High pressurE super-heroes

When it comes to working at high pressures, Clark Kent has nothing on modern day scientists who can now routinely achieve pressures of 200GPa – useful for investigating the chemistries of other planets, Anthony King reports

In the 1980s film, Superman compresses a piece of coal to create a diamond gem for Lois Lane. The pressures needed for this feat exist 150 to 200km below Earth’s surface, where diamonds form at high temperatures from carbon. Such conditions can now be recreated in a lab to make diamonds.

Nowadays, indeed, researchers can go well beyond Superman’s crunch pressure in their efforts to understand how materials inside some planets behave. And there’s a view that perhaps other materials with unique properties could be made if researchers can discover new arrangements of atoms from experiments.

Pressures of a couple of hundred gigapascals can now be routinely created in a lab by bringing together two diamonds, gradually forced towards one another, acting as an anvil. In 2022, the lab of Leonid Dubrovinsky at the University of Bayreuth, Germany, reported on a diamond anvil cell heated with a laser beam that allowed a chemical reaction at around 1000GPa; this was structurally characterised using single crystal synchrotron X-ray diffraction (C&I, 2022, 86 (6), 6). There are also multi-anvil cells, relying on a hydraulic press using tungsten-carbide alloys, that permit larger sample volumes, though at lower pressures.

An increasingly popular strategy is to use powerful lasers. Rather than slowly compressing materials between diamonds, a laser generates a sort of explosion and compression wave into the sample. At the National Ignition Facility in the US, 192 individual lasers can be trained on the centre of a spherical chamber, reaching mega-joules of energy. ‘We can generate very, very large explosions on the surface of our samples,’ says Amy Jenei, physicist at the National Ignition Facility of the Lawrence Livermore National Laboratory in the US. But keeping the material solid requires the pulses to be shaped accurately, she adds: ‘We need to keep that compression wave from steepening up into a shock and causing the material to melt.’ Using this approach, her group has reached pressures approaching a terapascal.

We would like to use shock experiments to probe the behaviour of matter inside planets, but we have to deal with the fact that the planets are there for billions of years, and the experiments are over in a flash.
Chris Pickard University of Cambridge, UK

Jenei is driven by a desire to understand the interiors of exoplanets – bodies orbiting distant stars – that may hold one of the keys to life elsewhere in the universe. A group at Lawrence Livermore recently used lasers to determine the melting point of iron under pressures up to 1000GPa (Science, 2022, 375 (6577), 202). They concluded that liquid metal cores lasted longest for super-Earths four to six times the mass of our planet, offering a strong magnetic shield against cosmic radiation. Sometimes, theoretical predictions are not borne out in experiments. The best predictions for diamond strength suggested it would melt at just above 1TPa, but Jenei’s group compressed it to 2TPa, without change of structure, due to the energy necessary to break its molecular bonds (Nature, 2021, 586, 532).

While the pressures above 350GPa at the centre of the Earth are considered extreme, relatively speaking this is not quite true, Jenei says: ‘Most of the mass of the Universe exists at high pressure, in the interiors of planets and stars.’ The reason being that gravity is such a strong force in the Universe.

Exoplanet excitement

The Kepler space telescope launched in 2009, revealing thousands of planets orbiting stars, with a range of masses and radii. This hinted at a huge diversity of interior compositions. But mass and radius deliver just an average density, leaving many questions about the nature of their insides. Even the interiors of planets such as Neptune and Uranus are up for debate, says Sara Seager, planetary scientist at Massachusetts Institute of Technology, who focuses on the discovery of ‘another earth’ and search for signs of life in exoplanet atmospheres. Planetary scientists such as Seager are eagerly awaiting measurements from the James Webb Space Telescope, launched December 2021, on exoplanets and their atmospheres, which might offer hints about their interiors.

Large planets may add mass, yet little volume. ‘There are planets the size of Jupiter, but with a range of mass over a factor of 10 to 100, which is pretty awesome physics,’ Seager says. ‘You have an object the same size yet 100 times more massive [heavier].’ One reason is that planetary interiors get so compressed that electrons pop off and nuclei get squeezed together.

NASA

The sorts of materials that can arise under such conditions offer frontier researchers access to a wilderness of exploration. Some small planets might be water worlds, for instance, where some of the water exists at supercritical phase due to high pressures – neither a gas nor a liquid. ‘Imagine having an exotic planet where there’s material that we have no experience of,’ says Seager. Another example could be planets dominated by carbides, not oxides. ‘That would be super different from any rocky planet that we’re familiar with,’ says Jenei.

A characteristic that matters for exoplanet habitability is the presence or absence of a magnetic field, which depends on its interior. Jupiter has the largest magnetic field in the Solar System, which Graeme Ackland at the University of Edinburgh, UK, notes is due to metallic hydrogen formed under high pressure (see Box). Earth’s magnetic field is generated by a molten iron-nickel core, meanwhile, which deflects charged particles from the sun, protecting life on Earth. Without it, we’d be fried, quips Ackland.

Superconductors

More than two decades ago, physicist Neil Ashcroft predicted that under high pressures sodium metal would become an insulator. This was shown experimentally around 2010. Under enough pressure, sodium switches from shiny metal to transparent insulator. ‘The standard explanation,’ says Eva Zurek, a chemist at State University of New York at Buffalo, US, ‘is that the valence electrons [on the outside of the atom], the ones that would be the conductor under normal pressure, get stuck in little cavities, because the core electrons start to overlap.’

