Hydrogen sublattice in LaH$_{12}$: structural stability and possible ordering motifs

Abstract

Lanthanum polyhydrides, synthesized under extreme pressure, exhibit near-room-temperature superconductivity. Finding a precise arrangement of the hydrogen sublattice that is critical for understanding the strong electron-phonon coupling remains experimentally unresolved due to the small X-ray scattering form factor of hydrogen atoms. In this work, we attempt to computationally reconstruct hydrogen sublattices of the LaH$_{12}$ at 100-200 GPa combining unconstrained evolutionary searches and structure generation with fixed lanthanum sublattice. By implementing a similarity metric, we screen the computationally generated configurations against experimental powder X-ray diffraction data. We specifically identify the trigonal $R\bar{3}$-LaH$_{12}$ and pseudo-hexagonal $C222_1$-La$_2$H$_{24}$ phases, which demonstrate close agreement with the experimental X-ray diffraction data. First-principles calculations confirm the metallic and superconducting nature of these hydrides. Crucially, our lattice dynamics analysis using the stochastic self-consistent harmonic approximation (SSCHA) indicates that these structures are dynamically stable. We reveal the presence of molecular H$_2$ units and soft phonon modes in the trigonal LaH$_{12}$ that suppresses superconductivity ($T_c \ge 30$ K), while the hydrogen clathrate network in the $C222_1$ phase drives strong electron-phonon interactions, yielding the coupling parameters of $λ\approx 1.83$, $ω_{\log} \approx 87.5$ meV and transition temperature of $T_c\approx 150$ K at 150 GPa.