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

Abstract

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...

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

“…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]. Other phases under extreme compression include the recent claimed (and still controversial) molecular conductor or atomic metal [12][22] [23], the predicted mobile solid state [2] [21], and superconducting superfluid quantum liquid [24][25] [26]. This wide range of behavior highlights the significance of dense hydrogen as an archetype of a manybody quantum system [23][24] [27].…”

“…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).…”

New possible candidate structure for phase IV of solid hydrogen

“…2 Therefore, much effort has been directed toward searching for candidate structures of solid hydrogen by combining DFT with optimization algorithms at high pressures and low temperatures, which has improved our understanding of the experimental observations [18][19][20][21][22] and predicted some peculiar properties for hydrogen at pressures beyond those explored experimentally. [23][24][25][26] The C2/c structure was proposed for phase III above 200 GPa and its Raman and IR vibrons exhibit good agreement with the experimental observations. Compared with phase III, the experimental Raman (infrared) spectrum of phase IV is more complex, and has two distinct vibron frequencies.…”

Dynamic phase transition theory

“…Palmer called such a region the "component" [3], and we note that conventional equilibrium statistics are exactly designed to handle such intracomponent motions. However, the temporal DOF becomes nontrivial when T is moderate: there are rare but significant inter-component motions for particles moving farther away beyond thermal fluctuations [18][19][20], which contributes to transport properties such as the diffusion coefficients and ionic mobility [21]. Instead of treating those motions as typical nonequilibrium phenomena, we found the dynamics for inter-component motions gradually become homogeneous towards the long time limit [17].…”