Analysis of high-energy x-ray diffraction data at high pressure: the case of vitreous As2O3 at 32 GPa

The x-ray structure factor of vitreous As₂O₃ has been measured at 32 GPa in a laser-perforated diamond anvil cell using a monochromatic, micro-focused high-energy x-ray beam. The experimental correction procedures are discussed in detail, and they yield reliable data over the range Q = 0.3-13.5 Å^-1. The use of modified form factors to analyse the scattering data is presented to account for charge transfer. Analysis of the radial distribution function yields an increase in the coordination number from 3.1 ± 0.3 oxygen atoms surrounding an arsenic atom at normal pressure to 4.8 ± 0.5 at 32 GPa with only a slight change in the As-O bond length. Substantial structural changes are observed at higher distances, extending up to 18 Å in real space.

Vitrification of a monatomic metallic liquid

Although the majority of glasses in use in technology are complex mixtures of oxides or chalcogenides, there are numerous examples of pure substances-'glassformers'-that also fail to crystallize during cooling. Most glassformers are organic molecular systems, but there are important inorganic examples too, such as silicon dioxide and elemental selenium (the latter being polymeric). Bulk metallic glasses can now be made; but, with the exception of Zr50Cu50 (ref. 4), they require multiple components to avoid crystallization during normal liquid cooling. Two-component 'metglasses' can often be achieved by hyperquenching, but this has not hitherto been achieved with a single-component system. Glasses form when crystal nucleation rates are slow, although the factors that create the slow nucleation conditions are not well understood. Here we apply the insights gained in a recent molecular dynamics simulation study to create conditions for successful vitrification of metallic liquid germanium. Our results also provide micrographic evidence for a rare polyamorphic transition preceding crystallization of the diamond cubic phase.