Ce-based oxyfluorides Ce1-xCaxO2-x-y/2F(y), with 0.13 <= x <= 0.29 and 0.03 <= y <= 0.24, adopting the fluorite structure, were prepared by coprecipitation in basic fluorinated medium followed by an annealing under air at T = 600 degrees C. XRD profile and Rietveld analyses allowed the determination of the crystallite sizes as well as the unit cell parameters. In this series, fluorine atoms are in tetrahedral environments of cations. F-19 magic angle spinning (MAS) NMR spectroscopy was used to study the local structure and fluoride ion environments. Four distinct F-19 resonances were observed and Assigned to four different types of environments, FCa4, FCa3Ce, FCa2Ce2, and FCaCe3, whose proportions vary with the calcium content. In these Ce-Ca oxyfluorides, F- anions have a great affinity for Ca2+ cations leading to an increase of the F amount with the Ca content. The absence of FCe4 environment is explained from a steric standpoint: the F-Ce bond lengths in the network are too short to accept fluorine ions in the vicinity of four Ce4+ cations. Finally, the increase of the average Ce-X (X = O, F) bond ionicity with the Ca and F contents was correlated to the evolution of the UV shielding properties of these new compounds, which exhibit outstanding UV absorption and scattering (in the visible range) properties.

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.