Leading components and pressure-induced color changes in N-doped lutetium hydride

Recent experimental study by Dasenbrock-Gammon et al. (Nature 2023;615:244) claims to have discovered room-temperature superconductivity in lutetium-nitrogen-hydrogen system at 1 GPa, which sheds light on the long-held dream of ambient superconductivity. However, all follow-up experiments found no evidence of superconductivity. The compositions and the crystal structures of the lutetium-nitrogen-hydrogen system remain unknown. By employing the density functional theory based structure prediction algorithm, we suggest that in lutetium-nitrogen-hydrogen the major component is LuH₂ (Fm3¯m), together with minor LuN (Fm3¯m). The blue LuH₂ at ambient pressure will turn into purple and red color at higher pressures, possibly accompanied by the formation of vacancies at hydrogen-sites. In LuH₂ and LuN, the density of states at the Fermi level is dominated by the Lu-5d orbitals, while those from hydrogen and nitrogen are very small, leading to the absence of superconductivity in these two compounds. Nitrogen-doping to LuH₂ fails to enhance the superconductivity as well. In this work, we identify the leading components in N-doped lutetium hydride, explain its intriguing color changes under pressure, and elucidate why superconductivity is absent in the follow-up experiments.