Direct MD simulation of liquid-solid phase equilibria for two-component plasmas

TL;DR

This study uses molecular dynamics simulations to accurately determine the liquid-solid phase diagrams of two-component plasmas, revealing detailed diffusion behaviors and validating previous models for carbon-oxygen systems, with new insights into oxygen-selenium mixtures relevant to neutron star crusts.

Contribution

First comprehensive MD simulation-based phase diagrams for two-component plasmas, including finite size effects and diffusion analysis, with validation against existing models and new findings for oxygen-selenium systems.

Findings

Carbon-oxygen phase diagram matches previous predictions, confirming accuracy.

Oxygen diffusion in selenium-rich solids is surprisingly fast.

Oxygen-selenium melting temperature is lower than prior estimates, likely due to electron screening.

Abstract

We determine the liquid-solid phase diagram for carbon-oxygen and oxygen-selenium plasma mixtures using two-phase MD simulations. We identified liquid, solid, and interface regions using a bond angle metric. To study finite size effects, we perform 27648 and 55296 ion simulations. To help monitor non-equilibrium effects, we calculate diffusion constants $D_i$. For the carbon-oxygen system we find that $D_O$ for oxygen ions in the solid is much smaller than $D_C$ for carbon ions and that both diffusion constants are 80 or more times smaller than diffusion constants in the liquid phase. There is excellent agreement between our carbon-oxygen phase diagram and that predicted by Medin and Cumming. This suggests that errors from finite size and non-equilibrium effects are small and that the carbon-oxygen phase diagram is now accurately known. The oxygen-selenium system is a simple two-component model for more complex rapid proton capture nucleosynthesis ash compositions for an accreting neutron star. Diffusion of oxygen, in a predominately selenium crystal, is remarkably fast, comparable to diffusion in the liquid phase. We find a somewhat lower melting temperature for the oxygen-selenium system than that predicted by Medin and Cumming. This is probably because of electron screening effects.