Synthesis and luminescent properties of rare-earth-doped CeO ₂-CaF₂ solid solutions via chemical solution routes

CeO₂-CaF₂ solid solutions were synthesized by a chemical solution method starting from metal acetates, trifluoroacetic acid as a fluorine source, and anhydrous ethanol as a solvent. Precursor gels, which were obtained by drying the resultant ethanolic solution at 110 °C, were heat-treated at a temperature in the range 400-1000 °C in air to obtain powdery products. Elemental analysis by energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy revealed that heating products actually contained cerium, calcium, oxygen, and fluorine. According to X-ray diffraction analysis, possible reaction pathways under high-temperature treatments were considered as initial formation of fluorides (CeF₃ and CaF ₂), subsequent oxidation of Ce³⁺ to Ce⁴⁺ in air, and final conversion to fluorite-type Ce-Ca-O-F solid solutions. Doping of Eu³⁺ or Sm³⁺ ions in the solid solutions led to occurrence of their characteristic photoluminescence due to intra-configurational f-f electronic transitions. Photo-excitation was achieved by irradiation with near ultraviolet light mainly through charge transfer from O^2- to Ce ⁴⁺ in the solid solutions and subsequent energy transfer to the doped ions. Spectral structures of photoluminescence suggested the occupation of Eu³⁺ or Sm³⁺ in Ce⁴⁺ sites with inversion symmetry in the solid solutions.