Density-driven structural transformations in network forming glasses: a high-pressure neutron diffraction study of GeO₂glass 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.

Pressure-induced structural changes in the network-forming isostatic glassGeSe4: An investigation by neutron diffraction and first-principles molecular dynamics

“…13(d). The presence of large proportions of AA, BB, and AB configurations is confirmed by recent investigations using 77 Se nuclear magnetic resonance (NMR) spectroscopy [22,56,61], and the presence of edge-sharing Ge(Se 1/2 ) 4 tetrahedra with AA configurations is supported by Raman spectroscopy experiments [12,15,21,22].…”

“…The pressure versus load calibration curves were taken from Ref. [77], and the measurement and data analysis procedures are described in detail elsewhere [35,[76][77][78]. In the case of the PEARL experiments, the data analysis procedure employed a Lorentzian function to extrapolate a measured S N (k) function to k = 0 for use in Eq.…”

“…In the case of the PEARL experiments, the data analysis procedure employed a Lorentzian function to extrapolate a measured S N (k) function to k = 0 for use in Eq. (4) [77].…”

Packing and the structural transformations in liquid and amorphous oxides from ambient to extreme conditions

“…To address the structural role played by oxide ions, we first consider the transformations associated with cold compression (i.e., pressurization at ambient temperature) of the prototypical network-forming glasses B 2 O 3 , SiO 2 , and GeO 2 , where significant changes to their ambient pressure structures have now been characterized by using a variety of in situ experimental methods (17)(18)(19)(20)(21)(22)(23). These transformations offer an ideal starting point because O-O interactions are expected to become ever more important with increasing density (12).…”

Atomistic insight into viscosity and density of silicate melts under pressure

“…This packing model is essentially the same as the normalized number density concept presented in ref. 33, which finds that GeO 2 and SiO 2 glasses increase coordination at the same packing fraction. The present work broadens the concept by accounting for volumes of modifier M (M ¼ Na, Mg, Ca and so on), so that the analysis can be applied to more complex systems.…”