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Results of experiments dedicated to the study of the structure under high pressure of amorphous Ge (a-Ge) and crystalline Ge (c-Ge) are reported. Energy-dispersive X-ray diffraction measurements of c-Ge have been collected at the DW11A beamline (DCI, LURE) using a heatable diamond anvil cell as pressure device up to 500K. The a-Ge measurements have been performed at the ESRF, using the advanced set-up available at the BM29 beamline, which allows the simultaneous collection of X-ray absorption spectroscopy data and diffraction patterns used to monitor pressure and crystallization of a sample in a Paris-Edinburgh large-volume cell. The new structural data allowed us to obtain a reliable determination of the lattice parameters as a function of pressure and temperature in c-Ge and of the first-neighbor distance distribution in a-Ge. [ABSTRACT FROM AUTHOR]

Metallic glasses are of recent interest worldwide due to their remarkable physicochemical properties which can be put in relation with their crystalline counterparts. Among them, cerium-based metallic glasses (Ce-MGs) have unique features such as the existence of polyamorphism under pressure, which is unexpected in these spatially compact systems. While a phase transition between amorphous phases with change of density and local structure has been previously detected, the corresponding structural variation under pressure was not clearly identified due to difficulties in performing accurate measurements and reliable analysis. In this work, angle dispersive x-ray diffraction experiments of Ce69Al10Cu20Co1 bulk metallic glass have been performed up to 16 GPa along two distinct isotherms (300 and 340 K). All of the diffuse signals have then been processed in order to extract the structure factor S(Q), the pair distribution g(r), the atomic density ?, and the compressibility as a function of pressure and temperature. These are crucial probes to fully characterize the phase diagram, and they clearly confirm the existence of a link between polyamorphism in Ce-MGs and the γ ⇆ γ transition in pure cerium. Finally, owing to the presence of a critical point in pure solid Ce, the existence of such a feature is discussed here for Ce-MGs. [ABSTRACT FROM AUTHOR]

Abstract: Numerous investigations have been performed on Ce metal since the discovery of the γ→α phase transformation, where a face-centered cubic structure is believed to collapse isostructurally with a volume change of ∼17%. However, two questions have yet to be answered definitively. First, is the transformation truly isostructural or is the face-centered cubic structure lost in α-Ce due to symmetry breaking? Second, if the transformation is isostructural does the face-centered cubic structure stay in crystallographic orientation through the volume collapse? Here, we use high-pressure and high-temperature X-ray diffraction measurements to examine single and polycrystalline samples of Ce in the vicinity of the γ↔α transformation. This was achieved by successive continuous compression and decompression in a diamond anvil cell at temperatures under, at and above the critical point. Our results show that the crystal structure remains face-centered cubic for both the γ and α phases. The results also show that the face-centered cubic structure retains its crystallographic orientation, simply reducing in volume during the γ→α phase transformation. Upon transformation to α, polycrystalline samples show increased diffraction peak broadening, while single crystals show increased streaking. These changes in diffraction can be attributed to increased damage and lattice misorientation from the transformation. Using a simple atomic lattice model, we show that a periodic array of misfit edge dislocation is necessary to accommodate the large volume difference at the γ–α interface and this could act as a source of the edge dislocations needed to produced previously observed deformation bands. [Copyright &y& Elsevier]