New Highly Divided Ce−Ca-Based Oxyfluorides with UV-Shielding Properties:  Study of the Ce_1-<i>_x</i>Ca<i>_x</i>O_2-<i>_x</i> and Ce_1-<i>_x</i>Ca<i>_x</i>O_2-<i>_x</i>_-<i>_y</i>_/2F<i>_y</i> Series

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 °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: Fm3̄m) 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 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/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 19F 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-x 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.