The effects of alkali cation mass and radii on the density of alkali germanate and alkali germano-phosphate glasses

A quantitative density model for alkali germanate and alkali germano-phosphate glasses has been developed based on Raman spectra and density data. The model demonstrates that for lighter/smaller cations such as lithium and sodium, the transition from 4- to 3-membered GeO4 rings is the prominent structural change responsible for changes in the density trends with increasing alkali oxide concentrations. For the heavier/larger cations such as potassium and rubidium, the 4- to 3-membered ring transition remains important but other factors must be considered. These include increases in density due to additional mass of the alkali oxides and decreases in density due to depolymerization of the glass network. The model shows that these combined factors can account for the ‘germanate anomaly’ noted in these glasses.