Molecular dynamics study of structural reorganization by electro-thermal poling in sodium diborate glass

This work explores by molecular dynamics simulation the basic mechanisms and pathways that pertain to the induced optical nonlinearity in an ionic oxide glass due to electro-thermal poling. To the best of our knowledge, this is the first time where the kinetics combining the ion migration and the structural rearrangements is studied by using atomistic simulation. Most of the models published so far deal mainly with the charge transport effects, with no information provided on the kinetics of structural reorganization. To this aim, a simple borate glass doped with sodium cations was employed. Notwithstanding the small size of the specimen, due to computer time limitations, we were able to identify most of the experimental trends observed in different glass systems and under different poling conditions and configurations. They include the conversion of negatively charged structural units into neutral ones in the charge depletion region, a process reducing the excess negative charge after the migration of the mobile cations to the cathode, and the increased polymerization of the network close to the anodic surface. Taking into account the restrictions of our model, we also showed a mechanism for the generation of free oxygen species, due to the partial polarization of the Boron-Non-bridging oxygen (B-NBO) bonds along the embedded internal Direct Current (DC) electric field and their subsequent elongation and weakening.