Tailoring the Synthesis of LnF₃ (Ln = La–Lu and Y) Nanocrystals via Mechanistic Study of the Coprecipitation Method

Here, 15 LnF₃ nanocrystals are synthesized using coprecipitation method with citrate stabilization to allow the fast, easy, and reproducible synthesis of several nanoscaled structures in water. General trends related to the behavior of LnF₃ nanocrystals are highlighted due to their broad range of application in several fields (e.g., medical applications). The same nature for all Ln³⁺ cations is expected due to the internal role of f orbitals. However, we found that the use of different lanthanide elements is crucial in the final size, shape, assembly, and crystalline structure. In addition, the decrease of the cation size of the lanthanide series changes the behavior of these compounds, resulting in hexagonal, orthorhombic, and cubic crystalline structures. In addition, we are able to tune the cubic crystalline phase to pure orthorhombic by modifying the pH of the system using HBF₄ instead of tetramethylammonium citrate. Via ¹¹B NMR, we demonstrated the mechanism of HBF₄ as fluorinating agent if an additional source of F^– is not added during the synthesis. ¹H NMR and IR techniques were performed to unravel the picture of the surface chemistry of the two representative metal cations (Y and La). Finally, HRTEM and SAED were performed to uncover the shape of the obtained nanocrystals and the preferential orientation of the assembled particles, giving crucial information on the involved mechanisms. This study reveals not only the dependence of the crystalline structure on the used metal and pH but also ability to achieve LnF₃ assembled particles depending on the final shape and temperature