First-principles simulations of liquid silica: Structural and dynamical behavior at high pressure

Abstract

We have carried out first-principles molecular dynamics simulations of silica liquid over a wide range of pressure ͑from 0 to ϳ 150 GPa͒ and temperature ͑3000-6000 K͒ within density functional theory and the pseudopotential approximation. Our results show that the liquid structure is highly sensitive to compression: the average Si-O coordination number increases from 4 at zero pressure initially slowly on compression and then more rapidly after 30% compression, reaching 6.5 at 150 GPa. At low compression, nearly all Si-O coordination environments are fourfold and relatively undistorted, whereas at high compression several coordination types ͑five-, six-, and sevenfold͒ coexist and the polyhedra are significantly distorted. The heat capacity and Grüneisen parameter show little variation with compression within the low-pressure regime and vary rapidly with compression in the high-pressure regime. Results are successfully fitted to the Mie-Grüneisen equation of state and show no evidence of spinodal instability or a temperature of maximum density. The behavior of the self-diffusion coefficient is consistent with a crossover from strong to fragile liquid behavior with increasing temperature and increasing pressure. Both Si and O self-diffusion coefficients vary anomalously at 4000 K-they initially increase with pressure and then decrease upon further compression. This anomalous behavior is absent at higher temperatures.

Structure of molten titanium dioxide

“…We can therefore estimate the pressure of the analogous SiO 2 melt as that at which silicon is 5-fold coordinated to oxygen. In the glass this is at about 27 GPa [65,66], and is comparable to existing ab initio calculations for the melt at 3500 K and 27 GPa [67] (n SiO = 4.76) or 3000 K and 20 GPa [68] (n SiO = 4.68). Such pressures for the liquid correspond to "molten Stishovite", being well above the equilibrium phase fields for Quartz and Coesite.…”

“…Liquid TiO 2 at ambient pressure has an average n TiO close to 5, while liquid silica has n SiO = 4 [56,57]. However, as pressure is increased, it has been shown both experimentally for SiO 2 glass [65,66], and computationally for SiO 2 liquid [67,68], that the average Si-O coordination increases. We can therefore estimate the pressure of the analogous SiO 2 melt as that at which silicon is 5-fold coordinated to oxygen.…”

“…Calculated coordination number distributions for molten silica are compared to those for TiO 2 in Fig. 7a, showing some similarity, although the comparison is rather qualitative given the mismatches in average coordination number (owing to the availability of liquid silica data only at discreet pressure values [67,68]). Fig.…”

Simulating liquid and amorphous silicon dioxide using real-space pseudopotentials

“…Previous of the a-SiO 2 simulations were conducted for systems less than hundred atoms [8][9][10][11] with a quenching rate that remained around 10 15 K/s. It has been reported that while the short range of interactions are not sensitively affected by periodic constraints, medium or long range interactions such as ring statistics are sensitive to the size of the ensemble.…”

Atomic diffusion in covalent liquids under pressure fromab initiomolecular dynamics

“…The diffusivity of atoms was shown to increase with pressure, as in other covalent liquids, such as SiO 2 [4] and B 2 O 3 [[5]. It is, however, unclear how the rearrangement process of the covalent bonds is affected by compression.…”