Structural Phase Transitions of Mg₂NiH₄ Induced by NiH₄ Molecular Ion Distortion and Metal Lattice Deformation under High Pressures

Experimental observations indicate that pressure treatments transform the monoclinic and orthorhombic low-temperature phases of Mg₂NiH₄ into an unresolved pseudocubic phase even at room temperatures, resembling the temperature-induced phase transformation of the material. This pressure-induced phase transformation is anticipated to involve deformations of the tetrahedrally bonded NiH₄ molecular ion and the metal lattice. However, the precise mechanism underlying this transformation remains unclear. To elucidate this behavior, this study adopted first-principles density functional theory calculations to investigate the effect of applied pressure on the observed phase transition. The results revealed that the phase transition from the low-temperature phases to the pseudocubic phase occurs at approximately 8-9 GPa. Furthermore, at this pressure, the metal lattice deforms into an antifluorite-like structure, and the NiH₄ molecular ion undergoes substantial distortion, resulting in the alignment of hydrogen atoms along the crystal axes of the metal lattice. Furthermore, through a comprehensive structural exploration, we successfully identified several tetragonal and orthorhombic models that are energetically more stable than the low-temperature phases at low pressures. These models are potential candidates for elucidating the pseudocubic phase observed in experiments. Overall, our findings are anticipated to enhance the current understanding of the structural changes occurring during pressure and temperature-induced phase transitions.