Synthesis And Luminescence Properties Of Rare Earths Doped Phosphate Phosphors

White light-emititng diodes?w-LEDs?have been widely used in lighting and display fields,and considered to be a fourth-generation lighting source owing to their environmentally friendly character,high efficiency,long lifetime,and small volume.However,the YAG:Ce method suffers from the problems of low color rendering index and high correlated color temperature because of the insufficient red light contribution.Thus,developments of new phosphors with high chemical stability and strong absorption in ultraviolet?UV?or near ultraviolet?NUV?region with high conversion efficiency are highly crucial.At present,phosphate luminescence materials have the advantages of high luminous efficiency,low synthesis conditions,low cost and high chemical stability.So,the phosphate luminescence materials have attracted much attention.In this work,a series of rare earth activated phosphate phosphors,such as A₃NaLn?PO₄?₃F?A=Sr,Ba;Ln=Gd,Y?and A₃Ln?PO₄?₃?A=Sr,Ba;Ln=Y,Lu?,were prepared by a conventional solid-state reaction technique.The photoluminescence excitation and emission spectra,luminescent decay behavior,thermal stability,concentration quenching of the obtained samples were discussed.?1?A series of single-phase and color-tunable Sr₃NaY?PO₄?₃F:Eu²⁺,Mn²⁺phosphors have been successfully prepared via the solid-state reaction.The crystal structure and phase of the prepared phosphors were investigated using XRD,SEM and TEM.The excitation and emission spectra,and fluorescence decays were measured and discussed systematically.The results revealed that Eu²⁺can efficiently transfer excitation energy to Mn²⁺via its 4f states and therefore sensitizes Mn²⁺emission under NUV excitation.In addition,the energy transfer mechanism was demonstrated to be a resonant type via a dipole–dipole process.The corresponding Commission Internationale de L'Eclairage?CIE?chromaticity coordinates showed that the emission colors of the phosphors can be tuned from blue?0.133,0.148?to white?0.327,0.351?by adjusting the ratio of Eu²⁺and Mn²⁺.Meanwhile,the good thermal stability of Sr₃NaY?PO₄?₃F:0.03Eu²⁺,0.12Mn²⁺sample showed that the emission intensity at 150°C was reduced form 100%to 72.3%measured at 25°C.?2?A series of Ce³⁺,Tb³⁺singly doped and Ce³⁺/Tb³⁺co-doped Ba₃GdNa?PO₄?₃F phosphors have been synthesized via the conventional high temperature solid-state reaction.The crystal structures,photoluminescence properties,fluorescence lifetimes,thermal properties and energy transfer of Ba₃GdNa?PO₄?₃F:Ce³⁺,Tb³⁺were systematically investigated in detail.Rietveld structure refinement indicates that Ba₃GdNa?PO₄?₃F crystallizes in a hexagonal crystal system with the space group P-6 and lattice constants of a=b=9.80587?and c=7.33931?.For the co-doped Ba₃GdNa?PO₄?₃F:Ce³⁺,Tb³⁺samples,tunable colors from blue to green can be realized by varying the doping concentration of the Tb³⁺ions.The intense green emission was realized in the Ba₃GdNa?PO₄?₃F:Ce³⁺,Tb³⁺phosphors on the basis of the highly efficient energy transfer from Ce³⁺to Tb³⁺with an efficiency of over 80%.The energy transfer mechanism has been confirmed to be quadrupole-quadrupole interaction,which can be validated via the agreement of critical distances obtained from the concentration quenching?13.84??.?3?Reddish-orange emitting phosphors,Sr₃Gd?PO₄?₃:Sm³⁺,were successfully synthesized by a conventional solid-state reaction.The crystal structure of the phosphors was characterized by X-ray diffraction?XRD?.The excitation spectra and emission spectra were utilized to characterize the luminescence properties of the as-prepared phosphors.The results show that the phosphor consisted of some sharp emission peaks of Sm³⁺ions centered at 564 nm,600 nm,647 nm and 707 nm,respectively.The critical distance of Sr₃Gd_0.93?PO₄?₃:0.07Sm³⁺was calculated to be 19.18?and the lifetime values of the sample was 1.63 ms.The band gap of Sr₃Gd?PO₄?₃ was estimated to be about 2.74 eV from the diffuse reflection spectrum.The optimum doping concentration is 7 mol%and the quenching occurs via dipole–dipole interaction according to Dexter's theory.The CIE value of Sr₃Gd?PO₄?₃:Sm³⁺phosphors presented it has high color purity.These results indicated that the Sr₃Gd?PO₄?₃:Sm³⁺may be a promising reddish-orange emitting phosphor for cost-effective NUV w-LEDs.?4?Reddish-orange emitting Eu³⁺-activated Sr₃Lu?PO₄?₃ phosphors have been successfully synthesizedby a conventional solid-state reaction.X-ray powder diffraction confirmed the phase formation.The photoluminescence excitation and emission spectra were investigated.The band gap of Sr₃Lu?PO₄?₃ was estimated to be about 3.68 eV from the diffuse reflection spectrum.The temperature-dependent luminescence of Sr₃Lu?PO₄?₃:Eu³⁺was discussed,and the activation energy for thermal quenching was calculated as 1.74 eV.The fluorescence decay curve and the CIE value of Sr₃Lu?PO₄?₃:Eu³⁺phosphors were investigated in detail.The phosphor presented high color purity.