First-order liquid-liquid phase transition in dense hydrogen

Winfried Lorenzen, Bastian Holst, and Ronald Redmer

Phys. Rev. B 82, 195107 – Published 9 November 2010

Abstract

We use ab initio molecular-dynamics simulations to study the nonmetal-to-metal transition in dense liquid hydrogen. By calculating the equation of state of hydrogen at high pressures up to several megabars and temperatures above the melting line up to 1500 K we confirm the first-order nature of this transition at these temperatures. We characterize both phases based on equation of state data, the electrical conductivity, and the pair-correlation functions, which are all derived self-consistently from these simulations. We locate the respective transition line in the phase diagram and give an estimate for its critical point. We compare with available experimental data and other theoretical predictions.

Received 28 May 2010

DOI:https://doi.org/10.1103/PhysRevB.82.195107

Authors & Affiliations

Winfried Lorenzen, Bastian Holst, and Ronald Redmer

Institut für Physik, Universität Rostock, D-18051 Rostock, Germany

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Issue

Vol. 82, Iss. 19 — 15 November 2010

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Images

Figure 1
(Color online) Convergence with respect to particle number at 1000 K and $1 g / {cm}^{3}$ for calculations at the $Γ$ point, the Baldereschi mean value point, and a $3 \times 3 \times 3$ Monkhorst-Pack grid. Each symbol corresponds to a full FT-DFT-MD simulation. The solid line corresponds to the pressure calculated with 512 atoms and a $3 \times 3 \times 3$ Monkhorst-Pack grid, assumed to be the most accurate result. The dashed lines indicate the region of 1% accuracy with respect to this value.Reuse & Permissions
Figure 2
(Color) Thermal EOS of dense hydrogen. A first-order phase transition is observed for temperatures below 1500 K and a Maxwell construction is performed (dashed line). The results are in good agreement with DFT and CEIMC calculations (Ref. 40).Reuse & Permissions
Figure 3
(Color) Isotherms of the electrical conductivity in dense liquid hydrogen as function of pressure. Each point in the diagram corresponds to one snapshot of the simulation. The solid lines are fits to these points. For temperatures below 1500 K pronounced jumps (dashed lines) indicate the discontinuous nonmetal-to-metal transition. Also shown are the results from Ref. 40.Reuse & Permissions
Figure 4
(Color) Variation in the proton-proton pair-correlation function $g (r)$ in liquid hydrogen in the coexistence region as function of the density along the 700 K isotherm. The abrupt transition from the nonmetallic phase with a pronounced molecular peak to a metalliclike liquid occurs at $1.03 g / {cm}^{3}$ .Reuse & Permissions
Figure 5
(Color) Snapshots of the simulation box at 700 K and $1.03 g / {cm}^{3}$ , visualized with the VMD program (Ref. 56). Shown are the ions (red spheres) and the isosurfaces of the electron density $(n_{e} = 2 / Å^{3})$ (green) in (a) the molecular phase and (b) the metallic phase.Reuse & Permissions
Figure 6
(Color) High-pressure phase diagram of hydrogen with the predicted first-order liquid-liquid phase transition (solid line with crosses), including an estimate for the critical point. The results are in agreement with the DFT results by Morales et al. (Ref. 40). The CEIMC results have a similar slope but are located at slightly higher temperatures, coinciding with the molecular-to-atomic transition line given in Ref. 38. The melting lines are taken from Refs. 4, 40. The extrapolation given in Ref. 4 is consistent with QMC calculations for the liquid (Ref. 57). Also shown are possible paths of the shock-wave experiments (Ref. 33), depending on the EOS (partial dissociation model and Sesame tables) used to derive the temperatures (Ref. 58).Reuse & Permissions

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