Photoluminescence tuning of Ca-5(PO4)(3)Cl:Ce3+/Eu2+,Tb3+/Mn2+ phosphors: structure refinement, site occupancy, energy transfer and thermal stability

A series of Ca-5(PO4)(3)Cl (CPOCl):Ce3+/Eu2+, Tb3+/Mn2+ phosphors with apatite structures have been prepared via the Pechini sol-gel process. The structure refinement indicates that the as-prepared phosphors crystallized in a hexagonal phase with the space group of P63/m (176), and there are two kinds of cation sites (4f and 6h) in the host lattice to accommodate the doping ions. The emissions of Eu2+ and Ce3+ at different lattice sites in the CPOCl host have been identified and discussed. The red shifted emission of CPOCl:Ce3+ with increasing Ce3+ doping concentrations has been explained, which is mainly attributed to the occupation of 6h sites of Ce3+ ions at a high doping level. In addition, the transformation from chlorapatite structures to oxyapatite structures driven by charge balance with Ce3+ concentrations also contributes to this red shift. When codoping Tb3+/Mn2+ ions into these cation sites, efficient energy transfers from Ce3+/Eu2+ ions to Tb3+/Mn2+ ions were observed, and the corresponding energy transfer mechanisms have been revealed. Under 340-420 nm near-ultraviolet light (n-UV) excitation, highly efficient blue-green tunable emission from Ce3+/Eu2+ ions to Tb3+ ions and singlephase white emission from Ce3+, Mn2+-codoped CPOCl can be obtained. In addition, the thermal stability of CPOCl:Ce3+/Eu2+, Tb3+/Mn2+ phosphors has been investigated systematically. Based on these experimental results, the as-prepared CPOCl:Ce3+/Eu2+, Tb3+/Mn2+ phosphors can act as potential color-tunable and single-phase white emission phosphors for possible applications in n-UV based white LEDs.