High-pressure Cmca and hcp phases of germanium

Structural phase transitions of germanium at pressures up to 190 Cpa have b een investigated by means of angle-dispersive x-ray powder diffraction usin g synchrotron radiation. Near 100 GPa the primitive hexagonal modification of Ge transforms to an orthorhombic phase with 16 atoms per unit cell. The results of full-profile Rietveld refinements in space group Cmca indicate t hat the orthorhombic modification of Ge is isotypic to the phase Si-VI. Nea r 160-180 GPa Ge undergoes a transition to a structure assigned as hexagona l close packed (hcp). Ab initio calculations of pressure-dependent enthalpi es and optimized structural parameters are presented for different structur es of Ge. The calculated results clearly support the observed occurrence of the Cmca structure in Ge.