Pressure-induced phase transitions in amorphous and metastable crystalline germanium by Raman scattering, x-ray spectroscopy, and ab initio calculations

A detailed study of the pressure-induced phase transitions in amorphous Ge (a-Ge) up to 17 GPa is reported combining Raman scattering, x-ray absorption spectroscopy (XAS) measurements, and density-functional theory calculations. a-Ge samples were films obtained by evaporation and characterized by different density of voids. Specific and reproducible phase transitions (interpreted as disorder-disorder, disorder-order, and order-disorder) are observed on pressurization and depressurization, depending on the initial density of voids. Details of the structural and vibrational properties of the various phases have been obtained by XAS and Raman-scattering data analysis. Samples showing a low density of voids transform first into a metallic disordered phase (8 GPa) and to GeII at higher pressures while those with higher density of voids transform directly to the GeII phase (10.6 GPa). Upon depressurization, the first nucleates into the GeIII metastable phase in the 7.2-2.3 GPa pressure range, while the others return to the amorphous state below 5 GPa. The behavior upon depressurization shows that the initial morphology determines the transitions experienced by the pressurized sample down to ambient pressure.