H$_2$-Like Units and Pressure-Tunable High-$T_\mathrm{c}$ Superconductivity in NaH$_6$

Abstract

Hydrogen-rich materials offer a route to high-temperature conventional superconductivity, but often require extreme pressures. Here we report a first-principles study of NaH$_6$, identified in our large-scale hydride search and independently predicted in a subsequent high-throughput study. Several phases compete, with $R\bar{3}m$ and $Pm\bar{3}m$ dominant over broad pressure ranges. The $R\bar{3}m$ phase remains dynamically stable down to 10 GPa, where its calculated $T_\mathrm{c}$ is 34 K, rising to 99 K at 50 GPa. Cubic $Pm\bar{3}m$ becomes thermodynamically competitive at higher pressures and reaches a calculated $T_\mathrm{c}$ of 230 K at 50 GPa, decreasing only weakly upon further compression. Molecular dynamics and phonon-renormalization calculations support structural persistence over the sampled timescale and indicate relatively small anharmonic effects. Despite their high $T_\mathrm{c}$, both phases contain elongated H$_2$-like units, often considered unfavorable for high-$T_\mathrm{c}$ hydrides. Mode-resolved electron-phonon coupling links Raman-active hydrogen vibrations to superconductivity. In $R\bar{3}m$, the linewidths of these modes increase with the pressure-induced tilt of H$_2$ units relative to the threefold axis, alongside the rise in $T_\mathrm{c}$. A minimal symmetry-based model of the vibration-induced second-moment tensor of the hydrogen sublattice shows that these trends are consistent with a significant contribution from response components perpendicular to the threefold axis. NaH$_6$ thus represents an unusual molecular-like hydride in which hydrogen-sublattice reorganization may enable high-$T_\mathrm{c}$ superconductivity at comparatively moderate pressures.