First-principles investigation of rhodium hydrides under high pressure

Hydrogen-rich compounds are being extensively explored theoretically and experimentally as potential hydrogen storage materials. In this work, we predicted a hydrogen-rich compound rhodium trihydride ($\mathrm{Rh}{\mathrm{H}}_3$) with a high volumetric hydrogen density of 212.5 g/L by means of ab initio calculations. The $\mathrm{Rh}{\mathrm{H}}_3$ with Pnma symmetry is thermodynamically stable and accessible through synthesis above 40 GPa. The compound is dynamically and mechanically stable at ambient pressure. Further calculation suggests a probable dehydrogenation temperature $T_{\mathrm{des}}$ of $65{}^{\circ }\mathrm{C}$ at ambient pressure with decomposition route to $\mathrm{Rh}+{\mathrm{H}}_2$. High volumetric hydrogen density and moderate dehydrogenation temperature place $\mathrm{Rh}{\mathrm{H}}_3$ as one of the best hydrogen storage materials. Our work encourages the experimental synthesis of $\mathrm{Rh}{\mathrm{H}}_3$ at high pressure.

Figure 1

(a) Convex hulls of the Rh-H system with respect to elemental Rh and ${\mathrm{H}}_2$ from 1 atm to 300 GPa. The compounds located on the solid lines are thermodynamically stable. (b) The pressure-composition phase diagram of Rh-H system under high pressure.

Figure 2

The crystal structures of (a) $\mathit{Fm}-3m$-RhH, (b) $\mathit{Fm}-3m\text{−}\mathrm{Rh}{\mathrm{H}}_2$, (c) $P{2}_1/m\text{−}\mathrm{Rh}{\mathrm{H}}_2$, (d) Cmcm-$\mathrm{Rh}{\mathrm{H}}_2$,and (e) Pnma-$\mathrm{Rh}{\mathrm{H}}_3$. The big gray balls represent Rh atoms and small pink balls represent H atoms.

Figure 3

EOS of Rh, RhH, $\mathrm{Rh}{\mathrm{H}}_2$, and $\mathrm{Rh}{\mathrm{H}}_3$ in the pressure range of (a) 0–30 and (b) 30–300 GPa. The results of our calculation are shown in solid lines and experimental data are represented by dots [10].

Figure 4

The calculated 2D ELF of (a) $\mathit{Fm}-3m$-RhH at 2 GPa for the plane (001), (b) $\mathit{Fm}-3m\text{−}\mathrm{Rh}{\mathrm{H}}_2$ at 4 GPa for the plane (0–11), (c) $P{2}_1/m\text{−}\mathrm{Rh}{\mathrm{H}}_2$ at 40 GPa for the plane (010), (d) Cmcm-$\mathrm{Rh}{\mathrm{H}}_2$ at 240 GPa for the plane (001), and (e) Pnma-$\mathrm{Rh}{\mathrm{H}}_3$ at 50 GPa for the plane (103). (f)–(j) Their 3D ELFs. The isosurface is 0.6.

Figure 5

The phonon band structures (left), total and PHDOS (right) of Pnma-$\mathrm{Rh}{\mathrm{H}}_3$ at (a) 50 GPa and (b) 1 atm, respectively.