Abstract
The phase diagram of hydrogen is one of the most important challenges in high-pressure physics and astrophysics. Especially, the melting of dense hydrogen is complicated by dimer dissociation, metallization and nuclear quantum effect of protons, which together lead to a cold melting of dense hydrogen when above 500 GPa. Nonetheless, the variation of the melting curve at higher pressures is virtually uncharted. Here we report that using ab initio molecular dynamics and path integral simulations based on density functional theory, a new atomic phase is discovered, which gives an uplifting melting curve of dense hydrogen when beyond 2 TPa, and results in a reentrant solid-liquid transition before entering the Wigner crystalline phase of protons. The findings greatly extend the phase diagram of dense hydrogen, and put metallic hydrogen into the group of alkali metals, with its melting curve closely resembling those of lithium and sodium.
Highlights
Under compression conditions, hydrogen and alkali metals have unfolded exotic and fascinating properties, and have attracted broad attention and interests[1,2,3,4,5,6]
When classical nuclear motions are included by using ab initio molecular dynamics (AIMD) simulations, the Cmcm phase spontaneously collapses into C2221 phase at an equilibrated temperature as low as 20 K. This transition keeps the orthorhombic lattice unchanged but the symmetry is lowered by local atomic displacements, implying that dense hydrogen still disfavors highly symmetric structures even at such high pressures
The electron localization function (ELF) as shown in Fig. 2(a) clearly unveils that the stability of this phase closely relates to the competition between residual chemical bonding and renascent metallic interactions: the Cmcm phase shows obvious H2 bonding within the H3 unit, whereas such pairing does not present in the C2221 structure
Summary
Hydrogen and alkali metals have unfolded exotic and fascinating properties, and have attracted broad attention and interests[1,2,3,4,5,6]. This transition keeps the orthorhombic lattice unchanged but the symmetry is lowered by local atomic displacements, implying that dense hydrogen still disfavors highly symmetric structures even at such high pressures.
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