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 $65{\phantom{\rule{0.16em}{0ex}}}^{\ensuremath{\circ}}\mathrm{C}$ 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.