Liquid-phase Synthesis of CaF<sub>2</sub> Particles and Their Low Refractive Index Characterization

Abstract

The purpose of the present study was to reveal the details of the preparation of CaF2 particles with controllable size (30–900 nm) and shape (spherical, hexagonal, and cubical forms) using a liquid-phase synthesis method, and to demonstrate that a change in the composition of the reactants and crystalline structure of the CaF2 product could improve material performance. The particles were synthesized from the reaction of CaCl2 and NH4F in an aqueous solution in the absence of any additional components (e.g., chemicals, surfactants). Monodispersed particles were achieved by the optimization of the reaction condition parameters: temperature, mixing rate, and reaction time. Control of the particle size was accomplished mainly by changing the concentration of the reactants, which is qualitatively explained by the conventional nucleation theory. Flexibility of the process in controlling particle morphology, from a spherical to a hexagonal and/or a cubical form, was predominantly achieved by varying the concentration of CaCl2. Since the identical XRD pattern was detected in particles with varying morphologies, the shape transformation was due to changes in particle growth. A theoretical background to support how the particles changed was also added and was compared with an analysis of the number of nuclei. In addition, sufficient adjustment of the reactant compositions made it possible to produce a material with an ultralow refractive index (nCaF2 was near to ntheoretical CaF2), which was confirmed by the measurement of the refractive index and the material crystallinity.