Local structures of MD-modeled vitreous silica and sodium silicate glasses

Structures of vitreous silica and sodium silicate glasses have been simulated using different potential models and compared with the neutron diffraction data, with good agreement. The environments of O, Si and Na have been thoroughly examined and compared with those in both crystalline structures and experimental glasses. Considerable similarities in local structures exist between crystalline and simulated glass structures in terms of bond length, coordination number and bond angle distribution (BAD). No significant environment changes were found, except for the coordination number (CN) variation due to composition changes. Further analysis suggests that a weaker interaction between Na and non-bridging oxygen (NBO) exists in the modeled glass structures, perhaps due to use of the same charge for both the bridging and non-bridging oxygens. On the other hand, the existence of fivefold silicon sites in sodium silicate glasses implies that these glasses more resemble high-pressure structures. In addition, the observation of neighboring Na around the fivefold silicons suggests that these fivefold silicons are transition state complexes. Examination of 2- and 3-rings reveals that these smallest rings are closely associated with the fivefold silicons, and are only intermediate states where oxygens transfer from one silicon site to another.