Thermal and Pressure Ionization in Warm, Dense MgSiO$_3$ Studied with First-Principles Computer Simulations

Abstract

Using path integral Monte Carlo and density functional molecular dynamics (DFT-MD) simulations, we study the properties of MgSiO 3 enstatite in the regime of warm dense matter. We generate a consistent equation of state (EOS) that spans across a wide range of temperatures and densities (10 4 -10 7 K and 6.42-64.16 g cm −3 ). We derive the shock Hugoniot curve, that is in good agreement with the experiments. We identify the boundary between the regimes of thermal ionization and pressure ionization by locating where the internal energy at constant temperature attains a minimum as a function of density or pressure. At low density, the internal energy decreases with increasing density as the weight of free states changes. Thermal ionization dominates. Conversely, at high density, in the regime of pressure ionization, the internal energy increases with density. We determine the boundary between the two regimes and show that the compression maximum along the shock Hugoniot curve occurs because K shell electrons become thermally ionized rather than pressure ionized.

Nonideal Mixing Effects in Warm Dense Matter Studied with First-Principles Computer Simulations

“…In Figs. 2, 3, and 4, we compare the shock Hugoniot curves that were computed with the fully interacting internal energy and pressure for BN [69], MgO [111], and MgSiO 3 [118,130] with those derived from the linear mixing approximation using the elemental EOS tables. The agreement between the pairs of curves is remarkably good in pressure-density and temperature-density spaces.…”