Hydrogen is the most abundant element in the Universe. Our knowledge of celestial bodies such as the Sun, which is about 75% hydrogen1, relies on understanding the properties of this element at extreme temperature and pressure. Replicating these conditions in the laboratory is exceptionally challenging, and even the structure of high-pressure phases of hydrogen at low temperatures has been an open question. Writing in Nature, Ji et al.2 report experiments that probe this structure at unprecedented pressures, revealing a hexagonal close-packed arrangement of molecules.

Read the paper: Ultrahigh-pressure isostructural electronic transitions in hydrogen

The simplicity of the hydrogen atom, which comprises a single proton and a single electron, does not prevent the high-pressure phases of the element from being rich and complex. Hydrogen is an electrical insulator at ambient conditions, but becomes a metal under extreme compression3 — a state that could, for example, help to generate Jupiter’s magnetic field. Additionally, theoretical work suggests that metallic hydrogen might exhibit many exotic phenomena, such as high-temperature superconductivity4 (electrical conduction without resistance) or superfluidity5 (fluid flow without friction).

Over the past few decades, multiple solid phases of hydrogen have been identified by increasing the pressure to well above that at the centre of Earth. These experiments make use of devices called diamond anvil cells, in which a hydrogen sample is placed in a thin-foil gasket, which is in turn screwed between two diamonds to achieve extreme pressures in the centre of the sample.

The main approaches for analysing the compressed samples involve studying how the constituent molecules absorb infrared light (infrared spectroscopy), or observing how they scatter light (Raman spectroscopy). Such methods provide insights into the molecular structure. They have revealed that, as pressure increases, hydrogen transitions from a crystalline solid in which all of the molecules have similar bond lengths, to a mixed phase in which molecules of different bond lengths coexist6,7. The results are consistent with theoretical models8.

The predominant technique for examining long-range order in materials is X-ray diffraction, in which X-rays scattered by the electrons in a crystal interfere with each other. The resulting diffraction pattern contains bright spots, corresponding to waves that interfere constructively; and dark spots, coming from waves that interfere destructively. X-ray diffraction has been used to make many important scientific discoveries, including the double-helix structure of DNA.

A strange kind of liquid

Unfortunately, using this technique to study high-pressure hydrogen has, up to now, proved extremely challenging. A major difficulty is that the ability of X-rays to scatter off electrons decreases as the mass of the atoms that make up the material decreases. Hydrogen, being the lightest element, therefore gives rise to particularly weak signals. As a result, it is hard to distinguish between the X-rays scattered by the electrons in the hydrogen sample and those scattered by the surrounding gasket, which is typically made from heavy elements (such as tungsten or rhenium). A further challenge is that the diamonds that are used to pressurize the sample break easily when exposed to X-rays, leading to loss of pressure.

Because of these difficulties, X-ray diffraction studies of hydrogen had so far reached pressures of up to only 190 gigapascals9 (about 1.9 million times standard atmospheric pressure). This is about half the pressure that hydrogen can be subjected to in diamond anvil cells, and is not high enough to study some of the element’s most exotic phases, such as the mixed phase.

Ji and co-workers have addressed these challenges in a tour de force, carrying out more than a hundred experiments over a period of five years at pressures of up to 254 GPa. To increase the signal arising from hydrogen compared with that from its surroundings, the gaskets used were made of elements lighter than tungsten and rhenium. The authors also designed the experiments to yield useful data in the short time available before the inevitable diamond failure.

The results provide evidence of the long-range structure of molecular hydrogen across three high-pressure solid phases, including the mixed phase. In all three, the molecules adopt a hexagonal close-packed structure (Fig. 1) in which they are symmetrically arranged in the shape of a hexagonal prism. Furthermore, increasing the pressure squeezes the prism, causing it to become flatter and fatter.

Figure 1 | Structure of hydrogen under extreme pressure. Ji et al.2 demonstrate that the molecules in three high-pressure solid phases of hydrogen adopt a hexagonal close-packed structure. The drawing is a snapshot of where the two constantly moving protons in each molecule might be located. It also shows the charge density of the two electrons in each molecule, averaged over many snapshots.

Some questions remain. Unlike all of the elements heavier than helium, hydrogen has no electrons tightly bound to its nucleus, and the electrons in a hydrogen molecule are situated in the molecular bond. As a result, the scattering of X-rays by these electrons cannot be used to directly probe the location of the nuclei in the molecule or the molecule’s orientation, but instead the location of the bond.

Consequently, Ji and colleagues’ X-ray results will need to be combined with those from other experimental techniques, such as infrared and Raman spectroscopy, and possibly also nuclear magnetic resonance spectroscopy, which has only in the past year become available at the extreme pressures being studied here10. Combining these experimental insights with theoretical models will make the full characterization of high-pressure hydrogen phases a reality.

The pressures reached in this X-ray study correspond to electrically insulating molecular hydrogen. In the next few years, experiments will probably focus on even higher pressures. However, it will prove a challenge for X-ray techniques to study the pressures at which the element becomes atomic and metallic. In this phase, the electrons are no longer in the molecular bond; instead, they are shared by all of the atoms in the structure, so it is unknown what the corresponding X-ray diffraction pattern would look like. Exciting times lie ahead for the study of the lightest and most abundant element in the Universe.