Lattice stability and high pressure melting mechanism of dense hydrogen up to 1.5 TPa

📝 Abstract

Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reach absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

💡 Analysis

Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reach absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

📄 Content

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Lattice stability and high pressure melting mechanism of dense hydrogen up to 1.5 TPa Hua Y. Geng,1,2 R. Hoffmann,2 Q. Wu1 1 National Key Laboratory of Shock Wave and Detonation Physics, Institute of Fluid Physics, CAEP; P.O.Box 919-102 Mianyang, Sichuan, P. R. China, 621900 2 Department of Chemistry and Chemical Biology, Cornell University, Baker Laboratory, Ithaca, New York 14853, USA Abstract Lattice stability and metastability, as well as melting, are important features of the physics and chemistry of dense hydrogen. Using ab initio molecular dynamics (AIMD), the classical superheating limit and melting line of metallic hydrogen are investigated up to 1.5 TPa. The computations show that the classical superheating degree is about 100 K, and the classical melting curve becomes flat at a level of 350 K when beyond 500 GPa. This information allows us to estimate the well depth and the potential barriers that must be overcome when the crystal melts. Inclusion of nuclear quantum effects (NQE) using path integral molecular dynamics (PIMD) predicts that both superheating limit and melting temperature are lowered to below room temperature, but the latter never reach absolute zero. Detailed analysis indicates that the melting is thermally activated, rather than driven by pure zero-point motion (ZPM). This argument was further supported by extensive PIMD simulations, demonstrating the stability of Fddd structure against liquefaction at low temperatures.

PACS numbers: 67.80.F-, 64.70.dj, 62.50.-p, 71.15.Nc, 64.30.+t Keywords: hydrogen, high pressure, melting, quantum solid, ab initio method

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I. INTRODUCTION Hydrogen, the simplest element, shows complex behavior under compression [1-6]. It has at least four allotropes in the solid state that were already known, and exhibits an anomalous melting temperature (Tm) that peaks at about 100 GPa [7-13] and then decreases downwards [12-15]. It was speculated that at higher pressures dense hydrogen in a metallic state might melt driven not by thermal motion of nuclei (as other elements usually are) but rather by pure nuclear quantum effects (NQE), or equivalently, by the zero-point motion (ZPM) of nuclei [16,17]. This conjecture is tantalizing and hints the possibility of a quantum liquid in its ground state as 0 K is approached [18].
Recent numerical simulations predicted that this descent might continue beyond 1 TPa [19]. However, there are two fundamental questions yet to be answered: (i) does dense hydrogen really melt at 0 Kelvin? (ii) What are the respective role played by the softening of the interaction potential, as well as that played by the NQE in this decline? Namely, does the low-temperature melting originate from the flatness of the potential energy surface [20] or simply because of the enormous ZPM? This query is important, because an analogous decrease of Tm has also been observed in the alkali metals such as Li [21,22] and Na [23], where NQE is insignificant. For these two elements, the Tm rises again at higher pressures. Considering the similarity of metallic hydrogen (HM) with the alkali metals [24], it is reasonable to expect that hydrogen should also follow a similar trend. A consequent supposition is that the potential softening could be limited, and the energy surface (ES) of HM in this pressure range might still be rough, with noticeable energy wells and barriers. If true, this will provide profound insight into the phase stability of solid HM, because thermally driven forces will diminish with decreasing temperature if the destabilization (or melting) of 3

a crystal is thermally activated (TA). On the other hand, when near the groundstate, the only possible dynamical forces that can destabilize a lattice are ZPM or quantum tunneling, the latter a mechanism in quantum melting that has received some attention only very recently [25]. In this hypothetical scenario, the particle tunneling length and the height and width of the barriers on the ES are the key parameters that dictate the melting behavior.
In this article, we will demonstrate for the first time that within the pressure range from 500 to 1500 GPa, HM does fall in this regime (i.e., with limited softening in the potential) and have noticeable energy barriers. One of the consequences is a strong meta-stability of crystalline or glass phases at low temperatures. Furthermore, the solid groundstate of dense hydrogen has also been established at the level of density functional theory (DFT) with the first direct numerical evidence obtained by using extensive AI-PIMD simulations.
II. METHOD AND THEORETICAL DETAILS
A. First-principles calculations In our calculations, the many-body electron problem is treated with DFT, and periodic boundary conditions (PBC) are used to model the solid and/or liquid phases. AIMD simulations are carried out in a micro canonical ensemble (NVE), in which the particle number N, internal e

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