Origin of the decoupling of oxygen and silicon dynamics in liquid silica as expressed by its potential energy landscape.

The oxygen and silicon dynamics in silica are compared via computer simulations. In agreement with experimental data and previous simulations a decoupling of oxygen and silicon dynamics is observed upon cooling. The origin of this decoupling is studied in the framework of the potential energy landscape. From analysis of the transition features between neighboring superstructures of minima, denoted metabasins, the differences between the oxygen and silicon dynamics can be quantified. The decoupling can be explicitly related to the presence of generalized rotational processes, giving rise to oxygen but not to silicon displacement. Closer analysis of these processes yields important insight into the nature of the potential energy landscape of silica. The physical picture of relaxation processes in silica, obtained in previous work for oxygen dynamics, is consistent with decoupling effects, elucidated here.