Phase diagram of hydrogen at high pressure: melting line and plasma phase transition

Miguel Morales, University of Illinois, Urbana-Champaign

Although hydrogen is the simplest atom in the periodic table, its phase diagram has many diverse and interesting phases. At low pressures it is stable in a molecular form, but as pressure in increased it dissociates into its monoatomic state. Despite intense work and debate from theory and experiment during the last 30 years, there is still no agreement regarding the nature and the location of this dissociation regime, both in the solid as well as in the liquid phase. I will present my work on the dissociation regime using first-principles simulations with Quantum Monte Carlo and Density Functional Theory (DFT). My results agree with the existence of a first-order phase transition at low temperatures ending in a critical point at a temperature of approximately 2000K and a pressure of 85 GPa. Above this temperature, dissociation occurs smoothly without any discontinuities in thermodynamic properties, but below there is a volume discontinuity associated with the sharp transition. I will also present calculations of the melting line of solid molecular hydrogen from free energy calculations in both liquid and solid states using DFT and including zero-point energy of the nuclei.

Abstract Author(s): Miguel A. Morales