Ab initio study of the structural, vibrational, and optical properties of potential parent structures of nitrogen-doped lutetium hydride

The recent report of near-ambient conditions superconductivity in a nitrogen-doped lutetium hydride has inspired a large number of experimental studies with contradictory results. We model from first principles the physical properties of the possible parent structures of the reported superconductors, LuH2 and LuH3. We show that only the phonon band structure of LuH3 can explain the reported Raman spectra due to the presence of hydrogens at the interstitial octahedral sites. However, this structure is stabilized by anharmonicity only above 6 GPa. We find that the intriguing color change with pressure in the reported superconductor is consistent with the optical properties of LuH2, which are determined by the presence of an undamped interband plasmon. The plasmon blueshifts with pressure and modifies the color of the sample without requiring any structural phase transition. Our findings suggest that the main component in the experiments is LuH2 with some extra hydrogen atoms at octahedral sites. Neither LuH2 nor LuH3 superconducts at high temperatures. © 2023 American Physical Society.