Phase Transformations in MgH₂–TiH₂ Hydrogen Storage System by High-Pressure Torsion Process

Magnesium hydride (MgH₂) and titanium hydride (TiH₂) are two potential candidates for solid-state hydrogen storage, but strong hydride formation energy in these hydrides undesirably results in their high dehydrogenation temperature. First-principles calculations show that the metastable hydrides in the MgH₂–TiH₂ system have low hydrogen binding energy, which makes them more appropriate for low-temperature hydrogen storage. In this study, severe plastic deformation (SPD) via the high-pressure torsion (HPT) method is applied to the MgH₂–TiH₂ system to synthesize metastable hydrides. While MgH₂ transforms to a high-pressure orthorhombic γ phase, TiH₂ does not exhibit any cubic-to-tetragonal phase transformation even by HPT processing at cryogenic temperature. Application of large strains by 400 HPT turns to the immiscible MgH₂/TiH₂ composite results in atomic-scale mixing and formation of nanostructured ternary Mg–Ti–H hydride with the metastable FCC structure and lower dehydrogenation temperature than TiH₂.