Structural anomaly in sodium germanate glasses by molecular dynamics simulation

The structures of GeO2–Na2O glasses with different Na2O contents, from 0–30 mol%, have been simulated by molecular dynamics (MD) and were found to be in good agreement with the existing experimental results. The Ge–O coordination number was found to increase initially with the addition of alkali oxide. The results indicate that this increase in coordination number was due to the conversion of some of the tetrahedral germanate units into 5-fold coordinated trigonal bipyramidal units. Further additions of alkali oxide, beyond the glass composition range where maxima (or minima) in certain physical properties are experimentally observed (∼18 mol% Na2O), led to the formation of non-bridging oxygen (NBO) atoms and to a decrease of the average Ge–O coordination number. The reconversion to tetrahedral geometry was found to be responsible for the observed decrease in Ge–O coordination. A simple scheme was used to analyse the populations of various structural entities present in the glass system. It is shown that the conversion of 4-fold to 5-fold coordinated units and their back-conversion, with increasing alkali oxide contents, are feasible and consistent.