The 4th International Conference on Matter and Radiation at Extremes (ICMRE 2019)

538 / 2019-03-20 10:40:23

Multifaceted nature of liquid-liquid transition in warm dense hydrogen

Liquid-liquid transition; High pressure; Hydrogen dissociation; Metallization

Plenary track > High Pressure Physics and Materials Science

The behavior of liquid hydrogen at high pressure has wide implications in astrophysics and high pressure physics and chemistry. The phase transformation in the liquid is variously described as a metallization, dissociation, density discontinuity or plasma phase transition (PPT). It has been tacitly assumed that these phenomena coincide at the “liquid-liquid” phase transition (LLT). In this work, the relevant pressure-temperature conditions are thoroughly explored with first-principles molecular dynamics. We show that the dissociation is a continuous process, not identical to the first-order LLT indicated by density discontinuity. Molecular dissociation is mainly governed by a pseudo-transition (thermal excitation cross-over), across a range of P and T where molecular and atomic hydrogen coexist. The width of the region gradually narrows at lower temperature, and eventually merges into the first-order LLT with its sharp volume collapse. The implication of this observation is that there is no critical phenomenon at the vanishing point of the corresponding LLT line. We also show that H/H2 are continually miscible, and that metallization may be associated with a percolation transition of atomic H at low pressure, allowing fluctuation/sloshing of charges across the system. One consequence is that partially dissociated hydrogen might exhibit strong noise in infrared and/or Raman spectra, with large transient clusters being persistently formed and destroyed. These findings could have a significant impact on the modeling of the metallic and molecular hydrogen interface/transition layer in giant gas planets, as well as on the design of H-rich compounds.