Critical point and mechanism of the fluid–fluid phase transition in warm dense hydrogen

Abstract

The density functional theory is used to calculate the equation of state and the proton–proton pair correlation functions in the range of hydrogen temperatures and densities where the fluid–fluid phase transition is expected. The metastable states are considered. The critical temperature has been estimated to be ~4000 K. We propose a two-step mechanism: the partial ionization of molecules to produce H₂⁺ ions at the phase transition followed by the formation of H₃⁺ ions.