Nonaqueous Synthesis of Colloidal Cs ₂ ZrF ₆ , K ₂ SiF ₆ , Na ₂ SiF ₆ , and Related A ₂ BF ₆ Nanocrystals via Fluoride Salt Precursors

We report the synthesis of a library of colloidal Zr⁴⁺-, Ti⁴⁺-, and Si⁴⁺-based A₂BF₆ (A = Na⁺, Cs⁺, NH₄⁺, and N(CH₃)₄⁺) nanocrystals. These lattices are synthesized without the use of aqueous HF but instead are prepared using safer fluoride salt precursors dissolved in nonpolar solutions. We show that nanocrystal morphologies, including shape anisotropy, are altered when various synthetic parameters are adjusted, including reaction temperature, time, and solvent polarity. Nucleation and growth pathways of A₂ZrF₆ compositions are compared. An in situ A-site cation exchange requiring internal anion sublattice reorganization is described that allows for conversion of anisotropic (NH₄)₂ZrF₆ nanorods into Cs₂ZrF₆ nanorods that could not be prepared directly. The development of colloidal nanocrystals as a new form factor for A₂BF₆ lattices addresses fundamental synthesis challenges in inorganic nanoscience and lays the groundwork for future work into doping such nanocrystals for solution-phase photonic applications, processing, or printing.