Diamonds can handle a little pressure. Actually, revise it – diamonds can handle a lot of pressure. In a series of new experiments, scientists have found that diamonds retain their crystal structure at pressures five times higher than that of the earth’s core.
This contradicts the predictions that diamond should transform into an even more stable structure under extremely high pressure, suggesting that diamond sticks to a mold under conditions where another structure would be more stable, called ‘metastable’.
The discovery has implications for modeling high-pressure environments such as the core 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 would not exist; therefore, we refer to ourselves as carbon-based life.
Carbon is therefore of great importance to scientists of all kinds. However, one place where carbon can be found – the core of carbon-rich exoplanets – is very difficult to study. The high pressure there is difficult to repeat, and once high pressure is reached, the material being pressed is difficult to investigate.
We know that carbon has several allotropes, or variant structures, at ambient pressures that have significantly different physical properties. Charcoal, graphite and diamond all form with different pressures, with diamonds occurring under higher pressure deep underground, starting at about 5 or 6 gigapascals.
The pressure in the core of the earth is about 360 gigapascals. At even higher pressures – about 1,000 gigapascals, just over 2.5 times the Earth’s core pressure, scientists have predicted that carbon would turn into several new structures, something we’ve never seen or achieved before.
One method of achieving insanely high pressure involves the use of a diamond image and shock compression. With this method, hydrocarbon was subjected to 45,000 gigapascals. This method tends to destroy the sample before its structure can be examined.
A team led by physicist Amy Lazicki Jenei of Lawrence Livermore National Laboratory has found another way to make it work. They use ramp-shaped laser pulses to press a sample of solid carbon to a pressure of 2000 gigapascals. At the same time, nanosecond time-resolved X-ray diffraction was used to examine the crystal structure of the sample.
It has more than doubled the previous pressure at which a material was examined using X-ray diffraction. And the results surprised the team.
“We discovered that under these conditions, carbon does not surprisingly change to any of the predicted phases, but that the diamond structure retains to the highest pressure,” Jenei said.
“The same ultra-strong interatomic bonds (which require high energy to break), which are responsible for the metastable diamond structure of carbon that remains indefinite at ambient pressure, are also likely to hinder its transformation above 1,000 gigapascals in our experiments.
In other words, diamond does not relax again in graphite when brought from deep underground: from higher pressure to lower. The strength that prevents the inversion may be the reason why diamond is not rearranged into another allotrope at even higher pressures than the one in which it formed.
This discovery could change how scientists model and analyze carbon-rich exoplanets, including the mythical diamond planets.
Meanwhile, there is more work to be done to understand the result. The team is not entirely sure why diamond is so strong – more research will be needed to find out why diamond is metastable over a wide range 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,” Jenei said.
“Further measurements using an alternative compression path or from an allotropic carbon with an atomic structure that requires less energy to rearrange will provide further insight.”
The research was published in Nature.