(Image: Daniel Stolle)
IT IS chemistry’s poster child. From copper’s conductivity to mercury’s mercurial liquidity, the periodic table assigns the chemical elements to neat columns and rows and so reveals their properties. It is chemists’ first reference point in all their endeavours, whether building better catalytic converters, making concrete set faster or looking for the best materials for medical implants.
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Yet this iconic picture of science is hopelessly parochial. Most of the known matter in the universe doesn’t exist under the cool, calm conditions of Earth’s surface that the periodic table assumes. By mass, more than 99.9 per cent of normal matter resides within planets and stars – environments of high temperatures, but above all unimaginable pressures.
Here, the elements’ familiar identities start to blur. “We essentially have a new periodic table at high pressures,” says materials scientist Paul Loubeyre of the Alternative Energies and Atomic Energy Commission (CEA) in Bruyères-le-Châtel, France. As yet, this high-pressure realm is one we know little about – proportionally as if, in thousands of years of Earth exploration, geographers had mapped out a region no larger than Spain.
Slowly, however, modern-day alchemists are ratcheting up the pressure. As they do, they are transforming the familiar physical and chemical properties of elements from hydrogen to iron, turning liquids to solids, non-metals to metals and more besides. The aim is not just to understand more about the deep chemistry of our planet and others, but also to find materials that react more efficiently, store energy more effectively, or even perform that most yearningly sought-after of tricks: conducting electricity without resistance at room temperature.
Electron squash
It was the Russian chemist Dmitri Mendeleev who, back in 1869, produced the first recognisable periodic table. He showed that patterns begin to emerge in the properties of the chemical elements if you order them by their atomic weight, and used those patterns to predict the existence of undiscovered elements. But it wasn’t until the advent of quantum mechanics in the 20th century that those patterns were explained. Electrons circling an atom’s nucleus can only occupy discrete “orbitals”, each of which accepts a strict number of electrons. The distribution of electrons within these orbitals – especially the outermost ones – determines an element’s chemical character.
There are still niggles about the periodic table’s validity under normal conditions (New Scientist, 12 July 2014, p 38), but turning up the pressure changes things entirely. Atoms get squished, deforming the “unit cells” that define matter’s basic scale. Electrons squeeze into tighter orbitals, overlapping and forming more exotic configurations, and begin making chemical bonds with electrons in other atoms in entirely different ways.
Carbon provides a familiar example of the changes that can result. Coal is carbon formed from plant debris, compacted and heated for millions of years a few kilometres below ground. But go 100 kilometres or so down, and the high temperature, along with pressures 30,000 to 50,000 times those at Earth’s surface, transform carbon’s bonding to make an apparently different substance: diamond.
Occasionally, geological processes fortuitously bring diamonds closer to the surface, where they can be mined. Since the 1950s, however, researchers have been cutting out the billion-year geological middlemen. Large hydraulic presses weighing tens of tonnes now compress carbon to yield synthetic diamonds used in coatings, cutting tools and even jewellery.
But even these pressures are tiny compared with those deep in Earth’s interior (see illustration below). For a more complete picture of the planet, we’d love to track what goes on in this high-pressure realm. “Stuff is moving and reacting, causing fluxes in different elements such as carbon, and this has relevance for climate change over geological history and theories about the origin of life,” says geochemist of Bayreuth University in Germany.
Diamond to the core
With no direct access to such depths – our deepest drilled hole, the Kola Superdeep Borehole, penetrates just 12 kilometres beneath the north-west tip of Russia – simulating the pressures found there requires some cunning ruses. As anyone who has stepped on a polished wooden floor wearing high-heeled shoes knows, force concentrated into a small area produces very high pressures. A device known as an anvil cell takes advantage of this fact, producing the same effect as a monster hydraulic press by crushing tiny samples between two high-heel tips made of extremely hard materials.
Tungsten carbide is one such material, capable of delivering pressures equivalent to those in Earth’s upper mantle. Diamond tips go further, to pressures found in Earth’s core. They are also transparent, allowing researchers to observe in real time what happens as materials are crushed.
