Thermodynamic properties and atomistic structure of the dry amorphous silica surface from a reactive force field model

Abstract

A force field model of the Keating type supplemented by rules to break, form, and interchange bonds is applied to investigate thermodynamic and structural properties of the amorphous ${\text{SiO}}_{2}$ surface. A simulated quench from the liquid phase has been carried out for a silica sample made of 3888 silicon and 7776 oxygen atoms arranged on a slab $\ensuremath{\sim}40\text{ }\text{\AA{}}$ thick, periodically repeated along two directions. The quench results into an amorphous sample, exposing two parallel square surfaces of $\ensuremath{\sim}42\text{ }{\text{nm}}^{2}$ area each. Thermal averages computed during the quench allow us to determine the surface thermodynamic properties as a function of temperature. The surface tension turns out to be $\ensuremath{\gamma}=310\ifmmode\pm\else\textpm\fi{}20\text{ }\text{erg}/{\text{cm}}^{2}$ at room temperature and $\ensuremath{\gamma}=270\ifmmode\pm\else\textpm\fi{}30$ at $T=2000\text{ }\text{K}$, in fair agreement with available experimental estimates. The entropy contribution $T{s}_{s}$ to the surface tension is relatively low at all temperatures, representing at most $\ensuremath{\sim}20\mathrm{%}$ of the surface energy. Almost without exceptions, Si atoms are fourfold coordinated and oxygen atoms are twofold coordinated. Twofold and threefold rings appear only at low concentration and are preferentially found in proximity of the surface. Above the glass temperature ${T}_{g}=1660\ifmmode\pm\else\textpm\fi{}50\text{ }\text{K}$, the mobility of surface atoms is, as expected, slightly higher than that of bulk atoms. The computation of the height-height correlation function shows that the silica surface is rough in the equilibrium and undercooled liquid phase, becoming smooth below the glass temperature ${T}_{g}$.