Thermodynamic anomalies and three distinct liquid-liquid transitions in warm dense liquid hydrogen

Hua Y Geng, Q Wu, Miriam Marques, Graeme Ackland

Research output: Contribution to journalArticlepeer-review


The properties of hydrogen at high pressure have wide implications in astrophysics and high-pressure physics. Its phase change in the liquid is variously described as a metallization, H2-dissociation, density discontinuity or plasma phase transition. It has been tacitly assumed that these phenomena coincide at a first-order liquid-liquid transition (LLT). In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show there is a large dependency on exchange-correlation functional and significant finite size effects. We use hysteresis in a number of measurable quantities to demonstrate a first-order transition up to a critical point, above which molecular and atomic liquids are indistinguishable. At higher temperature beyond the critical point, H2-dissociation becomes a smooth cross-over in the supercritical region that can be modelled by a pseudo-transition, where the H2→2H transformation is localized and does not cause a density discontinuity at metallization. Thermodynamic anomalies and counter-intuitive transport behavior of protons are also discovered even far beyond the critical point, making this dissociative transition highly relevant to the interior dynamics of Jovian planets. Below the critical point, simulation also reveals a dynamic H2↔2H chemical equilibrium with rapid interconversion, showing that H2 and H are miscible. The predicted critical temperature lies well below the ionization temperature. Our calculations unequivocally demonstrate that there are three distinct regimes in the liquid-liquid transition of warm dense hydrogen: A first order thermodynamic transition with density discontinuity and metallization in the sub-critical region, a pseudo-transition cross-over in the super-critical region with metallization without density discontinuity, and finally a plasma transition characterized by ionization process at very high temperatures. This feature and the induced anomalies originate in the dissociative transition nature that has a negative slope in the phase boundary, which is not unique to hydrogen, but a general characteristic shared by most dense molecular liquid. The revealed multifaceted nature of this dissociative transition could have an impact on the modeling of gas planets, as well as for the design of H-rich compounds.
Original languageEnglish
JournalPhysical Review B
Issue number13
Publication statusPublished - 28 Oct 2019


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