Structural Phase Transitions of Mg<sub>2</sub>NiH<sub>4</sub> Induced by NiH<sub>4</sub> Molecular Ion Distortion and Metal\nLattice Deformation under High Pressures

Experimental\nobservations indicate that pressure treatments transform\nthe monoclinic and orthorhombic low-temperature phases of Mg<sub>2</sub>NiH<sub>4</sub> into an unresolved pseudocubic phase even at room\ntemperatures, resembling the temperature-induced phase transformation\nof the material. This pressure-induced phase transformation is anticipated\nto involve deformations of the tetrahedrally bonded NiH<sub>4</sub> molecular ion and the metal lattice. However, the precise mechanism\nunderlying this transformation remains unclear. To elucidate this\nbehavior, this study adopted first-principles density functional theory\ncalculations to investigate the effect of applied pressure on the\nobserved phase transition. The results revealed that the phase transition\nfrom the low-temperature phases to the pseudocubic phase occurs at\napproximately 8–9 GPa. Furthermore, at this pressure, the metal\nlattice deforms into an antifluorite-like structure, and the NiH<sub>4</sub> molecular ion undergoes substantial distortion, resulting\nin the alignment of hydrogen atoms along the crystal axes of the metal\nlattice. Furthermore, through a comprehensive structural exploration,\nwe successfully identified several tetragonal and orthorhombic models\nthat are energetically more stable than the low-temperature phases\nat low pressures. These models are potential candidates for elucidating\nthe pseudocubic phase observed in experiments. Overall, our findings\nare anticipated to enhance the current understanding of the structural\nchanges occurring during pressure and temperature-induced phase transitions.