Molecular dynamic simulation on the structure of sodium germanate glasses

The structure of Na₂O-GeO₂ glasses has been investigated, using molecular dynamics (MD) simulations based on an empirical three-body interaction model. A multi-body potential function with two energy minima in the angular term for a given three-atom unit was introduced to permit a transformation between different coordination states, such as 4- and 6-fold coordinated Ge sites. Parameters in the potential function were optimized with a non-linear least-squares (NLS) method. The optimal parameter set obtained with a Na₄Ge₉O₂₀ crystal was employed for the structural models of pure GeO₂ and 2Na₂O·9GeO₂ glasses. In the model for the binary glass, however, a number of 5-fold coordinated Ge, non-bridging O and 3-fold coordinated O formed. Another binary model free from 5-fold Ge was prepared using a simple scheme in which no three-body interaction was applied to the structural units containing 5-fold coordinated Ge. All the structural models produced showed good agreement with the experiments, including vibrational spectra and radial distribution functions (RDFs). No significant difference between the experimental observables of the two binary glass models was revealed, except for the coordination environment of Ge. This is because 5- and 6-fold Ge do not occur in a discrete arrangement through a corner-sharing of GeO₄, e.g. GeO_5,6-GeO₄-GeO_5,6, but in a dense configuration, e.g. as edge-sharing GeO_5,6-GeO_5,6, forming smaller rings than those in pure GeO₂ glass.