Diamonds can withstand some pressure. Actually, check this out: diamonds can withstand a lot of pressure. In a series of new experiments, scientists have found that diamonds maintain their crystal structure at pressures five times higher than the Earth’s core.
This contradicts the predictions that the diamond should transform into an even more stable structure under extremely high pressure, suggesting that the diamond adheres to a shape under conditions in which another structure would be more stable, which is known as “metastable”.
The discovery has implications for modeling high-pressure environments such as the nuclei of carbon-rich planets.
Carbon is almost as common. It is the fourth most abundant element in the Universe and can be found in exoplanets and stars and in intermediate space. It is also a major ingredient of all known life on Earth. Without it, we would not exist; this is why we refer to ourselves as a carbon-based life.
Therefore, carbon is of intense interest to scientists of all kinds. However, it is very difficult to study a place where carbon can be found, the nuclei of carbon-rich exoplanets. The high pressures there are difficult to replicate and, once the high pressures are reached, the material that is extracted is difficult to probe.
We know that carbon has several allotropes, or varying structures, at ambient pressures that have significantly different physical properties. Charcoal, graphite, and diamond are formed at different pressures, with diamonds occurring at higher pressures at underground depths, from about 5 or 6 gigapascals.
The pressure in the Earth’s core is about 360 gigapascals. At even higher pressures (about 1,000 gigapascals, just over 2.5 times the Earth’s central pressure), scientists have predicted that carbon would be transformed back into several new structures, which we had never seen or achieved before.
One method of achieving incredibly high pressures is to use a diamond anvil and shock compression. With this method, the hydrocarbon has undergone 45,000 gigapascals. This method tends to destroy the sample before its structure can be investigated.
A team led by physicist Amy Lazicki Jenei of Lawrence Livermore National Laboratory found another way to make it work. They used ramp-shaped laser pulses to squeeze a sample of solid carbon, at a pressure of 2,000 gigapascals. Simultaneously, the resolved X-ray diffraction in nanoseconds was used to probe the crystal structure of the sample.
This more than doubled the previous pressure at which a material has been probed by X-ray diffraction. And the results surprised the team.
“We found that, surprisingly, under these conditions the carbon is not transformed in any of the predicted phases, but maintains the structure of the diamond to the highest pressure,” Jenei said.
“The same ultra-strong interatomic bonds (which require breaking high energies), which are responsible for the metastable diamond structure of carbon persisting indefinitely at ambient pressure, are also preventing its transformation above 1,000 gigapascals in our experiments.”
In other words, diamond does not relax back into graphite when it is removed from underground depths: from higher pressures to lower ones. The force preventing this reversal could be the reason why the diamond is not rearranged in another allotrope at pressures even higher than those formed.
This discovery could change the way scientists model and analyze carbon-rich exoplanets, including the mythical diamond planets.
In the meantime, more work needs to be done to understand the outcome. The team doesn’t quite know why the diamond is so strong – more research will be needed to find out why the diamond is metastable at a wide range of pressures.
“Whether nature has found a way to overcome the high energy barrier to the formation of the predicted phases inside exoplanets is still an open question,” Jenei said.
“Additional measurements using an alternative compression pathway or starting from a carbon allotrope with an atomic structure that requires less energy to rearrange will provide more information.”
The research has been published in Nature.