Pressure-induced structural phase transition, irreversible amorphization and upconversion luminescence enhancement in Ln3+-codoped LiYF4 and LiLuF4

Lanthanide ion (Ln(3+))-codoped inorganic materials have been widely studied owing to their unique upconversion photoluminescence (UC PL) performance, which can be employed for promising applications in imaging, sensing, detection, and treatment. However, the forbidden transition of 4f inner shell and the nonradiative relaxation from the interaction of Ln(3+) and a host lattice lead to low emission efficiency, limiting the industrial application. In this study, we report pressure-induced 1.2-2.6 times UC PL enhancement in the range of 10.0-25.0 GPa in Ln(3+) (19%Yb3+/1%Er3+, 19%Yb3+/1%Ho3+, and 19.9%Yb3+/0.1%Tm3+)-codoped LiYF4 and LiLuF4. In situ X-ray diffraction pattern and Raman spectra indicated that both LiYF4 and LiLuF4 underwent structure transitions at a pressure around 10.0 GPa. Moreover, improved UC emissions were observed for the released sample in a completely amorphous state. The luminescence spectra of Eu3+-doped LiYF4 and LiLuF4 probes revealed that the phase transitions resulted in the reduction of Ln(3+) site symmetry. Our research shows that pressure would be a powerful implement to design high-efficiency UC PL materials. Simultaneously, research on the "structure-luminescence" relationship can provide a deeper insight into the exploitation of potential UC PL materials and optimization of current luminescent materials.