Luminescence of NaGdFPO4:Ln3+ after VUV excitation: A comparison with GdPO4:Ln3+ (Ln=Ce, Tb)

The phosphors NaGdFPO4:Ln3+ and GdPO4:Ln3+ (for Ln3+=Ce3+ and Tb3+) were prepared by solid-state reaction technique, the VUV–vis spectroscopic properties of the phosphors were investigated, and we vividly compare the luminescence of Ce3+ and Tb3+ in the hosts. For phosphors GdPO4:Ln3+, the band near 155 nm in VUV excitation spectrum is assumed to be the host-related absorption, and for NaGdFPO4:Ln3+ the absorption is moved to longer wavelength, near 170 nm, showing the P–O bond covalency increased after fluoridation. The f–d transitions of Ce3+ and Tb3+ in the host lattices are assigned and corroborated, and it was found that the 5d states are with lower energy in NaGdFPO4:Ln3+ than those in GdPO4:Ln3+. For fluoridation of GdPO4:Ln3+ to NaGdFPO4:Ln3+, the energy change of Ln3+ (Ln=Ce, Tb) 5d states is consistent with that of host-related absorption.