Diamonds can take a bit of pressure. Actually revise that – diamonds can take a lot of pressure. In a series of new experiments, scientists have found that diamonds maintain their crystal structure at pressures five times those of the Earth’s core.
This contradicts predictions that diamond should transform under extremely high pressure into an even more stable structure, suggesting that diamond will stick to one shape under conditions where another structure would be more stable, which is called ‘metastable’.
The discovery has implications for the modeling of high-pressure environments, such as the cores of planets rich in carbon.
Carbon is almost as common as it gets. It is the fourth most abundant element in the universe and can be found in exoplanets and stars and the space in between. It is also a primary ingredient of all known life on Earth. Without it we wouldn’t exist; that’s why we call ourselves carbon-based life.
Carbon is therefore of great importance to all types of scientists. However, one place where carbon can be found – the nuclei of carbon-rich exoplanets – is very difficult to study. The high pressures present there are difficult to replicate, and once high pressures are achieved, the pressing of material is difficult to investigate.
We know that carbon has different allotropes or variant structures at ambient pressures that have significantly different physical properties. Charcoal, graphite, and diamond are all formed at different pressures, with diamond occurring at higher pressures deep underground, starting at about 5 or 6 gigapascals.
The pressure in the core of the Earth is up to about 360 gigapascals. At even higher pressures – about 1,000 gigapascals, just over 2.5 times Earth’s nuclear pressure, scientists predicted that carbon would turn back into several new structures that we have never seen or reached before.
One method of achieving insanely high pressures is to use a diamond anvil and shock compression. With this method, hydrocarbon has been subjected to 45,000 gigapascals. That method tends to destroy the sample before its structure can be examined.
A team led by physicist Amy Lazicki Jenei from Lawrence Livermore National Laboratory found another way to make it work. They used laser pulses in the form of a slope to squeeze a sample of solid carbon to a pressure of 2,000 gigapascals. At the same time, time-resolved X-ray diffraction with nanosecond duration was used to examine the crystal structure of the sample.
This is more than double the previous pressure at which a material was examined by X-ray diffraction. And the results took the team by surprise.
“We found that, surprisingly, under these conditions, carbon does not transform to any of the predicted phases, but retains the diamond structure to the highest pressure,” said Jenei.
“The same ultra-strong interatomic bonds (which require high energies to break) that are responsible for carbon’s metastable diamond structure that persists indefinitely at ambient pressure also likely hinder its transformation above 1,000 gigapascals in our experiments.”
In other words, diamond does not relax back into graphite when it is removed from the deep underground: from higher pressures to lower. The force preventing that reversal could be why diamond doesn’t rearrange into another allotrope at even higher pressures than the one in which it formed.
This discovery could change the way scientists model and analyze carbon-rich exoplanets, including the mythical diamond planets.
Meanwhile, there is more work to be done to understand the outcome. The team isn’t quite sure why diamonds are so strong – more research will be needed to find out why diamonds are metastable at a wide variety of pressures.
“Whether nature has found a way to overcome the high energy barrier for the formation of the predicted phases in the interior of exoplanets is still an open question,” said Jenei.
“Further measurements using an alternative compression route or from an allotrope of carbon with an atomic structure that requires less energy to rearrange will provide further insight.”
The research is published in Nature.