Crystallographic Site Engineering Of Doped Rare Earth Ions Toward The Modification Of Luminescent Properties Of Phosphors

With the enhanced efficiency of blue light-and near-ultraviolet light-emitting diodes(LEDs)chips,the "LED chips+rare earth doped phosphors" technology have promoted the research of rare earth luminescent materials,and also led to the rapid development in versatile applications.In the field of LED lighting,high-power lighting and full-spectrum lighting become the focus in white LED industry.Therefore,the research on phosphors with high thermal stability and red emission have become the main trend in the development of luminescent materials.In the field of special LED light sources,the lack of near-infrared(NIR)luminescent materials that can be excited by blue light and shows broad NIR emission have become technical bottleneck in the field of infrared detectors.Based on this,this paper focuses on the research of rare earth doped phosphors for LED applications.From the perspective of crystallographic lattice engineering of rare earth doped ions,we take ABPO4,β-Ca3(PO4)2 and A3BSi2O7 as structural prototypes and studies the relationship between sites occupation of the rare earth ions and luminescent properties in several typical Eu-doped phosphors.It can be categorized into the following aspects.(1)A new K2Ba(CaPO4)2:Eu2-phosphor with high thermal stability was designed and synthesized by using β-K2SO4 as the structural prototype and substituted it with cations and anions.By analysing of the sites occupation of Eu2+in K2Ba(CaPO4)2:Eu2+,it was found that the defect energy levels introduced by the heterovalent substitution take part in the photothermal ionization process of Eu2+at high temperature,which makes phosphors remain excellent thermal stability without emission quenching even at 200℃.This study provides a design principle for the development of rare earth doped phosphors used for high power lighting.(2)A new β-Ca3(PO4)2 type solid solution phosphors(Ca9-xSrx)MgK(PO4)7:Eu2+were synthesized by Ca→Sr substitution.It is found that the substitution of neighbor atoms can be acted as an effective strategy to control the sits occupation of rare earth ions.The substitution effectively controls the distribution of Eu2+ions in different cationic lattices,and result in the color-tunable white light emission.(3)We synthesized two types of Eu-doped A3LnSi2O7 phosphors,Rb3YSi2O7:Eu2+and K3YSi2O7:Eu2+.The selective occupation of Eu2+ions in polyhedrons with small coordination numbers(YO6 and Rb2O6)result in the red emission in Rb3YSi2O7 phosphor;and Eu2+selectively occupies K1O8 and Y2O6 polyhedrons in the K3YSi2O7 lattice,realizing orange-red light emission in K3YSi2O7:Eu2+phosphor.This study certified that the occupation of Eu2+in polyhedrons with low coordination numbers tend to achieve red light emission.This idea benefits to explore new oxide-based red phosphors.(4)By substituting Rb and Y ions with smaller K and Lu ions in the Rb3YSi2O7:Eu2+red phosphor,an Eu2+-doped near-infrared K3LuSi2O7:Eu2+phosphor was designed and synthesized.The selective sites occupation of Eu2+in LuO6 and K2O6 polyhedrons with small coordination numbers and small ionic radius leads to the unexpected near infrared emission,which shows broad NIR emission band peaking at 740 nm with a FWHM of 160 nm under 460 nm blue light excitation.This research opened a new chapter in the exploration of Eu2+ doped NIR phosphors.