The Thermodynamical Instability Induced by Pressure Ionization in Fluid Helium

TL;DR

This study investigates pressure ionization in fluid helium using chemical models, revealing that the thermodynamical instability depends on the free energy formulation, with implications for understanding phase transitions in dense fluids.

Contribution

It demonstrates how different constructions of the free energy function affect the manifestation of pressure ionization and thermodynamical instability in fluid helium.

Findings

Pressure ionization can induce thermodynamical instability with a first-order phase transition signature.

Modified Coulomb free energy models can suppress the instability, aligning results with first principles simulations.

The behavior of pressure ionization in helium varies significantly with the chosen chemical model.

Abstract

A systematic study of pressure ionization is carried out in the chemical picture by the example of fluid helium. By comparing the variants of the chemical model, it is demonstrated that the behavior of pressure ionization depends on the construction of the free energy function. In the chemical model with the Coulomb free energy described by the Pad\'e interpolation formula, thermodynamical instability induced by pressure ionization is found to be manifested by a discontinuous drop or a continuous fall and rise along the pressure-density curve as well as the pressure-temperature curve, which is very much like the first order liquid-liquid phase transition of fluid hydrogen from the first principles simulations. In contrast, in the variant chemical model with the Coulomb free energy term empirically weakened, no thermodynamical instability is induced when pressure ionization occurs, and the resulting equation of state achieves good agreement with the first principles simulations of fluid helium.