Preparation And Luminescent Properties Of White-emitting Ba₂TiP₂O₉ Luminescent Materials

In recent years,single-matrix white phosphors for LED have attracted much attention of researchers,and become a research hotspot of new high-efficiency phosphors.This kind of phosphors can effectively solve the problems of low color index and high color temperature because of the lack of red light in commercial Y₃Al₅O₁₂:Ce³⁺yellow phosphors.At the same time,it can effectively avoid the problems of unreasonable ratio,color reabsorption,difficult regulation of luminous peak intensity and thermal degradation difference of red,green and blue?RGB?trichrome phosphors when they are combined to produce white light.Single-matrix white phosphor can greatly reduce the cost of preparing white LED,and improve its luminous efficiency and color stability.Therefore,it is increasingly important to develop new and efficient single-matrix white phosphors.In this paper,Ba₂TiP₂O₉ single-matrix white phosphor was prepared by high temperature solid-state method.The effects of different preparation temperature and holding time on its phase structure and luminescent properties were studied.The phase transformation mechanism of its phase structure and micro-morphology was analyzed,and the appropriate firing system was determined.The different concentrations of Eu³⁺,Dy³⁺and Tb³⁺were doped in Ba₂TiP₂O₉ single-substrate white phosphors.The effect of doping ion concentration on matrix luminescence was studied,and the energy transfer process between matrix and activator ion was discussed.In this paper,Ba₂TiP₂O₉ phosphors were prepared at different temperatures and holding time in the temperature range of 900-1150?.X-ray diffraction?XRD?,scanning electron microscopy?SEM?and fluorescence spectroscopy?PL?were used to characterize the phase structure,morphology and luminescent properties of the phosphors.According to the test results,the phase transition temperature and process of Ba₂TiP₂O₉ were analyzed.The low-temperature phase beta-Ba₂TiP₂O₉transformed into high-temperature phase alpha-Ba₂TiP₂O₉ when the preparation temperature reaches 1100?and the holding time was more than 3h.With the increase of sintering temperature or the extension of holding time,the granular low-temperature phase,beta-Ba₂TiP₂O₉,grew in the direction parallel to?100?crystal plane,and transits through the massive mesophase Ba₂Ti P₂O₈.The high-temperature phase,alpha-Ba₂TiP₂O₉ with lamellar fault structure,was obtained by stacking layer by layer.Under the excitation of 254 nm ultraviolet light,a Ti⁴⁺-O^2-charge transfer transition occurred in the conical polyhedron of titanium dioxide in Ba₂TiP₂O₉matrix,resulting in white light emission.The spectral characteristics of beta-Ba₂TiP₂O₉and alpha-Ba₂TiP₂O₉ were basically the same,and the luminescence intensity of the former was slightly lower than that of the latter.Considering comprehensively,the optimum preparation condition of Ba₂TiP₂O₉ was to produce low-temperature phase beta-Ba₂TiP₂O₉at 1100?for 2 hours.In this paper,Eu³⁺,Dy³⁺,Tb³⁺doped with 1-7%,9%doping concentration of beta-Ba₂TiP₂O₉ were prepared under the above conditions.XRD and PL were used to study the effect of doping on the phase structure and luminescent properties of the matrix.The energy transfer mechanism between the matrix and rare earth ions was discussed and analyzed.After doping,the crystal phase of the phosphor was still mainly Ba₂TiP₂O₉.At the same time,there was a diffraction peak which belongs to?Ti O?₂P₂O₇ phase.The peak intensity increases with the increase of doping concentration,which could be attributed to the lattice distortion caused by unequal substitution.Under 480nm monitoring,the excitation spectra of the phosphors before and after doping show a wide excitation band in the range of 200-300nm.When 613,575 and 545nm were used as monitoring wavelength,M³⁺(Eu³⁺,Dy³⁺,Tb³⁺)-O^2-was absorbed in the excitation spectra of doped phosphors,and the characteristic absorption peaks of rare earth ions were observed at 393,475 and 380nm(Tb³⁺),respectively.Under the excitation of 254nm ultraviolet light,the emission spectrum of phosphor shows a white broad-band emission in the range of 350-700nm,which belongs to the charge transfer transition of Ti⁴⁺-O^2-.The Eu³⁺,Dy³⁺and Tb³⁺separately doped phosphors exhibited strong red characteristic emission at 613 nm,weak yellow characteristic emission at 575nm and weak green characteristic emission at 545nm,respectively.With the increase of doping concentration,the luminescence intensity of phosphors showed a downward trend as a whole,and then increased first and then decreased.The optimum doping concentration was 2%(Eu³⁺),4%(Dy³⁺)and 4%(Tb³⁺),respectively.The emission spectra under 254nm excitation overlap with those under613nm(Eu³⁺),575nm(Dy³⁺)and 545nm(Tb³⁺),respectively,indicating that there was an energy transfer process between matrix luminescence and rare earth ion luminescence.The transfer energy of matrix luminescence to rare earth ions decreased the white light intensity of matrix luminescence.With the increase of doping concentration,the luminescence intensity of rare earth ions increased,and the overall luminescence intensity of phosphors increased slightly due to the superposition of rare earth ions with matrix luminescence.Continuous increase of doped ion concentration would result in concentration quenching due to energy transfer between rare earth ions,non-radiative transition and lattice distortion.The Eu³⁺doping could effectively change the chroma coordinates of phosphors,while Dy³⁺and Tb³⁺have relatively weak characteristic emission,and their doping had little effect on the chroma coordinates.