Predicted reentrant melting of dense hydrogen at ultra-high pressures

Geng, Hua-Yun; Wu, Qiang

doi:10.1038/srep36745

Cited by 17 publications

(24 citation statements)

References 38 publications

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“…Its value of 950 K is lower than the classical case (as suggested in the Figure 3 of the 24 main text) where NQE has been excluded, and is consistent with the general expectation that zero-point motion (or generally the NQE) prefers isotropic states, thus would lower the transition temperature.…”

Section: Projected Pair Correlation Functionsupporting

confidence: 85%

Prediction of a Mobile Solid State in Dense Hydrogen under High Pressures

Geng

Sun

2016

J. Phys. Chem. Lett.

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The solid state of matter is usually characterized by the structural rigidity and spontaneous resistance to the change in its shape against external forces. The particles in a typical solid are tightly bound to each other, and vibrate around their equilibrium sites. In a crystalline state, particles distribute in a regular geometric lattice, thus giving long-range positional ordering.1 Amorphous or glassy states are also considered as solid, except there particles distribute irregularly. Usually a crystalline state must be a solid, and to crystallize is always equivalent to make a solid. The liquid state, on the other hand, is flowing, thus being homogeneous and isotropic. It is absence of any long-range spatial ordering, which is a consequence of the free mobility of particles at both microscopic and macroscopic scales in this state. with regular solids with defects or dislocations at similar temperatures. 11-13 By carefully inspecting the atomic structure and MD trajectories, we failed to find any well-defined dislocations or grain boundaries. Instead, it looks like all atoms participate in the diffusion process concertedly, though not simultaneously. Thus it is a bulk behavior, rather than migration confined to dislocations or boundaries. Because we know that bulk flowing is an intrinsic characteristic of liquid, this raises the speculation that in this phase the protons could be considered as a flow as in a liquid.This observation is consistent with previous simulations, where it was taken as a normal liquid. 14 However, we noticed that though the calculated radial distribution function (RDF) gives a feature very similar to a typical liquid, a distorted lattice pattern still presents through the long-time averaged proton density distribution (the inset in the left panel of Fig. 1). This is at odds with the assumption that it is in an isotropic liquid state. Furthermore, the projected RDFs 15 (right panel in Fig. 1) unveil a strong anisotropy and long-range positional ordering. This is quite different from our understanding about a typical liquid, in which the particle density distribution must not have any local or long-range features and the system must be isotropic. The unexpected emergence of anisotropy implies that the matter still have some crystalline 6 features, which is a strong indicator for a solid state. For these reasons, it seems plausible to tentatively interpret it as a novel matter state in which the solid and liquid features coexist harmonically in one pure phase.On the other hand, the observed structure pattern and anisotropy are maintained over a wide temperature range. As shown in Fig. 1 that was calculated with AI-PIMD simulations at 100 K, the increase of temperature does not change the structure pattern very much, and the accumulated RDF and projected RDFs are almost the same as that at 50 K. But interestingly, the rMSD (thus the self-diffusion mobility) is greatly enhanced. In other words, in this phase the atomic diffusivity can be enhanced without at the expense of the solid framew...

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Section: Projected Pair Correlation Functionsupporting

confidence: 85%

Prediction of a Mobile Solid State in Dense Hydrogen under High Pressures

Geng

Sun

2016

J. Phys. Chem. Lett.

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“…The observed anomalous plummet as shown in Fig. 5 not only originates in the H2 dissociation (the small and light atomic H drifts faster than the big and heavy H2 molecule), but also relates to the fact that the studied dissociation region is very close to the anomalous melting curve of dense hydrogen that continuously decreases with increasing pressure [3]. When far away from this anomalous region, the proton viscosity returns to normal behavior, increasing with pressure.…”

Section: Assessment Of the Viscosity In The Vicinity Of Dissociatmentioning

confidence: 85%

“…Rich physics and chemistry have been discovered, and are still being predicted in both pure hydrogen [1][2] [3] and hydrogen-rich compounds [4][5] [6] [7]. Dense solid hydrogen shows an unexpectedly complicated phase diagram [8][9][10] [11][12] [13][14] [15] with an anomalous melting curve maximum and minimum [3][16] [17][18] [19][20] [21], embodying solid states based around free rotating molecules (Phase I), broken symmetry due to quadrupole interactions (Phase II), packing of weakly bonded molecules (Phase III), and proposed "mixed" state (phases IV and V). The latter two phases have alternating layers of rotating molecules similar to Phase I, and weak molecules akin to Phase III [1] [13][14] [15].…”

Section: Introductionmentioning

confidence: 99%

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

Geng

Wu²,

Marqués

et al. 2019

Phys. Rev. B

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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 Supplementary Material. Results of dimer-dimer bond-length distribution (DDLD) calculations, comparison between PBE and vdW-DF results, the thermodynamic modelling of pseudo-transition and phase boundaries, calculated isotherms (equation of state, EOS), transient clusters and their lifetime and charge state, the angular distribution function of H3 clusters, mixing of H and H2 liquid, thermal fluctuation analysis, isotope effect, finite size effects, and the assessment of proton selfdiffusivity and viscosity.

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“…The discovery of phase IV [12,13] has recently reignited interest in the field. As a result of this discovery, the phase diagrams of hydrogen and deuterium above 180 GPa and at 300 K have been extensively studied experimentally [14][15][16][17][18][19][20][21] and theoretically [22][23][24]. The experimental studies radically expanded both phase diagrams by pushing the achievable P-T conditions to new limits cf.…”

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confidence: 99%