Fig. 1. Synthesis and characterization of Lu metal under a H₂:N₂ (99:1) gas mixture following established protocol. (a) Pre-heating phase of the lutetium sample at 2.0 GPa with a 99% H₂ and 1% N₂ gas mixture at 1.0 GPa. (b) Post-heating phase for the same sample, maintained at 65 °C and 2.0 GPa for 24 h, conspicuously devoid of the blue hue reported by Dasenbrock-Gammon et al. (c) Raman spectrum of the gas mixture in contact with the lutetium sample. (d) XRD patterns for pure lutetium metal compared with the sample post 24-h heating at 65 °C and 2.0 GPa. (e) Resistance variations with temperature for the sample post-heating at 1.0 GPa. (f) Resistance variations with temperature for the sample post-heating at 3.0 GPa.

Fig. 2. Characterization of nitrogen-doped LuH₃ synthesized at 200 °C and 2.0 GPa for 24 h. (a)–(d) Sequential images documenting the sample’s color evolution during synthesis, with no observed blue-to-pink color changes. (e) Comparative Raman spectra of LuH₃ before and after pressurized heating; the undoped LuH₃ spectrum is included for reference. (f) XRD patterns of LuH₃ pre- and post-heating, complemented by the undoped LuH₃ pattern at 0 GPa and a simulated LuH₃ pattern. (g) Resistance as a function of temperature for nitrogen-doped LuH₃ at pressures from 2.0 to 9.3 GPa. The inset illustrates the four-probe measurement.

Fig. 3. Characterization of nitrogen-doped LuH₂ produced from single-crystal LuH₂ treated in N₂ gas at 200 °C and 2.0 GPa. (a) Sequential photographs capturing the sample’s color evolution as the pressure ranges from 0 to 20.0 GPa. (b) XRD patterns of both undoped and nitrogen-doped LuH₂ single crystals. (c) Raman spectra of LuH₂ pre- and post-heat treatment. (d) Electrical transport behavior of nitrogen-doped LuH₂ powders under 0.4 GPa.

Fig. 4. Detailed comparison of structure and properties of LuH₂ and LuH₃. (a) XRD patterns of LuH₂ and LuH₃ before and after heating, along with the diffraction pattern of the Lu–N–H compound. (b) Simulated XRD patterns of LuH₂ (Fm3̄m), LuN (Fm3̄m), and LuH₃ (P3̄c1). (c) Raman spectra of LuH₃ before and after heating. (d) Raman spectra of LuH₂ before and after heating. (e) Raman spectra of as-synthesized LuH_2±xN_y compounds from Ming et al.¹⁹ (f) Raman spectrum of the Lu–H–N compound.¹¹

Fig. 5. Comparative electrical resistance measurements of nitrogen-doped and pristine LuH₂ in various forms and under pressures in the range 1–2 GPa. (a) Electrical resistance of nitrogen-doped polycrystalline LuH₂, illustrating a drop at 296 K and 1.0 GPa. (b) Electrical resistance of pristine polycrystalline LuH₂, showing a drop at 297 K and 2.0 GPa. (c) Electrical resistance of single-crystal LuH₂ in nitrogen gas, indicating a drop at 270 K and 1.6 GPa. The inset shows an optical photograph of the four-probe electrical resistance setup in a DAC. (d) Data from Dasenbrock-Gammon et al., exhibiting a resistance drop at 296 K and 1.0 GPa. (e)–(h) Resistance profiles after background subtraction, following the methodology of Dasenbrock-Gammon et al. (i) Andreev reflection measurements from single-crystal LuH₂ under 1.6 GPa in nitrogen (j) Andreev reflection measurements from standard MgB₂ for comparison, conducted under identical experimental conditions.