Fig. 1. Formation and synthesis diagram of the Lu + H₂ system as function of time and pressure. The insets are images of different rounds of lutetium hydride synthesis, corresponding to Lu at 28.5 GPa, LuH₂ at 5.0 GPa, and LuH₃ at 3.0 GPa, respectively, with the Y-axis position corresponding to the time at which the inset photograph was taken. The hollow red, solid red, hollow black, and solid black symbols indicate the start of LuH₂ generation, its completion, the start of LuH₃ generation, and its full completion, respectively.

Fig. 2. Photographs and Raman spectra of the Lu + H₂ system as function of time after loading. (a)–(d) Images captured during the reaction process. The circles and the square in these photographs indicate where the corresponding Raman spectra were measured. The Raman spectrum detected at the position of the blue square corresponds to “H₂-Rich” in (e), and the H₂ rotons are marked with a black plus (+) symbol. The red and black components in the sample images correspond to LuH₂, and LuH₃, respectively. (e) Raman spectra of the Lu + H₂ system as functions of time at 3.0 GPa and 298 K. The gray curve represents the spectrum of LuH₃ from the literature.⁴⁷ (f) Raman spectra of the Lu + H₂ system as functions of time at 20.3 GPa and 298 K. The “growth” of lutetium hydrides occurs over a specific time scale, and “growth” can also be deduced from the sample size in the images. See the supplementary material for further details of the reaction conditions.

Fig. 3. Bandgaps of LuH₂ and LuH₃ under high pressure. (a) Plots of the transformed Kubelka–Munk function vs photon energy for LuH₂ and LuH₃ at high pressures. (b) Pressure-dependent bandgaps of LuH₂ and LuH₃ under compression. The bandgap at ambient pressure is shown in Fig. S8 (supplementary material).

Fig. 4. XRD spectra of the Lu + H₂ system as functions of time after loading at 3.0 GPa and 300 K: (a) Lu-P6₃mmc (day 0); (b) Lu-P6₃mmc + LuH₂-Fm3̄m (day 5); (c) LuH₂-Fm3̄m + LuH₃-P3̄c1 (day 20); (d) LuH₃-P3̄c1 (>30 days). In (d), Re and NaCl are from the gasket and pressure-transmitting medium, respectively.

Fig. 5. Phase transition of lutetium trihydride under high pressure. (a) XRD spectra of LuH₃ during both compression and decompression runs. The asterisk indicates the diffraction peak of NaCl. The spectra in orange, purple, and cyan correspond to LuH₃-P3̄c1, the intermediate state, and LuH₃-Fm3̄m, respectively. (b) Rietveld refinement of the Fm3̄m spectrum at 41.4 GPa. The experimental data, fitted line, and residues are shown in black, red, and gray, respectively. (c) Pressure dependence of the primitive cell volume. The solid orange circles represent the primitive cell volume of P3̄c1 and the solid blue circles the primitive cell volume of Fm3̄m. The theoretical predictions of Dangic et al.²⁶ and experimental data from Palasyuk and Tkacz¹ and Moulding et al.⁴⁷ are also shown for comparison.

Fig. 6. Raman spectra of the phase transition of lutetium trihydride under high pressure. (a) Raman spectra of LuH₃ during compression. (b) Pressure dependence of the vibrational frequencies of LuH₃ obtained upon compression. Different colors correspond to different phases, and the white region represents the coexisting phases.