Metal hydrides are typically nonstoichiometric compounds at ambient pressure, commonly exhibiting hydrogen vacancies. Recently, hydrogen-rich metal hydrides featuring clathrate hydrogen sublattices have attracted great attention as potential room-temperature superconductors under pressure. However, accurately determining their hydrogen stoichiometry remains experimentally challenging, and the impact of hydrogen vacancies on their superconductivity is unclear. Here, we investigate the superconductivity and thermodynamic properties of ${\mathrm{CeH}}_{10}$ with hydrogen vacancies. Our results reveal that the nonstoichiometric clathrate hydride ${\mathrm{CeH}}_{9.75}$, featuring hydrogen vacancies in a fcc lattice, exhibits lower thermodynamic stabilization pressure compared to the perfect ${\mathrm{CeH}}_{10}$ crystal. While hydrogen vacancies are generally known to induce superionic states in clathrate hydrides, we find that the superionic transition temperature for ${\mathrm{CeH}}_{9.75}$ is actually higher than that of ${\mathrm{CeH}}_{10}$, suggesting that hydrogen vacancies may increase the hydrogen diffusion barrier and enhance the stability of clathrate hydrides under pressure. Furthermore, the calculated superconducting transition temperature of ${\mathrm{CeH}}_{9.75}$ show excellent agreement with experimental data. These findings indicate that nonstoichiometry may be a common feature in the high-pressure phase diagrams of clathrate hydrides.

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• Crystal structure
• High-pressure studies
• Superconductivity
• Hydrides