Luminescence and Energy Transfer Properties of Ca₂Ba₃(PO₄)₃Cl and Ca₂Ba₃(PO₄)₃Cl:A (A = Eu²⁺/Ce³⁺/Dy³⁺/Tb³⁺) under UV and Low-Voltage Electron Beam Excitation

Pure Ca₂Ba₃(PO₄)₃Cl and rare earth ion (Eu²⁺/Ce³⁺/Dy³⁺/Tb³⁺) doped Ca₂Ba₃(PO₄)₃Cl phosphors with the apatite structure have been prepared via a Pechini-type sol–gel process. X-ray diffraction (XRD) and structure refinement, photoluminescence (PL) spectra, cathodoluminescence (CL) spectra, absolute quantum yield, as well as lifetimes were utilized to characterize samples. Under UV light excitation, the undoped Ca₂Ba₃(PO₄)₃Cl sample shows broad band photoluminescence centered near 480 nm after being reduced due to the defect structure. Eu²⁺ and Ce³⁺ ion doped Ca₂Ba₃(PO₄)₃Cl samples also show broad 5d → 4f transitions with cyan and blue colors and higher quantum yields (72% for Ca₂Ba₃(PO₄)₃Cl:0.04Eu²⁺; 67% for Ca₂Ba₃(PO₄)₃Cl:0.016Ce³⁺). For Dy³⁺ and Tb³⁺ doped Ca₂Ba₃(PO₄)₃Cl samples, they give strong line emissions coming from 4f → 4f transitions. Moreover, the Ce³⁺ ion can transfer its energy to the Tb³⁺ ion in the Ca₂Ba₃(PO₄)₃Cl host, and the energy transfer mechanism has been demonstrated to be a resonant type, via a dipole–quadrupole interaction. However, under the low voltage electron beam excitation, Tb³⁺ ion doped Ca₂Ba₃(PO₄)₃Cl samples present different luminescence properties compared with their PL spectra, which is ascribed to the different excitation mechanism. On the basis of the good PL and CL properties of the Ca₂Ba₃(PO₄)₃Cl:A (A = Ce³⁺/Eu²⁺/Tb³⁺/Dy³⁺), Ca₂Ba₃(PO₄)₃Cl might be promising for application in solid state lighting and field-emission displays