Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO2 glass up to 17.5 GPa

The structure of GeO2 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 sime 5 GPa, is associated with a reorganization of GeO4 units. The second, extending over the range from sime 5 to 10 GPa, corresponds to a regime where GeO4 units are replaced predominantly by GeO5 units. In the third, as the pressure increases beyond ~10 GPa, appreciable concentrations of GeO6 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 GeO2 occur at similar dimensionless number densities to those found for SiO2, indicating similar densification mechanisms for both glasses. This implies a regime from about 15 to 24 GPa where SiO4 units are replaced predominantly by SiO5 units, and a regime beyond ~24 GPa where appreciable concentrations of SiO6 units begin to form.