Even so, to really see what is going on at the atomic level requires special illumination: the intense beams of X-ray light produced at synchrotron accelerators such as the European Synchrotron Radiation Facility (ESRF) in Grenoble, France. McCammon and her colleagues recently placed their diamond anvil cell in the ESRF’s beams to address the puzzle of Earth’s core consistency. The core generates Earth’s magnetic field, and is mostly made of iron. But the way seismic waves pass through it suggest the core has a consistency rather like tyre rubber – which is not what pure iron looks like under pressure. When mixed with carbon graphite powder and compressed, however, the researchers found iron forms an entirely different crystalline phase that mimics this plasticity, suggesting Earth has a vast, previously unknown carbon reservoir (). Similar experiments have also recently yielded new estimates for iron’s melting temperature under pressure, leading to the conclusion that the temperature of Earth’s inner core is 6000 °C, some 1000 degrees hotter than previously thought.
Iron’s large number of orbiting electrons make it particularly difficult to model under pressure. Undaunted, researchers such as Loubeyre are moving on to even more complex elements, such as the rare earths. These materials are essential for technology from screens for smartphones to magnets for wind turbines, and the hope is that exploring high-pressure forms will allow us to better exploit their properties.
This same motivation holds for simpler elements, too – and it’s here that the surprises have been greatest. Exert a pressure akin to those found in Earth’s outer core, and sodium – a soft, highly reactive metal at Earth’s surface – becomes transparent and something between a semiconductor and an insulator. Oxygen, meanwhile, turns into a solid metal at similar pressures, and if then cooled becomes a superconductor – in other words, it conducts electricity with no resistance.
The dramatic nature of these changes has caught many on the hop. “There have been simple rules guiding this field, but it turns out these things were wrong,” says Loubeyre. “We are seeing baffling changes in unit cell volumes and remarkable electrical properties that are highly promising for applications.”
Perhaps the most promising is the simplest atom of all: hydrogen. This is the universe’s most abundant element, consisting of just a single electron orbiting a single proton. It is a bit of a misfit in the periodic table, generally sitting uneasily above lithium and sodium at the top of metallic group 1.
Calculations since the 1930s have indicated that high pressures would indeed turn hydrogen into a metal, squeezing the electron out of each proton’s orbit and leaving them free to conduct. Not only that, but calculations suggested that the electrons’ skittish interactions with the lattice of proton cores might at some point allow them to team up and pass through the solid hydrogen unimpaired – hydrogen could be made a superconductor. That might even occur at room temperature and potentially persist even when the pressure is taken off, just as diamonds retain their structure at lower pressures.
This would be a trillion-dollar transition. The highest temperature so far seen for a superconductor, a copper-based compound called a cuprate, is -140 °C at ambient pressure, and -110 °C at high pressures. A room-temperature superconductor could allow the transmission of energy over distance without loss at a much lower price, transforming the electricity grid and consigning the internal combustion engine to history.
“A room-temperature superconductor could consign combustion engines to history”
No one is sure at what pressure hydrogen’s metallic and superconducting transitions might occur, although it seems likely that a metallic, although not superconducting, state makes up the core of Jupiter and other gas-giant planets. That’s tough to replicate: the pressure at Jupiter’s centre is 4000 gigapascals (GPa), more than 10 times that in Earth’s inner core and 40 million times greater than at Earth’s surface.
In 2011 and Ivan Troyan of the Max Planck Institute for Chemistry in Mainz, Germany, reported a metallic transition in hydrogen compressed to just 260 GPa in a diamond anvil cell (). The claim was met with scepticism, and later analysis suggested they had probably found something subtly different: a poorly conducting semi-metal state ().
