Oxygen Packing Fraction and the Structure of Silicon and Germanium Oxide Glasses

The recently proposed relationship between the oxygen volume fraction and topological ordering in solid and liquid oxide glasses at high pressure is examined with Bader's atoms-in-molecules (AIM) theory using glass structures generated from first principles molecular dynamics calculations. It is shown that the atomic (O/Si and O/Ge) volume ratio derived from AIM theory is not constant with pressure. This finding is due to the continuous change in the electron topology under compression. Unlike crystalline solids, there is no distinctive transition pressure for Si-O and Ge-O coordination in a glass; instead, the changes are gradual and continuous over a broad pressure range. Therefore, relating a unique Si-O or Ge-O coordination number to the properties of the glass at a given pressure is difficult.