Density-driven structural transformations in network forming glasses: A high-pressure neutron diffraction study of GeO ₂ glass up to 17.5GPa

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 5GPa, is associated with a reorganization of GeO ₄ units. The second, extending over the range from 5 to 10GPa, corresponds to a regime where GeO ₄ units are replaced predominantly by GeO ₅ units. In the third, as the pressure increases beyond 10GPa, 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 30GPa, 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 24GPa where SiO ₄ units are replaced predominantly by SiO ₅ units, and a regime beyond 24GPa where appreciable concentrations of SiO ₆ units begin to form.