Parent structures of near-ambient nitrogen-doped lutetium hydride superconductor

Recently, near-ambient superconductivity has been experimentally evidenced in a nitrogen-doped lutetium hydride [Nature (London) 615, 244 (2023)], which yields a remarkable maximum ${T}_c$ of 294 K at just 10 kbar. However, due to the difficulty of x-ray diffraction (XRD) in identifying light elements, such as hydrogen and nitrogen, the crystal structure of the superconductor remains elusive, in particular, for the actual stoichiometry of hydrogen and nitrogen and their atomistic positions. This holds even for its parent structure. Here, we set out to address this issue by performing a thorough density functional theory study on the structural, electronic, dynamical, and optical properties of lutetium hydrides. Through thermal and lattice dynamic analysis as well as XRD and superconductor color comparisons, we unambiguously clarified that the parent structures are a mixture of the dominant ${\mathrm{LuH}}₂$ phase of the ${\mathrm{CaF}}₂$ type (instead of the originally proposed ${\mathrm{LuH}}₃$ structure of the $Fm\overline{3}m$ space group) and the minor LuH phase of the NaCl type.