New high-pressure phase of BaH₂ predicted by ab initio studies

Pressure-induced phase transitions of BaH₂ have been studied by ab initio calculations. Our results show that BaH₂ transforms from the cotunnite structure to the InNi₂-type structure at about 2.3 GPa, which is in agreement with experimental results. The InNi₂ phase is predicted to be an insulator and transforms to a metallic phase with an AlB₂-type structure at about 34 GPa. Under higher pressure, a post-AlB₂ phase with the YbZn₂-type structure (space group Imma, 4 f.u./cell) is predicted, which is both dynamically and mechanically stable. Analysis of the enthalpies for both AlB₂ and YbZn₂ phases further supports the existence of this new phase. The [AlB₂ → YbZn₂] structural phase transition is identified as a second-order nature, driven by the softening of the transverse acoustic phonon mode at the L point (0.5, 0.0, 0.5).