We have investigated structural phase transitions of Ge at pressures up to
190 GPa using monochromatic synchrotron X-ray diffraction. The phase appear
ing near 100 GPa is found to have an orthorhombic structure (space group Cm
ca, 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 ne
w findings, the sequence of structural transitions observed fur Ge under pr
essure up to 190 GPa is similar to that of Si in the range up to 50 GPa. We
have performed lull potential linear muffin tin orbital calculations of th
e volume-dependent total energies of Ge in different crystal structures. Th
e calculated phase transitions are compared to the experimental results rep
orted here.