Photoluminescence tuning of Ca5(PO4)3Cl:Ce3+/Eu2+,Tb3+/Mn2+ phosphors: structure refinement, site occupancy, energy transfer and thermal stability

Abstract

A series of Ca₅(PO₄)₃Cl (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 single-phase 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.