Feasible Route to High-Temperature Ambient-Pressure Hydride Superconductivity

A key challenge in materials discovery is to find high-temperature superconductors. Hydrogen and hydride materials have long been considered promising materials displaying conventional phonon-mediated superconductivity. However, the high pressures required to stabilize these materials have restricted their application. Here, we present results from high-throughput computation, considering a wide range of high-symmetry ternary hydrides from across the periodic table at ambient pressure. This large composition space is then reduced by considering thermodynamic, dynamic, and magnetic stability before direct estimations of the superconducting critical temperature. This approach has revealed a metastable ambient-pressure hydride superconductor, ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$, with a predicted critical temperature of 160 K, comparable to the highest temperature superconducting cuprates. We propose a synthesis route via a structurally related insulator, ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$, which is thermodynamically stable above 15 GPa, and discuss the potential challenges in doing so.

Figure 1

(a) Pressure dependence of the formation enthalpies of Mg-Ir-H phases relative to the cubic ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ phase. Pressure synthesis route is indicated by arrows, starting from ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ at 15 GPa (1) and tracking decompression to 6 GPa, below which cubic ${\mathrm{Mg}}_2$${\mathrm{IrH}}_6+$ H should form (2). (b–d) Crystal structures of ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ and ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$: orange, green, pink, and maroon spheres denote the Mg, Ir, hydrido H, and interstitial H atoms, respectively.

Figure 2

Ternary convex hull at 20 GPa, calculated with PBEsol. Black lines are ridges connecting thermodynamically stable points. Star symbols indicate ${\mathrm{Mg}}_2{\mathrm{IrH}}_7$ and ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$. Only structures within 200 meV of the convex hull are shown. Symbols are smaller (and more blue) with increasing distance from the convex hull. At 20 GPa, before considering quantum nuclear effects, the $Amm2$ phase of ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$ is more stable than $Fm\overline{3}m$.

Figure 3

Histograms of the energy-dependent distribution of the anisotropic superconducting gap ${\mathrm{\Delta }}_{\mathbf{k}}$ on the Fermi surface of ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$, evaluated for each temperature solving the ME equations with ${\mu }^{*}=0.1$ (blue), 0.125 (red), and 0.16 (green). The solid dashed lines are guides to the eye tracking the average ${\mathrm{\Delta }}_{\mathbf{k}}(T)$. The inset shows the 3D Fermi surface (left) and $00\overline{1}$ projection (right) colored according to the $k$-dependent gap values ${\mathrm{\Delta }}_{\mathbf{k}}$ obtained from solving the anisotropic ME equation at 15 K with ${\mu }^{*}=0.125$.

Figure 4

(a) Electronic band structure of $Fm\overline{3}m$ ${\mathrm{Mg}}_2{\mathrm{IrH}}_6$ at ambient pressure. (b) The total electron density of states (eDOS) projected onto Mg (red), Ir (green), and H (blue) atoms. (c) The eDOS is projected onto Mg-$3s$ (red), Ir-$e_g$ (light green), Ir-$t_{2g}$ (dark green), and $\mathrm{H}\text{−}1s$ orbitals (blue). Dark green dashed line indicates the Fermi level.

Figure 5

(a) Phonon dispersion along a high-symmetry path in the Brillouin zone. The radius of the red and green circles in the panel (a) is proportional to the magnitude of ${\lambda }_{\mathbf{q}\nu }$ and ${\alpha }^{2}F({\omega }_{\mathbf{q}\nu }$) for each phonon mode, respectively. (b) Total (black) and projected phonon densities of states (phDOS) onto Mg (magenta), Ir (green), and H (blue) atoms. (c) Isotropic Eliashberg spectral function, ${\alpha }^{2}F(\omega )$, and cumulative frequency-dependent electron-phonon coupling strength $\lambda (\omega )$. The labeled red circles indicate the running total (and fraction) of $\lambda (\omega )$.