[Paper Review] On parent structures of near-ambient nitrogen-doped lutetium hydride superconductor

Mingfeng Liu, Xiangyang Liu+5 more|arXiv (Cornell University)|Mar 12, 2023

Inorganic Fluorides and Related CompoundsChemistry3 citations

TL;DR

This study identifies the parent phase of near-ambient superconducting nitrogen-doped lutetium hydride using density functional theory (DFT) calculations. It demonstrates that the dominant phase is fluorite-type LuH₂ (FL-LuH₂), not the originally proposed Fm\bar{3}m LuH₃, based on thermodynamic stability, phonon dynamics, XRD pattern matching, and optical absorption—particularly the lack of pink-color photon absorption, which aligns with experimental observations of pink-colored samples.

ABSTRACT

Recently, near-ambient superconductivity has been experimentally evidenced in a nitrogen-doped lutetium hydride by Dasenbrock-Gammon \emph{et al.} [Nature 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 dominant LuH$_2$ phase of the CaF$_2$-type (instead of originally proposed LuH$_3$ structure of $Fm\bar{3}m$ space group) and minor LuH phase of the NaCl-type.

Motivation & Objective

• To resolve the long-standing ambiguity in the crystal structure of the parent phase of nitrogen-doped lutetium hydride superconductor, which remains elusive due to XRD limitations in detecting light elements like H and N.
• To determine the thermodynamically and dynamically stable phases in the Lu-H system under ambient and moderate pressures.
• To reconcile experimental observations—particularly the pink color of the superconductor and XRD patterns—with theoretical predictions using optical spectroscopy and structural modeling.
• To clarify whether the originally proposed Fm\bar{3}m LuH₃ structure is viable or if alternative phases such as FL-LuH₂ or RS-LuH are more consistent with experimental data.
• To provide a reliable structural foundation for understanding the role of nitrogen doping in enabling near-ambient superconductivity at 10 kbar.

Proposed method

• Performed systematic DFT calculations on all possible Lu-H binary phases, including LuH, LuH₂, and LuH₃ in various polymorphs (fluorite, NaCl, zinc-blende, and Fm\bar{3}m structures).
• Calculated formation enthalpies and constructed convex hulls to assess thermodynamic stability, identifying phases with zero or low energy above hull.
• Conducted lattice dynamic analysis (phonon dispersion calculations) to evaluate dynamical stability at 0 GPa and under pressure.
• Simulated X-ray diffraction (XRD) patterns for each phase and compared them directly with experimental XRD data from Dasenbrock-Gammon et al. (2023).
• Computed optical absorption spectra within the independent-particle approximation to evaluate photon absorption in the visible range, particularly near pink color (1.9 eV).
• Used pressure-dependent structural and optical calculations (from 0 to 1 GPa) to assess the robustness of structural assignments under experimental conditions.

Experimental results

Research questions

• RQ1 Which Lu-H phase is thermodynamically stable and consistent with the experimental XRD pattern of the nitrogen-doped lutetium hydride superconductor?
• RQ2 Does the experimentally observed pink color of the superconductor sample correlate with the optical absorption behavior of candidate Lu-H phases?
• RQ3 Is the originally proposed Fm\bar{3}m LuH₃ structure dynamically stable, or does it exhibit phonon instabilities that rule it out?
• RQ4 Can the minor XRD peaks in the experimental data be explained by the presence of a secondary phase, and if so, which one?
• RQ5 To what extent do the simulated optical spectra of candidate phases match the experimental observation of low absorption in the pink region of the spectrum?

Key findings

• The fluorite-type LuH₂ (FL-LuH₂) phase is thermodynamically stable with zero energy above the convex hull (E_hull = 0 meV/atom), indicating it is the most favorable phase.
• FL-LuH₂ exhibits no phonon instabilities at 0 GPa and under pressure, confirming its dynamical stability, unlike Fm\bar{3}m LuH₃, which shows strong phonon softening.
• The simulated XRD pattern of FL-LuH₂ matches the main peaks of the experimental XRD data perfectly, outperforming other candidates including Fm\bar{3}m LuH₃.
• FL-LuH₂ shows vanishing absorption of photons near the pink color (1.9 eV), consistent with the experimentally observed pink color of the superconductor, whereas Fm\bar{3}m LuH₃ exhibits strong absorption.
• The minor XRD peaks are best reproduced by the NaCl-type LuH (RS-LuH) phase, which is also phonon stable and has a lattice constant (4.800 Å) close to the experimental value (4.753 Å) for compound B.
• The lattice constants of FL-LuH₂ (5.017 Å) and RS-LuH (4.800 Å) at 0 GPa agree well with experimental values (5.029 Å and 4.753 Å), respectively, further validating the structural assignment.