Silica is the predominant compound in the Earth’s interior, where it experiences extreme pressure and temperature conditions similar to those accessed at shock conditions. Therefore, the response of silica to shock compression is of great interest to researchers in many areas including physics, astrophysics, and geosciences. The microscopic origin of the anomalous behavior of silica subjected to high pressure compression, particularly, to shock compression has attracted significant research efforts, yet remains the subject of controversy. The existing theories predominantly invoke network topology and associated polymorphism considerations to explain the observed response of silica to extreme pressure and temperature conditions. In this work we present theoretical evidence of the existence of core-softening in the oxygen subsystem of silica and reveal its important role in forming the anomalous behavior of shocked silica. We utilize a soft-core (SC) interatomic potential for silica recently reported by us. The SC model is derived in numerical form by force-matching microscopic data sampled in <i>ab initio</i> MD simulations of liquid silica. The derived potential suggests softening of the oxygen-oxygen repulsion at short interatomic separations. The part of the global potential energy surface originating from O-O core-softening can be accessed at static or shock compressions to 9 GPa, which is close to Hugoniot elastic limit. To elucidate the role of the O-O core-softening more accurately we compared the SC model with its SC-free variant in which the SC is suppressed. We calculate the Hugoniot equation-of-state and room temperature compression isotherm up to 150 GPa and compare the results to available experimental data. The SC model shows much better performance compared to SC-free model with respect to reproducing the strong shock and other high pressure properties. We determine that the much improved performance of the SC model is due to a better description of high-density polymorphs which adopt higher oxygen coordination geometries due to O-O core-softening. Therefore, we conclude that the core-softening of the inter-ion repulsion within oxygen subsystem is an important factor in developing the anomalous shock-wave properties in silica.

Simulation of shock-wave properties in fused silica using soft-core potential

253rd National Meeting (2017)

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