Luminescence properties of Bi doped LaOF phosphor powder

Lanthanum oxyfluoride (LaOF) doped with bismuth (Bi) was prepared using the solid-state reaction method using ammonium fluoride, lanthanum oxide and bismuth oxide as precursors. Excess ammonium fluoride was used during the synthesis, resulting in non-stoichiometric samples for which the F:O ratio decreased as higher synthesis temperatures were used. This allowed assessment of the luminescence of LaOF:Bi phosphors as a function of the stoichiometry for the first time. The phosphor powders prepared at temperatures from 700 °C to 1000 °C produced a single broad white luminescence emission band centred at 518 nm when excited at 263 nm, for which the intensity increased with the synthesis temperature and the optimum Bi doping concentration was found to correspond to La1-xOF:Bix = 0.005. Increasing the synthesis temperature to 1100 °C caused the LaOF:Bi to convert from a tetragonal to a rhombohedral phase which exhibited negligible luminescence. X-ray photoelectron spectroscopy showed that strong segregation of Bi3+ ions from the bulk of the host to its surface occurred for this higher annealing temperature, to which we attribute quenching of the luminescence. The stability of the optimally luminescent tetragonal sample, prepared at 1000 °C, was investigated. It was found to be stable when exposed to the atmosphere for up to six months, although its luminescence degraded slowly under radiation from a 254 nm ultraviolet lamp which was attributed to segregation of Bi3+ ions to the surface and formation of Bi2O3.