[There, is] plenty of room for new phases at high pressure [!,?]

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Evidence for a new phase of dense hydrogen above 325 gigapascals
Philip Dalladay-Simpson, Ross T. Howie & Eugene Gregoryanz

Nature 529, 63–67 (07 January 2016)

Almost 80 years ago it was predicted that, under sufficient compression, the H–H bond in molecular hydrogen (H2) would break, forming a new, atomic, metallic, solid state of hydrogen. Reaching this predicted state experimentally has been one of the principal goals in high-pressure research for the past 30 years. Here, using in situ high-pressure Raman spectroscopy, we present evidence that at pressures greater than 325 gigapascals at 300 kelvin, H2 and hydrogen deuteride (HD) transform to a new phase—phase V. This new phase of hydrogen is characterized by substantial weakening of the vibrational Raman activity, a change in pressure dependence of the fundamental vibrational frequency and partial loss of the low-frequency excitations. We map out the domain in pressure–temperature space of the suggested phase V in H2 and HD up to 388 gigapascals at 300 kelvin, and up to 465 kelvin at 350 gigapascals; we do not observe phase V in deuterium (D2). However, we show that the transformation to phase IV′ in D2 occurs above 310 gigapascals and 300 kelvin. These values represent the largest known isotropic shift in pressure, and hence the largest possible pressure difference between the H2 and D2 phases, which implies that the appearance of phase V of D2 must occur at a pressure of above 380 gigapascals. These experimental data provide a glimpse of the physical properties of dense hydrogen above 325 gigapascals and constrain the pressure and temperature conditions at which the new phase exists. We speculate that phase V may be the precursor to the non-molecular (atomic and metallic) state of hydrogen that was predicted 80 years ago.

New low temperature phase in dense hydrogen: The phase diagram to 421 GPa
Ranga Dias, Ori Noked, Isaac F. Silvera
(Submitted on 7 Mar 2016 (v1), last revised 26 May 2016 (this version, v2))

In the quest to make metallic hydrogen at low temperatures a rich number of new phases have been found and the highest pressure ones have somewhat flat phase lines, around room temperature. We have studied hydrogen to static pressures of GPa in a diamond anvil cell and down to liquid helium temperatures, using infrared spectroscopy. We report a new phase at a pressure of GPa and T=5 K. Although we observe strong darkening of the sample in the visible, we have no evidence that this phase is metallic hydrogen.

No "Evidence for a new phase of dense hydrogen above 325 GPa"
Ranga P. Dias, Ori Noked, Isaac F. Silvera
(Submitted on 18 May 2016)

In recent years there has been intense experimental activity to observe solid metallic hydrogen. Wigner and Huntington predicted that under extreme pressures insulating molecular hydrogen would dissociate and transition to atomic metallic hydrogen. Recently Dalladay-Simpson, Howie, and Gregoryanz reported a phase transition to an insulating phase in molecular hydrogen at a pressure of 325 GPa and 300 K. Because of its scientific importance we have scrutinized their experimental evidence to determine if their claim is justified. Based on our analysis, we conclude that they have misinterpreted their data: there is no evidence for a phase transition at 325 GPa.

Nature of the Metallization Transition in Solid Hydrogen
Sam Azadi, N. D. Drummond, W. M. C. Foulkes
(Submitted on 2 Aug 2016)

Determining the metalization pressure of solid hydrogen is one of the great challenges of high-pressure physics. Since 1935, when it was predicted that molecular solid hydrogen would become a metallic atomic crystal at 25 GPa [1], compressed hydrogen has been studied intensively. Additional interest arises from the possible existence of room-temperature superconductivity [2], a metallic liquid ground state [3], and the relevance of solid hydrogen to astrophysics [4, 5].  

Early spectroscopic measurements at low temperature suggested the existence of three solid-hydrogen phases [4]. Phase I, which is stable up to 110 GPa, is a molecular solid composed of quantum rotors arranged in a hexagonal close-packed structure. Changes in the low-frequency regions of the Raman and infrared spectra imply the existence of phase II, also known as the broken-symmetry phase, above 110 GPa. The appearance of phase III at 150 GPa is accompanied by a large discontinuity in the Raman spectrum and a strong rise in the spectral weight of molecular vibrons. Phase IV, characterized by the two vibrons in its Raman spectrum, was discovered at 300 K and pressures above 230 GPa [6–8]. Another new phase has been claimed to exist at pressures above 200 GPa and higher temperatures (for example, 480 K at 255 GPa) [9]. This phase is thought to meet phases I and IV at a triple point, near which hydrogen retains its molecular character. The most recent experimental results [10] indicate that H2 and hydrogen deuteride at 300 K and pressures greater than 325 GPa transform to a new phase V, characterized by substantial weakening of the vibrational Raman activity. Other features include a change in the pressure dependence of the fundamental vibrational frequency and the partial loss of the low-frequency excitations.  

Although it is very difficult to reach the hydrostatic pressure of more than 400 GPa at which hydrogen is normally expected to metalize, some experimental results have been interpreted as indicating metalization at room temperature below 300 GPa [6]. However, other experiments show no evidence of the optical conductivity expected of a metal at any temperature up to the highest pressures explored [11]. Experimentally, it remains unclear whether or not the molecular phases III and IV are metallic, although it has been suggested that phase V may be non-molecular (atomic) [10]. Metalization is believed to occur either via the dissociation of hydrogen molecules and a structural transformation to an atomic metallic phase [6, 12], or via band-gap closure within the molecular phase [13, 14]. In this work we investigate the latter possibility using advanced computational electronic structure methods.

Structures of crystalline materials are normally determined by X-ray or neutron diffraction methods. These techniques are very challenging for low-atomic-number elements such as hydrogen [15]. Fortunately optical phonon modes disappear, appear, or experience sudden shifts in frequency when the crystal structure changes. It is therefore possible to identify the transitions between phases using optical methods.

Observation of the Wigner-Huntington Transition to Solid Metallic Hydrogen

Ranga Dias, Isaac F. Silvera

(Submitted on 5 Oct 2016)

We have studied solid hydrogen under pressure at low temperatures. With increasing pressure we observe changes in the sample, going from transparent, to black, to a reflective metal, the latter studied at a pressure of 495 GPa. We have measured the reflectance as a function of wavelength in the visible spectrum finding values as high as 0.90 from the metallic hydrogen. We have fit the reflectance using a Drude free electron model to determine the plasma frequency of 30.1 eV at T= 5.5 K, with a corresponding electron carrier density of 6.7x1023 particles/cm3 , consistent with theoretical estimates. The properties are those of a metal. Solid metallic hydrogen has been produced in the laboratory