Many common materials behave in extraordinary ways when subjected to extreme temperatures and pressures. For example the common barbeque gas propane becomes liquid when pressurised in gas bottles. Under the far more extreme pressures found at the centres of stars, gaseous hydrogen adopts a metallic state.
Theoretical physicists have for many years predicted that if subjected to sufficient pressure the common metal aluminium can change its crystal structure from the normal face centred cubic to a denser body centred cubic BCC form. Crystal structure matters a lot when it comes to chemical and physical properties, graphite, and diamond are both forms of pure carbon, yet their properties could hardly be more different. Theorists predict that aluminium with a BCC structure would be 41 per cent denser than the FCC metal and may have vastly different chemical and physical properties.
Generally when physicists want to study what happens to materials under extreme pressures they use a device known as a diamond anvil. This is a specially shaped pair of diamond points that can be set in the jaws of a large hydraulic press which generally also has facility to heat the compressed material too. However diamond - the strongest material available - yields below the pressure and temperature required to transform aluminium. As a result no one has ever created BCC aluminium until now.
A international team including scientists at the Australian National University have just released a paper in Nature Communications in which they have created BCC aluminium by an unusual method they describe as a top down temperature and pressure approach. Instead of a mechanical squeezing device, they used an ultrafast pulse laser to create a plasma.
When using lasers to heat matter the pulse length is of critical importance. A given amount of energy spread over a long pulse, heats the electrons and ions of the crystal relatively slowly allowing the heat to be transported from the focus by electrons and vibrations of the lattice known as phonons. As a result, an area far larger than the focal spot gets heated. However if the energy is concentrated into a short enough pulse – a few femtoseconds – the heat conduction mechanisms can’t remove it from the focus in time and the result is a massive heating of a tiny area deep within the crystal.
This heating turns the material of the crystal into plasma which expands with colossal force, compressing the atoms of the crystal around it. By using a sapphire, which is an oxide of aluminium, the team were able to generate conditions of such extreme pressure and temperature that tiny shells of BCC aluminium were formed within the crystal. “It’s not just the speed of heating that’s important here, the material also quenches or cools very rapidly, freezing in a new phase.” Professor Rode explains. “the pressures involved are enormous, over 50 million atmospheres.”
At present the new metal only exists in tiny crystals, about 20 nanometres across within the compressed sapphire. however, new materials are always exciting to scientists and engineers because they offer the potential for better devices that can do things that were previously impossible. And the ultra fast pulse laser method offers a relatively simple way to do just that.
There’s also a lot of excitement about work like this from a pure physics point of view. Generally scientists don’t have access to conditions like the centre of planets and stars in the laboratory. Moreover having the ability to create such ultra high pressures relatively easily using fairly small scale equipment offers the exciting potential of opening up this field of study.
“Knowing how materials behave under extreme pressures and especially having them available in the lab to study is a really helpful step in better understanding extreme physics such as that at the centre of the Earth as it was forming.” Professor Rode says.