Co-ordination and intermediate-range order alterations in densified germania

We analyze the structure of a-GeO2, pressure-densified from a starting density of 3660 kg/m3 to a final density of 6000 kg/m3, using a combination of molecular dynamics simulation and topological analysis employing efficient local cluster ring counting algorithms. The initial modeled configuration is dominated by fourfold germanium co-ordination and rings of six or seven germanium atoms (as in cristobalite- or tridymite-like a-SiO2). The first response to increasing density is a change to larger ring (e.g. 8-ring) configurations (as in quartz-like a-SiO2) that pack more compactly. At still higher pressure, an intermediate fivefold germanium co-ordination appears that progressively converts into almost entirely sixfold (octahedral) germanium at higher pressures, accompanied by growth of the 3- and 4-rings that characterize the rutile-GeO2 crystalline polymorph. The topology of the final densified structure does not entirely resemble a rutile-like glass analogue, however, because of the retention of 6-rings and almost 10% 5-co-ordinated Ge. Overall, the present study shows how pressure (or density) affects the structure of the glass at a range of length scales.