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, H 2 O 2, and HF as fluorinating agent. Powders have been annealed under air at 600 °C during 12 h. Oxides and oxyfluorides with Ce 1- x Ca x O 2- x and Ce 1- x Ca x O 2- x - y /2 F y 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 3̄ 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 α 1 radiation, which allows one to detect the occurrence of CaF 2 traces. Taking into account the F−Ce bond distance in tetrahedral coordination (bond valence model Ce−F = 2.50 Å; average bond distances in Ce 1- x Ca x O 2- x - y /2 F y: 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. Finally, an accurate EELS analysis on isolated particles (10−20 nm) with Ce 0.75 Ca 0.25 O 1.67 F 0.17 composition confirms the value of the Ca/Ce average atomic ratio, and preliminary 19 F MAS NMR investigations show the occurrence of various FCa 4- n Ce n 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 Ce 1- x Ca x O 2- x and Ce 1- x Ca x O 2- x - y /2 F y 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 Ce 4+ electropositive character in these two systems.