Probing the Defect Structure of Anion-Excess Ca1-xYxF2+x (x = 0.03−0.32) with High-Resolution 19F Magic-Angle Spinning Nuclear Magnetic Resonance Spectroscopy

Abstract

Fluorine-19 magic-angle spinning nuclear magnetic resonance spectroscopy (19F MAS NMR) was used to study the local structure and fluoride ion environments in the anion-excess cubic fluorite material Ca1-xYxF2+x (0 < x ≤ 0.32). Four distinct 19F resonances were observed at spinning speeds of >20 kHz; these have been assigned to three different types of sites: two interstitial sites, sites that have relaxed slightly from the normal sites of the fluorite structure, and the normal sites. Very similar spectra were observed for the mineral Tveitite (x ≈ 0.25−0.27), which contains an ordered calcium yttrium−fluoride phase with excess fluoride ions located in cuboctahedron clusters. Only three resonances were observed for this high-yttrium-content material, which were assigned to the normal, relaxed, and interstitial fluoride ions that make up the cuboctahedron. The similarity between the spectra of the synthesized materials and Tveitite indicate that similar clusters are present. The resonances due to the interstitial sites are assigned to fluoride ions in cuboctahedra or square antiprisms and the additional interstitial ions associated with these clusters. The changes in intensities of these resonances are consistent with the increase in the cuboctahedron:square antiprism ratio at higher doping levels. Variable-temperature NMR shows that the anions within the clusters are mobile and undergo exchange with each other. At higher temperatures and doping levels, exchange involving the relaxed and normal fluoride ions also occurs.