At pressures vastly greater than those found on the surface of the Earth, matter can behave in strange ways. Oxygen can become superconductive, while metals can turn into insulators. In a 2007 experiment, sodium was found to turn transparent when squeezed by a pressure of 200 GPa – two million times the pressure at the surface of the Earth. Theorists have predicted, and there have been several unconfirmed observations, that hydrogen can become metallic.

Up the pressure

The diamond-anvil cell is the tool of choice to generate extreme pressures as it squeezes a sample between two tiny, gem-grade diamond crystals – diamond being one of the hardest substances known. Unfortunately, hard as diamonds are, they always fail eventually. This makes it challenging to achieve static pressures above about 250 GPa, and almost impossible above 420 GPa.

The only way to achieve super-high pressures in the laboratory, until now, has been to bombard a sample with shock waves, which compress it suddenly to generate pressures of many hundreds of gigapascals. But there are two problems with this technique: first, it grants only nanoseconds of observation time; and second, it generates a lot of heat as well as very high pressure, which can make it difficult to disentangle the effects of the two as solids are often turned into liquids. This is problematic for geophysicists trying to study the reactions at the centre of the Earth, where the pressure is stable at above 350 GPa, and even more so for those studying the gas giants, for example, where the internal pressures are well above this. A tool to generate static pressures in these ranges would be useful in the geophysicist's arsenal.

Nanocrystalline upgrade

To try to develop such a tool, Leonid Dubrovinsky and Natalia Dubrovinskaia of the University of Bayreuth in Germany, together with colleagues from Belgium and the US, took a hard look at the existing diamond-anvil cell. When diamonds eventually fracture, they do so along one of the cleavage planes running through the material. Based on previous work, the researchers knew that a diamond made up of numerous small crystals would not have these well-defined cleavage planes.

The researchers fabricated nanocrystalline diamond hemispheres, about 12–20 μm diameter, from tiny carbon balls at 2200 K and 20 GPa pressure using a newly developed technique. They then made a two-stage diamond anvil. On the outside of the press, they arranged two flat, gem-grade diamond plates. Inside these they placed their nanocystalline hemispheres. The flat edges were placed against the plates, with the curved edges, giving the smallest contact area and the largest pressure, placed against the sample. After some optimization, the researchers were able to generate static pressures of up to 640 GPa of the nanocrystalline diamond on the sample, allowing them to measure the equations of state of rhenium and gold at these extraordinary pressures.

Increasing stability

Dubrovinskaia believes that by implementation of computer control it should be possible to improve the stability of the pressure in the device. "By increasing the stability, we should be able to reach higher pressures," she says. "That is one side – the other aspect is improvement of the materials themselves. The strength of nanocrystalline diamonds, like any polycrystalline material, is different depending on the size of nanoparticles. So if we play with particles with different sizes and different shapes of nanocrystals, it may play a significant role."

Katsuya Shimizu, an expert in the study of matter at high pressures from the University of Osaka, Japan, who led the team that demonstrated superconductivity in oxygen, believes the double-stage diamond-anvil technique proposed by the researchers, which requires a tiny amount of the sample to be placed between two "semi-balls", may limit its applicability. Nevertheless, he believes that "some researchers will perform experiments following this technique".

The researchers conclude that "achieving 1 TPa in static-compression experiments with double-stage diamond-anvil cells is a viable goal". The pressure in the cores of gas giants is about 700 GPa, so if Dubrovinskaia and colleagues further perfect their technique, astronomers may soon have a tool to study the conditions within these planets.

The research is published in Nature Communications.

•  This story first appeared on our sister website physicsworld.com