GeO2 vs SiO2: Glass transitions and thermodynamic properties of polymorphs

Relative-enthalpy measurements have been made on the hexagonal, tetragonal, glass and liquid phases of GeO₂. The glass transition is very sensitive to the impurity content, with a T _g ranging from 980 K for a pure product to 780 K for a Li-doped sample with 0.06 mol % Li. The relative C _p change at T _g of about 5% increases with the impurity content as a result of lower glass transition temperatures. Above 298 K the derived heat capacities are similar for all forms, with slightly higher values for the amorphous phases and two C _p cross-overs at 400 and 1000 K between the hexagonal and tetragonal modifications. For both GeO₂ and SiO₂ the coordination state markedly affects C _p and the entropy below 300 K, where the properties are much lower for the tetragonal than for the hexagonal modifications, i.e., S ₂₉₈ = 39.7 vs 55.3 J/mole K and 27.8 vs 41.4 J/ mole K for GeO₂ and SiO₂, respectively. The high-temperature C _p's of coesite and stishovite are likely similar to those of the low-pressure SiO₂ forms. Finally, these results, low-temperature C _p data and enthalpy-of-solution measurements have been used to derive a consistent set of thermodynamic properties for the GeO₂ modifications.