Abstract
Pressure induced structural modifications in vitreous GexSe100−x (where 10 ≤ x ≤ 25) are investigated using X-ray absorption spectroscopy (XAS) along with supplementary X-ray diffraction (XRD) experiments and ab initio molecular dynamics (AIMD) simulations. Universal changes in distances and angle distributions are observed when scaled to reduced densities. All compositions are observed to remain amorphous under pressure values up to 42 GPa. The Ge-Se interatomic distances extracted from XAS data show a two-step response to the applied pressure; a gradual decrease followed by an increase at around 15–20 GPa, depending on the composition. This increase is attributed to the metallization event that can be traced with the red shift in Ge K edge energy which is also identified by the principal peak position of the structure factor. The densification mechanisms are studied in details by means of AIMD simulations and compared to the experimental results. The evolution of bond angle distributions, interatomic distances and coordination numbers are examined and lead to similar pressure-induced structural changes for any composition.
Highlights
Suggests a transition from a low density amorphous semiconductor to a high density metallic amorphous between 10 and 15 GPa using X-ray diffraction (XRD) and electrical conductivity measurements[8]
A strong diamond diffraction peak due to diamond anvil cell (DAC) is observed at q = 3.04 Å−1, which was subtracted through the empty cell measurement from the measured XRD spectra of the samples in order to find the peak positions reliably
We have investigated the pressure-induced polyamorphism in vitreous GexSe100−x using a combination of EXAFS, X-ray absorption near edge structure (XANES), XRD experiments along with ab initio simulations
Summary
Suggests a transition from a low density amorphous semiconductor to a high density metallic amorphous between 10 and 15 GPa using X-ray diffraction (XRD) and electrical conductivity measurements[8]. Additional evidence for a semiconductor to metal transition in GeSe4 under pressure was noted recently using a combination of neutron diffraction and ab initio simulations[24]. That work confirmed that the average coordination number shows no significant changes up to ≈8 GPa and that non-negligible fraction of 5-fold Ge and 3-fold Se appear around 13 GPa. Despite the experimental findings and simulations carried out regarding the pressure-induced structural changes in GeSe2 and GeSe4 glasses, a complete understanding of the underlying densification mechanisms leading to the AATs in GexSe100−x glasses is still lacking. We report results of a comprehensive study of the polyamorphic transitions in vitreous GexSe100−x with a detailed description of the structural modifications using a combination of X-ray absorption spectroscopy (XAS), XRD and ab initio molecular dynamics (AIMD) simulations under pressure up to 41.4 GPa. Our results provide a thorough insight of the atomic packing rearrangements triggered during the AAT over a range of compositions including flexible and optimally rigid networks
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