The Research On Local Structure And Luminescent Properties Of Garnet Phosphors

LED devices prepared by phosphor-converted method have shown great application prospects in the fields of decorative lamps,plant growth lamps,and iris recognition,which due to their advantages of energy saving,environmental friendliness and long lifetime.Hence,the diversity of photoluminescent colors of phosphors is an important factor to realize its wide variety of applications.Therefore,controllable potoluminescent tuning is highly desirable.Herein,the relationship between the structure and luminescence properties of the YAG:Ce phosphor and the exploration of the new garnet phosphor materials were studied.The local microstructural evolutions and luminescent properties of Y3Al5O12:Ce via the substitution of the cations Ga3+/Sc3+ for Al3+ are investigated.The emission spectrum of Y3Al2-xGaxAl3O12:Ce has a blueshift from 559 nm to 531 nm with the substitution of Ga3+ for Al3+.This outcome can be attributed to a lengthened Ce-O bond,which leads to a smaller crystal splitting.In the above ending composition Y3Ga2Al3O12:Ce,an unusual redshift from 531 nm to 545 nm can be observed by further replacing Ga3+ with the larger Sc3+.Combining the first-principle calculations and the energy level position analysis,introduction of Sc3+ into Y3Al5O12 lattices causes a sharp increase in(Ce-O8)polyhedron distortion,leading to a larger additional distorted crystal-field splitting of Eg levels compared with that with Ga3+ dopant.Moreover,a relatively larger electronegativity of Sc3+ generally enhances the nephelauxetic effect and further lower the lowest 5d level.These two factors are responsible for the unusual redshift.A formula was proposed to describe the polyhedral distortion that can explain the unusual redshift very well.Secondly,a spinodal decomposition phenomenon was found as the Ca2+-Hf4+ions pairs were substituted Y3+-Al3+ pairs in YAG:Ce phosphor.The phase structure,composition and critical concentrations of phase separation are determined by XRD Rietveld refinement,respectively.The explanations of phase formation mechanisms are given based on the thermodynamics by first-principle calculations.Moreover,the relationship between the phase structure and the luminescent properties of samples are discussed in detail.In addition,the first-principles calculation method was introduced to investigate the effect of carbon interstitial defects on Y3Al5O12:Ce phosphor,in which we found that the carbon interstitials tend to occupy the next-nearest sites of Ce3+ ion in Y3Al5O12:Ce lattice.Specially,these interstitial defects can shorten the Ce3+-O2+ bond length,leading to a larger crystal field splitting of 5d orbital of the Ce3U atom and bigger 5d centroid shift.These two factors cause the emission spectrum of Y3Al5O12:Ce,C red shift compared with that of Y3A15O12:Ce phosphor.Moreover,with our comparison experiment,the experimental results are in accord with our first principles calculation data.This work systematically investigated the impacts of the carbon interstitial defect on Y3A15O12:Ce,and provided a new route to tune the emission spectrum in Y3A15O12:Ce.Finally,we designed a new garnet matrix by the aid of the Diagonal Relationship theory.And then a new garnet phosphor,Ca2LuScZrAl2GeO12,was invented and synthesized.The crystal structure,electronic structure,and luminescent properties of the amples were studied in detail.The electronic structure analysis showed that both the 4s orbital of Ge and 4d orbital of Zr an lower the bottom edge of the conductive band,which enhanced the photoioIization process of Ce3+ activated garnet phosphors.