Ab initio constraints on silica melting to 500 GPa

TL;DR

This study uses ab initio methods to determine silica's melting curve up to 500 GPa, revealing a smooth increase without abrupt transitions and insights into the structural changes in the liquid state under high pressure.

Contribution

It provides the first ab initio melting curve of silica up to 500 GPa, showing a gradual increase and detailed structural insights into the liquid phase at extreme pressures.

Findings

Melting curve increases smoothly from 50 to 500 GPa

No abrupt changes at 120 GPa and 300 GPa

Structural correlation in liquid is short-ranged with increased Si coordination

Abstract

The melting curve of pure silica (SiO$_2$) was determined using {\it ab initio} density functional theory together with the solid-liquid coexisting approach, thermodynamic integration and the Z method. The melting curves are consistent with a smooth slow increase in a large region from 50 GPa (dT/dP $\approx$ 15 K/GPa) to about 500 GPa (dT/dP $\approx$ 5 K/GPa) without any abrupt changes at around 120 GPa and 300 GPa as seen in some recent experimental and computational studies. The topography of the melting curve above 50 GPa is consistent with a gradual change in the distribution of the Si coordination numbers in the liquid state and the absence of large changes in the density following solid-solid phase transitions. The pair distribution functions show that the structural correlation in the liquid is mainly short-ranged and that the Si-O bond is stiff. The densification of the melt structure with pressure above 50 GPa is therefore due to an increase in 7- and 8-fold coordinated silicon.