Phase and morphology evolution of luminescent NaLnF₄ (Ln = La to Yb) micro-crystals : understanding the ionic radii and surface energy-dependent solution growth mechanism

Lanthanide (Ln)-based NaLnF₄ micro-crystals (MCs) exhibit great advantages and strong feasibility for integration into micron-sized photonic devices. However, there is still a lack of thermodynamic understanding of the solution growth mechanisms of NaLnF₄ MCs. Herein, we report the preparation of hexagonal NaLnF₄ MCs with various morphologies (rod-, disk- and flake-like) and crystal sizes (100 nm to 10 μm) via a hydrothermal route. The morphologies and sizes of NaLnF₄ MCs were revealed to be highly dependent on the ionic radius of different Ln³⁺ ions. We have proposed a ligand-adsorption-driven oriented growth model and a surface energy dominant nucleation model to interpret the above phenomena. First, the citrate (Cit^3-) adsorption on the {1010} lattice plane primarily leads to different morphologies of NaLnF₄ MCs. Second, different surface energies are required for nucleation and are responsible for crystal size evolution from NaYbF₄ to LaF₃. Third, alternative phase types from NaLnF₄ to LnF₃ are related to the Ln-F bond energy and Goldschmidt's law of crystal chemistry. In addition, the surface ligand removal is beneficial for the luminescence enhancement of NaLnF₄ MCs. These results provide a thermodynamic understanding for researchers to develop NaLnF₄ MCs with targeted morphology and size.