Density of alkali germanate glasses related to structure

The structural origin of the `germanate anomaly' effect remains a matter of controversy. We employ in this study a quantitative description of the germanate network structure to simulate the experimental density of potassium germanate glasses with K 2O contents up to 40 mol%. The results show that the short range order structure, including the change of germanium co-ordination number from four to six, cannot reproduce the maximum in density. The `germanate anomaly' effect seems to involve both short and medium range order structural changes as metal oxide is added to GeO 2.