High-pressure science can produce surprises—as demonstrated by two recent discoveriesby Philip Ball / February 22, 2017 / Leave a comment
Normality is notoriously subjective, but from a cosmic perspective it looks nothing like our own experience. Most of the observable matter in our universe is either much more tenuous or dense than our surroundings, and much colder or hotter. This leaves us more parochial in our preconceptions than the most insular of Little Englanders (however difficult that might be to imagine).
The past several weeks have delivered a couple of stark reminders. One report in the journal Science presented us with an image (literally) of hydrogen not as the invisible, lightweight gas that held the Hindenburg aloft on its fateful transatlantic voyage in 1937, but as a shiny metallic solid. Another, in Nature Chemistry, shattered the reputation of helium (hydrogen’s safer replacement in buoyant balloons) as the most chemically inert element by showing it as a component of a chemical compound of sorts, in a union with sodium.
What both of these discoveries had in common was that they transformed chemical intuitions by subjecting the materials to extremely high pressures. Of course it’s all relative: the squeezing achieved by the researchers who made these things was feeble indeed compared to, say, the pressure at the centre of the sun, let alone the surreal compression inside neutron stars that squeezes atoms themselves out of discrete existence. All the same, making hydrogen dense enough to turn it into a solid metal pushes the capabilities of high-pressure science to its limits.
Physics and chemistry at tremendous pressures might seem like an esoteric pursuit—until you recognise that the roughly one atmosphere pressure under which we live is a very rare circumstance. Most of planet Earth exists at far greater compression: at the planet’s core, iron and nickel are squeezed to around 3.6m atmospheres. If we want to understand the mineralogy of our planet, we need presses capable of enormous pressures. In one early example of such a machine, at the General Electric laboratories in Schenectady, New York, the first artificial diamonds were made in 1954 by squeezing graphite at temperatures and pressures comparable to those at which diamonds form about 150km or so beneath the Earth’s surface.
High-pressure science produces all kinds of surprises. Around sixteen different crystalline forms of ice have now been discovered at high pressure. Other chemical substances can be transformed beyond recognition by compression—as the work on hydrogen implies. Researchers have been seeking a high-pressure, metallic form of hydrogen (meaning that it conducts electricity, although we should also anticipate that such a material will have a shiny metallic lustre) ever since it was predicted in 1935.
It’s no surprise that hydrogen can form a solid: all chemical substances are expected to do that if you make them cold enough (although for helium you need to squeeze it a little too). It’s the metallic part that seems more exotic. Crudely speaking, what this means is that the atoms can’t any longer hold on to their electrons: these can drift through the crystal, making them able to carry an electric current. To be even more hand-waving about it: squeeze an orange hard enough, and its pips will fall out.
To understand that behaviour fully, you need quantum theory. As physicists became better at doing the quantum calculations, a new and even more remarkable prediction emerged. In 1968, Neil Ashcroft of Cornell University showed that if enough pressure was piled on this hypothetical metallic hydrogen, it could be transformed into an exotic electrical conductor called a superconductor, which has no electrical resistance. A current may (in theory, anyway) circulate around a ring of such a material forever without losing its energy by warming up the material.
Plenty of superconductors are known—mostly metals, but also some ceramics. Yet to attain this unusual state, they need to be kept ultra-cold, chilled with liquid nitrogen or helium refrigerant. This makes it complicated to take advantage of superconductivity in technologies, although it can be done at a cost. Ashcroft, however, predicted that superconducting solid hydrogen might exist even at room temperature. That was a goal worth chasing.
It’s been a competitive and controversial race. Mostly, researchers have tried to make metallic hydrogen in presses called diamond anvil cells, in which tiny samples of material are squeezed between the beveled teeth of two diamonds, like a thumbscrew made by Cartier. The pressures achievable in such devices have slowly crept up—the challenge is to prevent the diamonds cracking. It’s now possible to reach pressures greater than those at the Earth’s core, and the latest claim, made by Harvard scientists Isaac Silvera and Ranga Dias, records pressures of almost five million atmospheres. They say that at this point, and with the hydrogen cooled to within five degrees or so of absolute zero, the material between the diamond teeth becomes reflective and electrically conducting. Metallic hydrogen at last!
Or is it? Others are sceptical. Not only is the claim based on a single experimental observation, but it’s not clear how reliable the pressure estimate is, and the silvery stuff could be instead the thin layer of aluminium oxide that coats the diamonds to stop hydrogen atoms from finding their way into the gems and making them more brittle.
If the sceptics are right, this wouldn’t be the first false sighting. Nor would it be the first time Silvera himself, a veteran of high-pressure physics, has courted controversy. My advice would be to reserve judgement on whether the quest is over yet.
However, it’s just a reflection of my own parochialism that I call this stuff exotic at all. Most planetary scientists consider it likely that there’s an immense blob of metallic hydrogen in our solar system, existing deep inside the gas giant Jupiter, where the pressure at the planetary core reaches perhaps ten times what Silvera and Dias have claimed to produce. There, the higher temperature means that the metal would be liquid, not solid.
The claim of a chemical compound of helium seems more solid, so to speak. It comes from an international collaboration led by Xiao Dong of Nankai University in China and Stony Brook University in the US. The team squeezed sodium and helium to a pressure of 1.1m atmospheres in a diamond anvil cell and found that these elements form a crystalline compound containing two sodium atoms for each one of helium.
Helium is usually regarded as the archetypal “inert gas,” reacting with nothing. That doesn’t make it rare or obscure—it’s the second most abundant element in the cosmos, after hydrogen, and there’s plenty of it in stars and gas giants like Jupiter. But it seems a dull fish indeed as far as chemistry goes. It’s not quite fair to say that helium has “no chemistry”—a few peculiar molecules have been reported, all barely stable under any conditions. And a few solid materials containing helium were also known already, although the atoms generally rattle around in these without significantly affecting any others, like politely ignored guests. In contrast, the presence of helium in the new high-pressure compound with sodium completely changes the nature of its host, turning normally metallic sodium into an electrical insulator. That’s why the researchers feel justified in speaking here of a genuine chemical combination of the two elements, not just a forced and indifferent marriage.
Recreating the cores of planets—or at least, conditions like them—in the laboratory is a marvel of ingenuity, but it’s not just that. It’s a reminder too that extremes are often the best place to look for new physics. And unlike the case of high-energy physics, where the cutting edge demands herculean equipment at a commensurate cost, in high-pressure physics you can do an awful lot for not so much more, really, than the price of a couple of wedding rings.