High-Pressure Structures of Ge above 100 GPa

We have investigated structural phase transitions of Ge at pressures up to 190 GPa using monochromatic synchrotron X-ray diffraction. The phase appearing near 100 GPa is found to have an orthorhombic structure (space group Cmca, Z = 16 atoms per unit cell, isotypic to Si–VI). Ge further undergoes a sluggish transition to a hcp structure at around 160–180 GPa. With these new findings, the sequence of structural transitions observed for Ge under pressure up to 190 GPa is similar to that of Si in the range up to 50 GPa. We have performed full potential linear muffin tin orbital calculations of the volume-dependent total energies of Ge in different crystal structures. The calculated phase transitions are compared to the experimental results reported here.