In this work, the complex hydrogen sorption behaviors in a 3NaBH 4 /HoF 3 composite prepared through mechanical milling were carefully investigated, including the reactions occurred during ball milling and de-/rehydrogenation processes. Different from other rear earth fluorides, the HoF 3 can react with NaBH 4 during ball milling, leading to the formations of Na–Ho–F and Na–Ho–BH 4 complex compounds. The first dehydriding of the 3NaBH 4 /HoF 3 composite can be divided into 4 steps, including the ion exchange between H − and F − , the formation of NaHo(BH 4 ) 4 , the decomposition of NaHo(BH 4 ) 4 and reaction of NaBH 4 with Na–Ho–F compounds. The final products, HoB 4 , HoH 3 and NaF, can be rehydrogenated to generate NaBH 4 and NaHoF 4 with an absorption capacity of 2.3 wt% obtained at 400 °C. Based on the Pressure–Composition–Temperature measurements, the de-/rehydrogenation enthalpies of the 3NaBH 4 /HoF 3 composite are determined to be 88.3 kJ mol −1 H 2 and −27.1 kJ mol −1 H 2 , respectively.
HoF3 crystals of varying sizes and morphologies were synthesized by controlling the amount of water. As the water content gradually decreased, the size of the crystals reduced, transforming from microcrystals to approximately 100 nm nanocrystals with unique morphologies. At room temperature, the pressure-induced phase transitions, and photoluminescence (PL) properties of HoF3 micro/nanocrystals are studied using in situ high-pressure PL technique. The PL spectra show that the HoF3 micro/nanocrystals exhibit two structural phase transformations at 5 (6 GPa for NCs) and 12 GPa. Nanoparticles have higher fluorescence intensity, which initially increases and then decreases with changes in pressure. Based on first-principles calculations, HoF3 transforms from an orthorhombic structure to a hexagonal structure during the phase transition, with the coordination number of holmium atoms increasing from 9 to 11. The high-pressure Raman spectra on lattice modes also confirmed the existence of the two phase transitions. This work not only provides precise structural changes but also facilitates the understanding of two typical structures of rare-earth trifluoride (REF3), which may play an important role in the application of the RE family.
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