Correlation effect for dynamics in silica liquid

We study numerically the diffusion mechanism in silica liquid via molecular dynamics simulation. For this purpose we examine the evolution of structural units SiO${}_{x}$ ($x=4\ensuremath{-}6$) for different times and at temperatures from 3000 to 4500 K. Simulation shows that the diffusivity of the silicon particle is performed through the transition ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$, i.e., the bond-breaking and bond-reformation events. As a ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$ transition occurs, one oxygen particle moves out of or into the coordination shell, leading to a collective movement of Si particles. Other types of transitions, for instance, SiO${}_{4}$ \ensuremath{\rightarrow} SiO${}_{6}$ or SiO${}_{6}$ \ensuremath{\rightarrow} SiO${}_{4}$, are negligible. We establish an expression for the diffusion coefficient that shows that the diffusivity is not proportional to the rate of ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$ because it is strongly localized in the network structure. A high degree of localization of ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$ leads to a heterogeneous dynamics. We find that the dynamics slowdown is determined by two terms: The first one concerns the change in the statistic property related to the fraction of non-four-coordinated units (SiO${}_{3}$, SiO${}_{5}$, SiO${}_{6}$, and SiO${}_{7}$) and the second term concerns the correlation effect. Furthermore, we show that the correlation coefficient depends on both the fraction of the back-forth ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$ transition and the degree of localization of ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$. Our finding qualitatively supports the ideal that anomalously slow dynamics near the glass-transition point is caused by a strong localization of ${\mathrm{SiO}}_{x}\ensuremath{\rightarrow}{\mathrm{SiO}}_{x\ifmmode\pm\else\textpm\fi{}1}$.