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…