Ashcroft and others also predicted that under pressure hydrogen would form special electron pairs that couple – known as a Cooper pair – via lattice vibrations, a quantum effect that underpins superconductivity. What’s more, since hydrogen has a proton and an electron, whose interaction is not diminished by shielding via other electrons, it is especially suited to such coupling, says Zurek. Experiments have since ‘metallised’ hydrogen at high temperatures and pressures, and theoretical calculations predict high-temperature conductivity near 500GPa. ‘Metallising hydrogen is right on the edge of what can be achieved,’ says Ackland, of lab experiments. But any superconducting materials produced under high pressure serve as a proof of concept, rather than for practical uses.

One way is to instead subject hydrogen to another kind of pressure. ‘The addition of an extra element can be used to add chemical pressure, and chemical pressure can lower the external pressure needed to do ‘metallisation’ of hydrogen,’ explains Zurek. Also, ‘if we can add extra electrons, we can disrupt the bonding and lower the pressure for metallised hydrogen,’ says Ackland. Heavy metallic transition elements such as the lanthanides are of interest here.

A group at the Max Planck Institute for Chemistry in Mainz, Germany, led by Mikhail Eremets, is arguably the leader in exploring superconductivity in metal hydrides. They reported superconductivity in lanthanum hydride (LaH10) at a pressure of 170GPa and 250K, the highest critical temperature in a superconducting material until then reported (Nature, 2019, 569, 528). A US group independently reported a similar result (Phys. Rev. Lett., 2019, 122, 027011). Notably, the LaH10 superconductivity was predicted computationally before being synthesised.

Stormy science

Conducting high-pressure experiments is no easy task. Pressure equals force divided by area, so the same force exerted over a small area generates more pressure. ‘Our field has had to develop ways of measuring superconductivity in ridiculously small samples, inside diamonds,’ says Ackland; people working on superconductivity generally handle bigger samples, which allow for more reliable measurements. ‘They expect a much higher level of proof, but it can’t be done with the apparatus that we’ve got at the moment.’

Shock compression experiments achieve high pressures but are constrained by brevity. ‘We would like to use these shock experiments to probe the behaviour of matter inside planets,’ says theoretical physicist Chris Pickard at the University of Cambridge, ‘but we have to deal with the fact that the planets are there for billions of years, and the experiments are over in a flash.’ One approach is to use computer simulations, something Pickard contributed to by calculating the quantum mechanical behaviour of materials at high pressures. Experimentalists often must lean on theorists for predictions, but some of these are viewed as necessarily unmoored from observation. ‘We really need to develop new experimental techniques to benchmark theoretical predictions,’ says Jenei.

Controversy sometimes jolts the high pressure research community. In September 2022, Nature retracted a paper from the University of Rochester, published in October 2020, which reported room temperature superconductivity in a carbonaceous sulfur hydride. The authors disagreed with the retraction.

Others say a tandem push from practical experiments and theoretical is crucial. Given the lofty objectives – to better understand our planetary neighbours, distant exoplanets and the fundamentals of the Universe – the struggles are viewed as worthwhile. And while scientists can replicate the pressure that Superman exacted on coal to create diamond, their experiments still fall somewhat short of pressures at the centre of large planets such as Jupiter’s intense 4000GPa. This leaves plenty of headroom for experimentalists to improve their techniques. In exploring ever higher pressures, recipes for materials such as room temperature superconductors might well be discovered, to transform 21st-century technology.

Metallic hydrogen

Hydrogen is the most abundant element in the Universe, and it is especially interesting under high pressure. On Earth, hydrogen atoms with one electron come together to form molecular H2. High pressures inside gas giant planets like Saturn and Jupiter, however, can squeeze the electrons out of the covalent bonds and turn it into a metal, with roaming electrons. ‘The atoms are floating around in a soup of electrons,’ says Graeme Ackland, physicist at the University of Edinburgh, UK, who studies hydrogen in extreme conditions. ‘When hydrogen becomes metallic, all of the chemistry changes.’

Journey to the centre of Earth and you would encounter pressures above 350GPa. That’s about three million times the atmospheric pressure, 100kPa, on our planet’s surface. Jupiter, however, is 11 times the size of Earth, over 300 times as massive, with an interior consisting of about 95% hydrogen and helium. Its core pressure is a staggering 3000 to 4500GPa. The chemistry of hydrogen shifts from molecular hydrogen to metallic hydrogen at around 200 to 400GPa. This has profound implications for Jupiter’s interior structure and has relevance to exoplanets orbiting around distant stars. Helium, an inert gas, is comfortable mixing with molecular hydrogen. But since helium is heavier than hydrogen, on Jupiter it is predicted to sink into the interior – so-called helium rain.

Metallic hydrogen can also mix more easily with other metals, such as iron, and together this combination may sink into a planet’s interior. ‘It seems wide open in different exoplanets as to whether you can have a heavy solid core of iron, as on Earth, or whether that iron will be dissolved into metallic hydrogen all the way down,’ says Ackland. ‘We don’t really know how it will play out.’ A spinning core of iron, with its free electrons, is what generates a magnetic field, as found on Earth. What happens under extreme pressures inside many planets is open to debate, since no one yet has experimentally created such conditions.