Luminescent Properties Of Single-phase β-Ca₃（PO₄）₂ Based White Light-emitting Phosphor

White light-emitting diodes（WLEDs） as new solid-state light sources have a greatly promising application in the field of lighting and display. So far, much effort has been devoted to exploring novel luminescent materials for WLEDs. Currently, the major challenges in WLEDs are to achieve high luminous efficacy, high chromatic stability, brilliant color-rending properties, and price competitiveness against fluorescent lamps, which rely critically on the phosphor properties. In recent years, numerous efforts have been made to develop single-phase white-light-emitting phosphors for near-ultraviolet or ultraviolet excitation to solve the above challenges with certain achievements. Owing to their special crystal structure, compounds with the β-Ca2（PO₄）3 structure show excellent thermal stability and high tolerance to rare earth and transition-metal ions. Herein, much attention has been payed to them. In this doctoral dissertation, several series of Ce³⁺/Tb³⁺/Mn²⁺ and Dy³⁺-activated single-phase full-color phosphors whose hosts exhibit β-Ca2（PO₄）3 structure have been prepared by a solid-state reaction. X-ray diffraction（XRD）, field-emission scanning electron microscopy（FE-SEM）, diffuse reflectance spectra, and photoluminescence（PL） spectroscopy were used to characterize the samples. The crystal structures and luminescence properties, especially the energy transfer behavior, are discussed in detail.Ca₁₉Mg₂（PO₄）₁₄, Ca₁₉Zn2（PO₄）₁₄, Ca₉ Mg Li（PO₄）7, Ca₉ Zn Li（PO₄）7, Ca₉Bi（PO₄）7, Ca₈ Mg Bi（PO₄）7, Ca₉Gd（PO₄）7, Ca₉ Li Gd0.667（PO₄）7and Ca₉ Na Gd0.667（PO₄）7 have been synthesized by a solid-state reaction.It is indicated that Ca₈ Mg Bi（PO₄）7 is the optimal host among the nine kinds of host, and the Ca₁₉Zn2（PO₄）₁₄ takes second place.Ce³⁺ ions have been doped into Ca₁₉Mg₂（PO₄）₁₄, Ca₁₉Zn2（PO₄）₁₄, Ca₉ Mg Li（PO₄）7, Ca₉ Zn Li（PO₄）7, Ca₈ Mg Bi（PO₄）7, Ca₉Gd（PO₄）7, Ca₉ Li Gd0.667（PO₄）7and Ca₉ Na Gd0.667（PO₄）7. It is observed thatthe emissionspectra of these phosphors exhibit broad bands and the emissionspectra appear as an intense purplish-blue light with a peak at 360-370 nm, which originated from the 5d→4f transition of Ce³⁺.In addition, it is noticed that the emission peaks have a slightred shift with increasing Ce³⁺ concentration which could be attributed to the enhancement of the crystalfield magnitude surrounding the Ce³⁺ ions.Ca₈Mg Bi（PO₄）7:Dy³⁺ and Ca₉Bi（PO₄）7:Dy³⁺ phosphor have been prepared and studied. The results show that for both Ca₈ Mg Bi（PO₄）7:Dy³⁺ and Ca₉Bi（PO₄）7:Dy³⁺ phosphors, on excitation at 350 nm, the emission transitions of Dy³⁺ ions consisted of double emission bands, which can be attributed to the 4F9/2 ?6H15/2（484 nm） and 4F9/2 ?6H13/2（572 nm） transition emissions.The dual energy transfer of Ce³⁺→Tb³⁺ and Ce³⁺→Mn²⁺ has been investigated.The energy transfer from Ce³⁺to Tb³⁺ in Ca₁₉Mg₂（PO₄）₁₄ and Ca₉ Li Gd_2/3（PO₄）7 phosphors has been demonstratedto be a resonant type via a dipole-quadrupole mechanism, meanwhile, the energy transfer from Ce³⁺to Tb³⁺ in other hosts by via a dipole-dipole mechanism. The spectral overlapbetween the emission band of Ce³⁺and the excitation band of Mn²⁺ in all the six hosts, which supports theoccurrence of the energy transfer from Ce³⁺to Mn²⁺, has been studied and demonstrated to be aresonant type via a dipole-quadrupole mechanism.For the above Ce³⁺, Tb³⁺, and Mn²⁺ co-doped phosphors, the energy transfer efficiency and the critical distance have also been estimated. It was found that all these series of phosphors can yield three major RGB emission bands in the visible spectral region: purplish blue emission from the 5d–4f transitions of Ce³⁺, green emission from the 5D4–7FJ（J = 3, 4, 5, 6） transitions of Tb³⁺ and red emission from the 4T1（4G）-6A1（6S） transition of Mn²⁺ ions. Based on the energy transfer, the emission colors of the obtained phosphors can be tuned from blue to green/red, and eventually to white emission by controlling the doping content of the Tb³⁺/Mn²⁺ ions. Based on the good PL properties and varied hues of the above hosts by adjusting the doping concentration of the activators(Ce³⁺, Tb³⁺, Mn²⁺), all the hosts might be promising as a host material for using in solid-state lighting.