Pressure-Induced\nStructural Phase Transitions and\nPhotoluminescence Properties of Micro/Nanocrystals HoF<sub>3</sub>

HoF<sub>3</sub> crystals of varying\nsizes and morphologies were\nsynthesized by controlling the amount of water. As the water content\ngradually decreased, the size of the crystals reduced, transforming\nfrom microcrystals to approximately 100 nm nanocrystals with unique\nmorphologies. At room temperature, the pressure-induced phase transitions,\nand photoluminescence (PL) properties of HoF<sub>3</sub> micro/nanocrystals\nare studied using in situ high-pressure PL technique. The PL spectra\nshow that the HoF<sub>3</sub> micro/nanocrystals exhibit two structural\nphase transformations at 5 (6 GPa for NCs) and 12 GPa. Nanoparticles\nhave higher fluorescence intensity, which initially increases and\nthen decreases with changes in pressure. Based on first-principles\ncalculations, HoF<sub>3</sub> transforms from an orthorhombic structure\nto a hexagonal structure during the phase transition, with the coordination\nnumber of holmium atoms increasing from 9 to 11. The high-pressure\nRaman spectra on lattice modes also confirmed the existence of the\ntwo phase transitions. This work not only provides precise structural\nchanges but also facilitates the understanding of two typical structures\nof rare-earth trifluoride (REF<sub>3</sub>), which may play an important\nrole in the application of the RE family.