Binary sesquioxides ($M_2{\mathrm{O}}_3$-type metal oxides) are considered potential candidates for hydrogen permeation barriers (HPBs) under both ambient and high-pressure conditions. However, our previous work revealed that the high-pressure ${\mathrm{Al}}_2{\mathrm{O}}_3$ phase [${\mathrm{Rh}}_2{\mathrm{O}}_3\left(\mathrm{II}\right)$ phase] exhibits ultrahigh hydrogen diffusivities. In this paper, we systematically surveyed the phase transition patterns of the binary sesquioxides and identified ${\mathrm{Er}}_2{\mathrm{O}}_3$ as a superior HPB candidate that exhibits a different phase transition pattern from that of ${\mathrm{Al}}_2{\mathrm{O}}_3$. Since only shared-edge octahedral voids are present in the high-pressure phase of the seven-coordinated erbium-centered polyhedra of ${\mathrm{Er}}_2{\mathrm{O}}_3$, hydrogen atom diffusion occurs across the shared edge between adjacent octahedral voids. This geometric confinement induces a shift from electrostatic to covalent interactions in hydrogen-oxygen covalent interactions during diffusion, thereby increasing the energy barrier; e.g., the diffusion energy barrier of a hydrogen atom in the C-RES phase is 0.50 eV at 0 GPa, and it drastically increases to 1.28 eV in the A-RES phase at 20 GPa. Our findings establish a selection principle for HPBs: choosing crystal phases with highly coordinated cation-centered polyhedra shifts hydrogen-oxygen interaction from electrostatic to covalent during diffusion, strongly suppressing hydrogen diffusion.

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• Diffusion
• Hydrogen storage
• Pressure effects
• Density functional calculations