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A detailed study of the pressure-induced phase transitions in amorphous Ge $(a\text{-Ge})$ up to 17 GPa is reported combining Raman scattering, x-ray absorption spectroscopy (XAS) measurements, and density-functional theory calculations. $a\text{-Ge}$ 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.

International audience; We present spectacular evidence of metallization of amorphous Ge films under high pressure using optical micrographs and Raman-scattering and x-ray-absorption spectroscopies. This transformation is identified as a low-density amorphous LDA to high-density amorphous HDA transition, relevant to a large class of systems including amorphous ice, semiconductors, and oxides. We have discovered that this transition initiates at the surface of the LDA sample, characterized intrinsically by a thickness dependent density of voids. Contrary to the case of transitions involving stable crystalline solid phases, our observations show that pressure-induced phase transitions in inhomogeneous amorphous samples are morphology driven and are favored for lower defect void densities. The metal disordered phase is observed first at 8 GPa for a lower density of voids, transforming to a metastable ordered phase upon depressurization. The local HDA structure is characterized by an increase in the first-neighbor coordination number, average distance, and variance.