First-Principles Prediction of the Softening of the Silicon Shock Hugoniot Curve

TL;DR

This study uses first-principles methods to predict silicon's shock response at extremely high pressures, revealing it is significantly softer than previous models suggested, with implications for high-pressure physics.

Contribution

It combines OFMD, PIMC, and Kohn-Sham DFT to accurately predict silicon's EOS and shock behavior at pressures up to above 10 Gbar, highlighting a previously unaccounted softening.

Findings

Silicon is 20% softer than existing EOS models predict.

PIMC reveals a second compression maximum due to 1s electron ionization.

Experimental data at 1-2 Mbar support the predicted softening.

Abstract

Shock compression of silicon (Si) under extremely high pressures (>100 Mbar) was investigated by using two first-principles methods of orbital-free molecular dynamics (OFMD) and path integral Monte Carlo (PIMC). While pressures from the two methods agree very well, PIMC predicts a second compression maximum because of 1s electron ionization that is absent in OFMD calculations since Thomas-Fermi-based theories lack shell structure. The Kohn-Sham density functional theory is used to calculate the equation of state (EOS) of warm dense silicon for low-pressure loadings (P < 100 Mbar). Combining these first-principles EOS results, the principal shock Hugoniot curve of silicon for pressures varying from 1 Mbar to above 10 Gbar was derived. We find that silicon is 20% or more softer than what was predicted by widely-used EOS models. Existing high-pressure experimental data (P = 1 - 2 Mbar) seem to indicate this softening behavior of Si, which calls for future strong-shock experiments (P > 10 Mbar) to benchmark our results.