Comprehensive Solid-State Characterization of Rare Earth Fluoride Nanoparticles

The combination of multinuclear solid-state NMR spectroscopy and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles revealed their general formula to be (H₃O)­Y₃F₁₀·<i>x</i>H₂O. ¹H magic-angle spinning (MAS) NMR experiments provided information on sample purity and served as an excellent probe of the zeolithic incorporation of atmospheric water. ¹⁹F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yielded spectra featuring three unique ¹⁹F resonances arising from three different fluorine sites within the (H₃O)­Y₃F₁₀·<i>x</i>H₂O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles led to changes in the integrated peak intensities in the ¹⁹F MAS NMR spectra; the origin of this behavior is discussed in terms of ¹⁹F longitudinal relaxation. ¹⁹F–⁸⁹Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under MAS conditions indicated that two distinct yttrium environments are present, and on the basis of the relative peak intensities, the population of one of the two sites is closely linked to the nanoparticle size. Both ¹⁹F MAS and ¹⁹F–⁸⁹Y VACP/MAS experiments indicated small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF₃ nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application