First-principles molecular dynamics simulations of MgSiO3 glass: Structure, density, and elasticity at high pressure

Abstract

We report a first-principles molecular dynamics study of the equation of state, structural, and elastic properties of MgSiO 3 glass at 300 K as a function of pressure up to 170 GPa. We explore two different compression paths: cold compression, in which the zero pressure quenched glass is compressed at 300 K, and hot compression, in which the liquid is quenched in situ at high pressure to 300 K. We also study decompression and associated irreversible densification. Our simulations show that the glass at the zero pressure is composed of primarily Si-O tetrahedra, partially linked with each other via the bridging O atoms (present in 35%; the remaining being the non-bridging O atoms). With increasing pressure, the mean Si-O coordination number gradually increases to 6, with fivefold and subsequently sixfold replacing tetrahedra as the most abundant coordination environment. The Mg-O coordination comprising of a mixture of four-, five-, and sixfold species at zero pressure picks up more high-coordination (seven-to ninefold) species on compression and its mean value increases from 4.5 to 8 over the entire pressure range studied. Consistently, the anion-cation coordination numbers increase on compression with appearance of oxygen tri-clusters (three silicon coordinated O atoms) and mean O-Si coordination eventually reaching 2. Hot compression produces greater densities and higher coordination numbers at all pressures as compared with cold compression, reflecting kinetic hindrances to structural changes. On decompression from 6 GPa, the glass regains its initial uncompressed structure with almost no residual density. Decompression from 27 GPa produces significant irreversible compaction, and the peak-pressure of decompression significantly influences the degree of density retention with as high as 15% residual density on decompression from 170 GPa. Irreversibility arises from the survival of high coordination species to zero pressure on decompression. With increasing pressure, the calculated compressional and shear wave velocities (about 5 and 3 km/s at the ambient conditions) of MgSiO 3 glass increase initially rapidly and then more gradually at high pressures. Our results suggest that hot-compressed glasses perhaps provide closer analog to high-pressure silicate melts than the glass on cold compression.

Fate of MgSiO 3 melts at core–mantle boundary conditions

“…In contrast, our data agree well with recent ab initio molecular dynamic (MD) simulations ( Fig. 2A) (19). MgSiO 3 glass is as compressible as MgSiO 3 melt with a K T0 = 16.9 ± 3.2 GPa vs. K T0 = 20 GPa (20), respectively.…”

“…S10). Our experimental isotherms agree with the isotherms from MD simulations along different isotherms (19,21,23) (Fig. 3A).…”

“…The bulk moduli also exhibit a large discrepancy: K T0 = 16.9 ± 3.2 GPa in our study compared with K T0 = 78.4 ± 0.6 GPa determined from the Brillouin data (18). The densities recalculated from Brillouin data make use of the unrelaxed sound velocities (19). As a result, only elastic, but not the configurational, contributions to the compressibility are probed by Brillouin scattering, and the compressibility is underestimated.…”

“Free” oxide ions in silicate melts: Thermodynamic considerations and probable effects of temperature

“…100-1000) potentially leading to direct observability of both concentrations and T effects. As for any "high energy" melt species that may become much more abundant in high temperature liquids relative to the glass, FO are expected to become more prominent in molecular dynamics (MD) computer simulations, which generally represent structures above 2500 to 3000 K (Ghosh et al, 2014;Nesbitt et al, 2015a). This kind of rough thermodynamic calculation may indeed be one way to better link MD results to observations in real glasses.…”

“…6000 K vs. 3000 K (Bajgain et al, 2015). In both ab initio (Ghosh et al, 2014;Spiekermann et al, 2013) and classical (Al-Hasni and Mountjoy, 2014; Kapoor et al, 2015;Spera et al, 2011) MD simulations of liquids such as MgSiO 3 and CaMgSi 2 O 6 , a few % of FO are typically reported at high temperatures or in simulated quenched glasses. Because "cooling" rates in such calculations are typically many orders of magnitude higher than any obtainable in the laboratory (e.g.…”