New phase transitions in Ge were observed by energy-dispersive x-ray diffraction techniques for pressures up to 125 GPa (1.25 Mbar) as follows: the -Sn structure to the simple hexagonal (sh) phase at 75 ± 3 GPa and to the double hexagonal close-packed structure (dhcp) at 102 ± 5 GPa. These are the highest pressures for which a crystalline structure change has been directly observed by x-ray diffraction. Total-energy pseudopotential calculations predict 84 ± 10 GPa for the -Sn to sh phase transition and 105 ± 2 GPa for sh to hcp (not dhcp) transition. The role of 3 d core electrons in increasing the transformation pressures in Ge, as compared to Si, is emphasized. © 1986 The American Physical Society.