr³⁺:K₃Lu（PO₄）₂and Eu³⁺:K₃Lu（PO₄）₂samples were prepared,and the modulation mechanism of Pr³⁺and Eu³⁺ions induced by K₃Lu（PO₄）₂crystal difference was studied.The crystal evolution of K₃Lu（PO₄）₂was realized by ion doping method.On this basis,the application potential of relevant samples in the fields of fluorescence enhancement and information encryption was explored.The main experimental conclusions are as follows:（1）Pr³⁺:K₃Lu（PO₄）₂（0.1 at.%）phosphors were successfully synthesized by traditional high-temperature solid state reaction.Based on differential scanning calorimetry（DSC）analysis,Pr³⁺:K₃Lu（PO₄）₂samples showed three different crystal structures from low temperature to high temperature,corresponding to phaseⅠ[P2₁/m,coordination number（CN）of Lu³⁺=7],phaseⅡ（P2₁/m,CN=6）,and phaseⅢ（P3,CN=6）,respectively.For the 4f→4f transition region,the fluorescence emission of Pr³⁺:K₃Lu（PO₄）₂sample is dominated by the¹D₂→³H₄magnetic dipole transition,and the fluorescence intensity reaches the highest when Pr³⁺-doped concentration is 0.6 at.%.The fluorescence lifetime of Pr³⁺ion in phaseⅢis longer than that those in the other two phases（～1 ms）,which is related to the dominant ¹D₂→³H₄transition.The fluorescence lifetime of Pr³⁺:K₃Lu（PO₄）₂at room temperature decreases with the increase of Pr³⁺-doped concentration,which is mainly ascribed to the decrease of the distance between adjacent Pr³⁺ions and the increase of the probability of non-radiative transition and energy transfer.In addition,the fluorescence intensity of Pr³⁺ions in phaseⅠ（78 K）is about 170 times stronger than that in phaseⅢ（300 K）,which is mainly dependent on the change of local coordination environment caused by the crystal transformation of Pr³⁺:K₃Lu（PO₄）₂.Subsequently,a series of Pr³⁺:K₃Lu_0.999-xGd_x（PO₄）₂（x=0-0.2）phosphors were synthesized by high-temperature solid state reaction.The Pr³⁺-doped concentration was fixed at 0.1 at.%.The introduction of Gd³⁺ions successfully stabilized phaseⅠ（or phaseⅡ）to room temperature.Compared with undoped K₃Lu（PO₄）₂,the fluorescence intensity of Pr³⁺:K₃Lu_0.999-xGd_x（PO₄）₂sample with x=11 at.%increased by 27 times.（2）The Eu³⁺:K₃Lu（PO₄）₂sample（0.1 at.%）was successfully sintered by high-temperature sintering method.The temperature-induced reversible phase transitions of K₃Lu（PO₄）₂（phase I?phaseⅡand phaseⅡ?phaseⅢ,below room temperature）give rise to an obvious photoluminescence difference of Eu³⁺ions.The Eu³⁺emission mainly focused on the ⁵D₀→⁷F₁transition in phaseⅢbut manifested the comparable ⁵D₀→⁷F_1,2transitions in the two low-temperature phases.In addition,the steady-state and transient spectral analysis shows that Eu³⁺ions tend to occupy inversion Lu（1）site in phaseⅢ,while entering the non-inversion ones in phases I andⅡ.Next,y Eu³⁺:K₃Lu_1-y（PO₄）₂（y=0.1-15 at.%）samples were successfully sintered by high temperature sintering method.Based on the changes of Eu³⁺ions,different Eu³⁺:K₃Lu（PO₄）₂phases are regulated,so that phase I（or phaseⅡ）can be successfully stabilized to room temperature.In addition,the samples with Eu³⁺-doped concentration of5.5 at.%and 8.75 at.%show obvious photoluminescence hysteresis and excellent stability and reproducibility in the temperature range of 270-350 K.Based on that basis,the application potential of Eu³⁺:K₃Lu（PO₄）₂sample in the field of optical information encryption is explored.This project systematically studied the influence of different K₃Lu（PO₄）₂phases on the fluorescence properties of Pr³⁺and Eu³⁺ions,and realized the modulation of different crystal structures of K₃Lu（PO₄）₂through the chemical substitution of Lu³⁺sites.The research results not only highlighted the significance of luminescent ion site engineering,but also demonstrating the application value of phase-change hosts in the development of high-performance luminescent materials.