Still, just last month Eremets and Troyan pulled off something close to the desired trick with hydrogen atoms in a slightly more complex chemical form. They compressed hydrogen sulphide – that smelliest of gases, which gives off a strong whiff of rotten eggs – to just 90 GPa and transformed it to a solid metal. Taken to 150 GPa, it seems to become a superconductor when cooled to just -70 °C, the highest temperature ever recorded for a superconductor (Nature, ). This superconductor also seems to work by a conventional mechanism that has been understood for 50 years, in contrast to the still-mysterious workings of the cuprates.
Pure hydrogen still remains the ultimate goal – after all, says Patrick Bruno of the ESRF, the discovery of metallic hydrogen would probably be worth a Nobel prize. Besides a potential room-temperature superconductor, metallic hydrogen might form the basis of a revolutionary fuel technology: the molecular bonds of pure hydrogen are broken at high pressure, leaving a system that can store a huge amount of chemical energy.
There are still high-pressure depths to plumb to get there. “Recent work on sophisticated simulations cast doubts that there is any metallic state below 400 GPa – the current edge for experimental studies,” says Eremets.
Loubeyre and his colleagues are working with a two-stage anvil – essentially a diamond crushing a diamond crushing something else – that lets them achieve pressures of 600 GPa, and he is confident of results soon. “Metallic hydrogen will probably be discovered in the next two to three years,” he says.
Meanwhile new pressure records continue to be set – most recently osmium, Earth’s least compressible metal, . As we progress we are beginning to find out just how much the 99.9 per cent of chemistry we don’t see differs from the 0.1 per cent we do. Our techniques currently limit us to observing the properties of small samples of high-pressure materials, but the hope is that better synchrotrons, for example, will allow us to observe and perhaps even mimic the chemical reactions between different compounds that must go on deep beneath our feet.
That truly would be an eye-opener. Chemists already know and routinely exploit the transformations in chemical reactivity that even relatively small pressure increases bring. Molecular nitrogen in the atmosphere, for example, is a thankfully unreactive gas. But up the pressure 200-fold, and it readily reacts to make the fertiliser ammonia – the basis of the Haber-Bosch reaction that, by transforming agricultural productivity, has fed many a hungry mouth since the early 20th century. With such successes in mind, the pressure’s on to discover more.
Limits of life
While researchers strive to recreate pressure extremes in the lab (see main story), human physiology fixes us firmly in a very thin shell of survivable existence. With training, we may reach Earth’s highest points without breathing machines to assist with the thin, low pressure air. In water, rapidly increasing pressures limit even the most highly trained divers to a hundred or so metres.
But in general, Earth’s life seems strangely pressure-resistant. Fish have been sighted by submersibles at depths of over 8000 metres, while the deepest microbial life found to date lies some 11 kilometres beneath the ocean surface in the Marianas and Tonga trenches, at pressures of 110 megapascals (MPa), some 1100 times those at Earth’s surface.
Lab experiments suggest complex life becomes unviable if you go much deeper. The complex molecules used to make cells start to disintegrate at 200 to 300 MPa, and between 700 and 800 MPa even the most resistant biomolecules cease to function. Weirdly, though, some simple bacteria such as E. coli and Shewanella appear to resist pressures up to four times as high as this, and certain seeds, spores and freshwater shrimp eggs have been shown to resume normal behaviour after being subjected to even higher pressures. “Take a seed, squeeze it to 75,000 atmospheres (7600 MPa) and it can still sprout, although the plant might look a bit different,” says chemist of University College London. “We don’t really have a clue what is going on.”
And with unicellular bacteria at least, survivors from one high-pressure experiment are able to resist even higher pressures in subsequent steps. Perhaps, says McMillan, that’s because the DNA of the survivors contains protein-generating domains that help confer extreme pressure-resistance – which could give a clue as to their ultimate origin. “Maybe these could even be related to a distant past where life began in a deep rock-hosted environment, or sheltered there when the young Earth was blasted by asteroid impacts,” he says.
As things stand, it seems temperature, rather than pressure, is the main limitation to life on Earth. “If life were present on other planets or satellites that remain colder at depth, then the laboratory results reveal that organisms could survive at even more crushing pressures,” says McMillan.
This article appeared in print under the headline “Elements at the extreme”
