Comprehensive model and investigation of F− ions-induced cubic-to-hexagonal phase transformation in NaYF4

The physical mechanism of the structural phase transformation in NaYF4 has a great significance for the synthesis and processing of high luminescence efficiency hexagonal NaYF4 materials. In this work, we proposed a new physical model of the phase transformation in NaYF4, in which F− ions induce structural phase transformation. We also conducted a set of experiments to prove this. Using density functional theory (DFT) simulations, TEM, and XRD analysis, we confirmed the validity of our structural phase transformation model. We also described a two-step methodology for the comprehensive investigation of the cubic-to-hexagonal phase transformation. In the first step, the cubic-to-hexagonal phase transformation in NaYF4 is considered. In the second step, hexagonal NaYF4 is converted through the oxidation process into YOF, which is also known in the literature as pseudo “cubic phase” NaYF4. Our research could also guide the development of technological processes for fine-tuning the nanocrystal fluorescence characteristics of NaYF4 for future applications.