Energy Transfer Processes in NASICON-Type Phosphates under Synchrotron Radiation Excitation

The luminescence properties of NASICON-type Na3.6M1.8(PO4)(3) (M = Y, Lu) and Na3Sc2(PO4)(3) phosphates, undoped and rare earth-doped (RE = Tb3+, Dy3+, Eu3+, Ce3+), were studied using synchrotron radiation in a wide energy region of 4.5-45 eV. Intrinsic emission originating from self-trapped excitons with electron component localized at the 3d Sc states was detected in both doped and undoped Na3Sc2(PO4)(3) while only defect-related emission was registered in Na3.6M1.8(PO4)(3). Emission of RE ions substituting three-valent cations in low-symmetry sites was detected in all doped phosphates. The efficiency and pass ways of energy transfer from the host to emission centres were analysed based on luminescence excitation spectra. It is shown that the most efficient energy transfer is realized in Tb3+-doped phosphors, while it was poor for other RE ions. The differences in energy transfer efficiencies are explained by different position of RE f states in the crystal electronic band structure influencing the efficiency of charge carrier trapping in the substance. Based on excitation spectra analysis, the bandgap values were estimated to similar to 8 eV for all studied phosphates.