title: Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO2 glass up to 17.5 GPa.
abstract: The structure of GeO(2) glass was investigated at pressures up to 17.5(5) GPa using in situ time-of-flight neutron diffraction with a Paris-Edinburgh press employing sintered diamond anvils. A new methodology and data correction procedure were developed, enabling a reliable measurement of structure factors that are largely free from diamond Bragg peaks. Calibration curves, which are important for neutron diffraction work on disordered materials, were constructed for pressure as a function of applied load for both single and double toroid anvil geometries. The diffraction data are compared to new molecular-dynamics simulations made using transferrable interaction potentials that include dipole-polarization effects. The results, when taken together with those from other experimental methods, are consistent with four densification mechanisms. The first, at pressures up to approximately equal 5 GPa, is associated with a reorganization of GeO(4) units. The second, extending over the range from approximately equal 5 to 10 GPa, corresponds to a regime where GeO(4) units are replaced predominantly by GeO(5) units. In the third, as the pressure increases beyond ~10 GPa, appreciable concentrations of GeO(6) units begin to form and there is a decrease in the rate of change of the intermediate-range order as measured by the pressure dependence of the position of the first sharp diffraction peak. In the fourth, at about 30 GPa, the transformation to a predominantly octahedral glass is achieved and further densification proceeds via compression of the Ge-O bonds. The observed changes in the measured diffraction patterns for GeO(2) occur at similar dimensionless number densities to those found for SiO(2), indicating similar densification mechanisms for both glasses. This implies a regime from about 15 to 24 GPa where SiO(4) units are replaced predominantly by SiO(5) units, and a regime beyond ~24 GPa where appreciable concentrations of SiO(6) units begin to form.
authors.affiliation: Department of Physics, University of Bath, Bath, UK. p.s.salmon@bath.ac.uk
authors.forename: Philip S
authors.initials: PS
authors.lastname: Salmon
authors.forename: James W E
authors.initials: JW
authors.lastname: Drewitt
authors.forename: Dean A J
authors.initials: DA
authors.lastname: Whittaker
authors.forename: Anita
authors.initials: A
authors.lastname: Zeidler
authors.forename: Kamil
authors.initials: K
authors.lastname: Wezka
authors.forename: Craig L
authors.initials: CL
authors.lastname: Bull
authors.forename: Matthew G
authors.initials: MG
authors.lastname: Tucker
authors.forename: Martin C
authors.initials: MC
authors.lastname: Wilding
authors.forename: Malcolm
authors.initials: M
authors.lastname: Guthrie
authors.forename: Dario
authors.initials: D
authors.lastname: Marrocchelli