Mechanisms of network collapse in GeO sub(2) glass: high-pressure neutron diffraction with isotope substitution as arbitrator of competing models

The structure of the network forming glass GeO sub(2) is investigated by making the first application of the method of in situ neutron diffraction with isotope substitution at pressures increasing from ambient to 8 GPa. Of the various models, the experimental results are in quantitative agreement only with molecular dynamics simulations made using interaction potentials that include dipole-polarization effects. When the reduced density [rho]/[rho] sub(0) [>, ~] 1:16, where [rho] sub(0) is the value at ambient pressure, network collapse proceeds via an interplay between the predominance of distorted square pyramidal GeO sub(5) units versus octahedral GeO sub(6) units as they replace tetrahedral GeO sub(4) units. This replacement necessitates the formation of threefold coordinated oxygen atoms and leads to an increase with density in the number of small rings, where a preference is shown for sixfold rings when [rho]/[rho] sub(0) = 1 and fourfold rings when [rho]/[rho] sub(0) = 1.64.