Pressure-induced phase transition of BaH2 : Post Ni2In phase

Up to now a Ni2In structure is a final step in the structural sequence of ionic AX2 compounds under high pressure. Powder X-ray diffraction experiments on BaH2 were performed at room temperature and high pressures up to 69 GPa. Successive phase transformations were observed to occur in two stages. The first was from the cotunnite to the Ni2In structure at 2.5 GPa. The second transition commenced at pressures around 50 GPa and was completed at 65 GPa. At the transition the arrangement of a cation sublattice changes from an hcp to a simple hexagonal lattice. This is the first observation of the post Ni2In phase.