Molecular dynamics study of liquid silica under high pressure

Abstract

Structural changes of liquid silica are investigated under high pressure by molecular dynamics simulation. It is well known that high-silica liquids display anomalous pressure-dependent behaviour in their diffusivities. The potential model, the so-called 'soft potential', is used, as it is expected to simulate the structural changes of silica at high temperature well. With increasing pressure, above the glass transition temperature, the simulated silica melt shows the so-called diffusivity maximum under a pressure of 20 GPa, as already shown by the previous studies. However, it is also found that this diffusivity maximum disappears above 2800 K. The analysis of Si coordination number suggests that the competition between the increase of five-fold and that of three-fold controls the extent of the anomaly. Secondly, the analysis of 'local oxygen packing number (LOPN)', that had been developed to investigate geometrical features in amorphous structures, is applied. In a complementary manner to the analysis of the Si coordination number, the local structure in the silica melt shows the gradual structural transformation from a low-density to high density packing on compression. Finally, a model explaining the two types of change of diffusivity in silica melt was proposed in combination with the LOPN analysis and the structon analysis that had been developed to investigate the thermal change of local structures.

Melting behavior of SiO2 up to 120 GPa

“…The relative sharpness of the structure change in the melt suggests a dominant role of the covalent character of Si-O bonds in the melt (Lin et al 2007), which could induce a significant difference in energy between the tetrahedral "SiO4" and octahedal "SiO6" electronic configurations as a function of pressure and temperature. The two different melt structures could be relatively incompatible with each other, in agreement with previous reports (Brazhkin et al 2011;Lin et al 2007;Takada et al 2016)). Figure 1: Typical temperature measurements.…”

“…We finally estimate the SiO2 LDM and HDM densities at ~3.26 10 3 and ~4.17 10 3 kg/m 3 , respectively, at 45 GPa and 5400 K (Table 2). Such LDM density is consistent with a previous theoretical report considering less than 10% Si in 6-fold coordination in the melt (Takada et al 2016).…”

“…Polyamorphism in liquid SiO2 has been extensively discussed in the past by theoretical approaches (e.g. (Brazhkin et al 2011;Lin et al 2007;San et al 2016;Takada et al 2016)). To address experimentally the structural evolution of liquid SiO2 under high pressures, we performed LH-DAC experiments coupled with the Soller slits system installed at the ID27 beamline (Weck et al 2013).…”

“…The pressure-induced Si coordination change in amorphous SiO2 was investigated experimentally at room temperature (Benmore et al 2010;Lin et al 2007;Murakami and Bass 2010;Sato and Funamori 2010) and theoretically (Meade et al 1992;Stixrude and Karki 2005;Takada et al 2016; Usui and Tsuchiya 2010). In both cases, the transformation to an octahedral-based SiO2 structure is clearly demonstrated.…”

Amorphous boron oxide at megabar pressures via inelastic X-ray scattering

“…amorphous boron oxide | extreme compression | megabar pressures | inelastic X-ray scattering W hen under extreme compression, amorphous oxides undergo structural transitions into more densely packed glassy networks that are significantly different from those at ambient pressure (e.g., refs. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. The detailed structures of the glasses at high pressure are among the remaining fundamental mysteries in modern physical sciences.…”