Network cavity, spatial distribution of sodium and dynamics in sodium silicate melts

Molecular dynamics simulation is carried out for studying the structure and dynamics of sodium silicate melts using the network cavity (NC), NF (network former) cluster and NC cluster. The simulation shows that an NC contains up to six Na, and its radius varies from 1.4 to 4.5 Å. The number of Na atoms located in NC depends strongly on the constituent content of NC-forming atoms. The simulation also reveals that Na and O form the chemical bond. The static structure is found to be heterogeneous with separate Na-poor and Na-rich regions formed by different-type NC clusters, the number and size of which vary with SiO2 content. We also find the sodium deficit around Si and sodium surplus around O. As the status of O changes, Na atoms are redistributed between vicinity spaces of network former (VSNFs). The dynamical structure is heterogeneous with separate regions occupied by an NF cluster of high-sodium-density atoms and a number of NF clusters of low-sodium-density atoms. During hundreds of picoseconds, the sodium atoms are not uniformly distributed throughout VSNFs, but they prefer to move along diffusion pathways. In the SiO2-rich model, the diffusion pathways emerge clearly, while in the SiO2-poor model, these diffusion pathways disappear.