Structural phase transition of rutile-type MgH2 at high pressures

The pressure-induced structural phase transition of MgH2 was examined up to 57GPa at room temperature. alpha-MgH2 with the rutile-type structure at ambient conditions transformed to gamma-MgH2 with the alpha-PbO2 structure. A complete conversion to the gamma-phase, however, did not take place and these two phases coexisted up to 9GPa. Above 9GPa, MgH2 transformed to the orthorhombic phase (HP1-phase), and further to another orthorhombic phase (HP2-phase) at around 17 GPa. Space groups of the HP1 phase and the HP2-phase were concluded to be Pbc2(1) and Pnma (cotunnite-type), respectively. Cotunnite-type MgH2 was stable at pressures up to 57 GPa. Thus the proposed structural sequence of MaH(2) under pressure is rutile-type -> alpha-PbO2 -> Pbc2(1) -> cotunnite-type, with an increase in the coordination number of magnesium ions from 6 to 9.