Local Structure of Rare-Earth Fluorides in Bulk and Core/Shell Nanocrystalline Materials

NaREF₄ (rare-earth, RE = Y, Lu) nanocrystals (NCs) containing Ln³⁺-dopants are of great interest due to their unique ability to downshift, downconvert, and upconvert light. While the luminescent properties and microscale structures of such NCs are well understood, relatively little is known about their molecular-level structures, the nature of the NC cores and shells, and the interactions of the stabilizing ligands at the NC surface. To address these issues, we present a comprehensive characterization of bulk β-NaYF₄, one of the most commonly used hosts for Ln³⁺-dopants in luminescent NCs. Using a combination of powder X-ray diffraction (pXRD) and multinuclear (⁸⁹Y, ²³Na, ¹⁹F) solid-state nuclear magnetic resonance (SSNMR) spectroscopy, we provide conclusive evidence that this compound crystallizes in a hexagonal <i>P</i>6₃/<i>m</i> structure, resolving a longstanding debate. The structure of this bulk form is related to the structure of NaYF₄/NaLuF₄ core/shell NCs. From the similarities between the ¹⁹F and ²³Na SSNMR spectra of the bulk and NC materials, it is concluded that the NCs have the same β-NaYF₄ and β-NaLuF₄ phases as the bulk compounds. A series of cross-polarization NMR experiments confirm the presence of oleates on the surface of the particle via their proximity to surface Na⁺ sites