Luminescent Properties of Rare Earth Fully Activated Apatites, LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb): Site Selective Crystal Field Effect

Novel LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 compounds have been successfully synthesized, and the site selectivity and occupancy of activator ions have been estimated including LiEu9(SiO4)6O2 compound. The rare earth (RE) fully occupied compounds, as well as the RE partially occupied congeners are required for the assessment of site selectivity of RE (activator) ions in apatite-type compounds. The splitting energies of the 6H and 4F Wycoff positions of LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds are calculated based on crystal field theory: ΔE Ce(6H) = 3849.3 cm–1, ΔE Ce(4F) = 4228.1 cm–1, ΔE Eu(6H) = 3870.0 cm–1, ΔE Eu(4F) = 4092.8 cm–1, ΔE Tb(6H) = 3637.6 cm–1, ΔE Tb(4F) = 4396.1 cm–1, indicating that the splitting energy for the 4F site is larger than that for the 6H site in all compounds; thus the absorption energy is higher for the 6H site. In apatite-type LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds, the Ce3+ ions predominantly occupy the 4F site associated with the absorption band around 300 nm at lower Ce3+ concentration, and then enter the 6H site associated the absorption band around 245 nm. For the Eu3+-doped compounds, the 4F site and 6H site are mixed within the charge transfer band (CTB) between 220 and 350 nm. Eu3+ ions initially preferentially occupy the 6H site (around 290 nm) at lower Eu3+ concentration and subsequently enter the 4F site (around 320 nm) with increasing Eu3+ concentration. For the Tb3+-doped compounds, the absorption due to the two different sites is mixed within f–d absorption band between 200 and 300 nm. At lower Tb3+ concentration, the Tb3+ ions enter favorably 6H site around 240 nm and then enter 4F site around 270 nm. These compounds may provide a platform for modeling a new phosphor and application in the solid-state lighting field.