New highly divided Ce-Ca-based oxyfluorides with UV-shielding properties:: Study of the Ce1-xCaxO2-x and Ce1-xCaxO2-x-y/2Fy series

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 degrees 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: Fm (3) over bar m) 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 K-alpha 1 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 angstrom; average bond distances in Ce1-xCaxO2-x-y/2Fy: Ce/Ca-O/F = 2.35 angstrom), the low value of the F/Ca atomic ratio is governed by steric effects and constrains present in the network. Finally, an accurate EELS analysis on isolated particles (10-20 nm) with Ce0.75Ca0.25O1.67F0.17 composition confirms the value of the Ca/Ce average atomic ratio, and preliminary F-19 MAS NMR investigations show the occurrence of various FCa4-nCen environments (0 <= n <= 0.4). TEM analysis reveals that particle size is around 10 nm, which corresponds to the coherent domain/crystallite size determined by XRD analysis taking into account a Thompson -Cox -Hasting function for the X-ray diffraction lines shape. Finally, the Ce1-xCaxO2 and Ce1-xCaxO2-x-y/2Fy series exhibit interesting UV-shielding properties with a decrease of the optical band gap with Ca content in the oxide series, whereas the band gap varies inversely with Ca and F content in the oxyfluoride series. This tendency has been interpreted considering the difference in the Ce4+ electropositive character in these two systems.