New highly divided Ce-Ca-based oxyfluorides with UV-Shielding properties: study of the Ce1-xCaxO2-x and Ce1-xCaxO2-x-y/2Fy series
Résumé
Coprecipitations of Ce(III) nitrates and Ca chlorides have been performed in basic medium at pH > 12 using NaOH, H2O2, and HF as fluorinating agent. Powders have been annealed under air at 600 C during 12 h. Oxides and oxyfluorides with Ce1-xCaxO2-x and Ce1-xCaxO2-x-y/2Fy formulas have been obtained. The Ce/Ca and F/Ca atomic ratios have been determined by elemental analysis (microprobe analysis and F titration with specific electrode), leading one to conclude that the higher is the Ca content, the higher is the F rate. The F/Ca atomic ratio is always lower than 1, but the F content strongly increases with the Ca rate, demonstrating the affinity of fluorine for Ca atoms. XRD data analysis confirmed that Ce-Ca-based oxides and oxyfluorides crystallize with the fluorite-type structure (SG: Fmm) and the a-cell parameter follows Vegard's law both for oxide and for oxyfluoride series. The solubility limit has been estimated to correspond to a Ca/Ce atomic ratio between 0.35 and 0.40 thanks to an accurate XRD analysis using monochromatic Cu K1 radiation, which allows one to detect the occurrence of CaF2 traces. Taking into account the F-Ce bond distance in tetrahedral coordination (bond valence model Ce-F = 2.50 Å; average bond distances in Ce1-xCaxO2-x-y/2Fy: Ce/Ca-O/F = 2.35 Å), the low value of the F/Ca atomic ratio is governed by steric effects and constrains present in the network...