The structure of amorphous, crystalline and liquid GeO2

Germanium dioxide (GeO2) is a chemical analogue of SiO2. Furthermore, it is also to some extent a structural analogue, as the low- and high-pressure short-range order ( tetrahedral and octahedral) is the same. However, a number of differences exist. For example, the GeO2 phase diagram exhibits a smaller number of polymorphs, and all three GeO2 phases ( crystalline, glass, liquid) have an increased sensitivity to pressure, undergoing pressure-induced changes at much lower pressures than their equivalent SiO2 analogues. In addition, differences exist in GeO2 glass in the medium-range order, resulting in the glass transition temperature of germania being much lower than for silica. This review highlights the structure of amorphous GeO2 by different experimental ( e. g., Raman and NMR spectroscopy, neutron and x-ray diffraction) and theoretical methods ( e. g., classical molecular dynamics, ab initio calculations). It also addresses the structures of liquid and crystalline GeO2, that have received much less attention. Furthermore, we compare and contrast the structures of GeO2 and SiO2, as well as along the GeO2 - SiO2 join. It is probably a very timely review, as interest in this compound, that can be investigated in the liquid state at relatively low temperatures and pressures, continues to increase.