Probing the local environments of fluorine in Ce-based fluorite-type oxyfluorides with 19F MAS NMR spectroscopy
Résumé
Ce-based oxyfluorides Ce1-xCaxO2-x-y/2Fy, 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 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. 19F magic angle spinning (MAS) NMR spectroscopy was used to study the local structure and fluoride ion environments. Four distinct 19F 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.