| Abstract
| - 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 °Cduring 12 h. Oxides and oxyfluorides with Ce1-xCaxO2-x and Ce1-xCaxO2-x-y/2Fy formulas have beenobtained. The Ce/Ca and F/Ca atomic ratios have been determined by elemental analysis (microprobeanalysis 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 increaseswith the Ca rate, demonstrating the affinity of fluorine for Ca atoms. XRD data analysis confirmed thatCe−Ca-based oxides and oxyfluorides crystallize with the fluorite-type structure (SG: Fm3̄m) and thea-cell parameter follows Vegard's law both for oxide and for oxyfluoride series. The solubility limit hasbeen estimated to correspond to a Ca/Ce atomic ratio between 0.35 and 0.40 thanks to an accurate XRDanalysis using monochromatic Cu Kα1 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/Caatomic ratio is governed by steric effects and constrains present in the network. Finally, an accurateEELS analysis on isolated particles (10−20 nm) with Ce0.75Ca0.25O1.67F0.17 composition confirms the valueof the Ca/Ce average atomic ratio, and preliminary 19F MAS NMR investigations show the occurrenceof various FCa4-nCen environments (0 ≤ n ≤ 0.4). TEM analysis reveals that particle size is around 10nm, which corresponds to the coherent domain/crystallite size determined by XRD analysis taking intoaccount a Thompson−Cox−Hasting function for the X-ray diffraction lines shape. Finally, theCe1-xCaxO2-x and Ce1-xCaxO2-x-y/2Fy series exhibit interesting UV-shielding properties with a decreaseof the optical band gap with Ca content in the oxide series, whereas the band gap varies inversely withCa and F content in the oxyfluoride series. This tendency has been interpreted considering the differencein the Ce4+ electropositive character in these two systems.
- Nanoparticles of Ce−Ca-based oxides and oxyfluorides have been prepared and characterized by XRD, TEM/EELS, and 19F MAS-NMR, showing the homogeneity of crystallites and the occurrence of FCa4-nCen environments (0 ≤ n ≤ 4). These materials exhibit UV-shielding properties with a reduction of the refractive index due to the presence of Ca2+ and F- ions.
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