Diamonds Do Very, Very Well Under Extreme Pressure, Scientists Confirm

February 25, 2021

American diplomat Henry Kissinger once joked that "a diamond is merely a lump of coal that did well under pressure." Now, scientists at the Lawrence Livermore National Laboratory in California have taken that notion to the extreme.

Researchers predicted that a diamond crystal would shift to a new, more stable structure when subjected to 2 trillion pascals of pressure. That's equivalent to five times the pressure found at the center of the Earth.

What they discovered, however, was that the diamond crystal remained perfectly stable. In their terminology, the diamond proved to be "metastable" due to the strong chemical bonds that hold its carbon atoms together.

The findings lend credence to the fantastical theory that carbon-rich exoplanets may have diamonds at their core.

During their experiments, physicist Amy Lazicki Jenei and her colleagues modeled high-pressure environments by pummeling the diamond crystal with powerful lasers and then used X-rays to examine the structure.

Scientists had theorized that the carbon-based material would transform again into several new structures, ones we have never seen or achieved before.

"We discovered that, surprisingly, under these conditions carbon does not transform to any of the predicted phases, but retains the diamond structure up to the highest pressure," Jenei said.

What's also super interesting about diamonds is that they form at intense pressures far below the Earth's surface, but retain their structure when relieved of that pressure. Carbon in its most stable state is graphite (the same material you find in a lead pencil). Theoretically, carbon under less pressure would revert to its most stable variant, but it doesn't.

The scientists reported that they are not entirely sure why diamond is metastable across a wide range of pressures and there is much more work ahead.

Jenei and her team published their findings in the January 27 edition of Nature.

In September of 2020, a team of researchers from Arizona State University and the University of Chicago suggested that the key factor in determining whether an exoplanet will be rich in diamonds is the chemical composition of the star that it orbits.

Exoplanets that orbit stars with a high carbon-to-oxygen ratio are more likely to be carbon-rich. Under the right conditions, such as the presence of water, heat and pressure, the highly concentrated carbon at the core of exoplanets could turn into diamonds, they predicted in a study published in The Planetary Science Journal.

While the prospects of finding a diamond planet are exciting, the scientists claim that the same characteristics that might make a planet diamond-rich would also make it uninhabitable. They believe that carbon-rich planets lack geologic activity and, therefore, have atmospheric conditions that would be inhospitable to life. Atmospheres are critical for life as they provide air to breathe, protection from the harsh environment of space and even pressure to allow for liquid water, said the scientists.

Credits: Image of 341-carat diamond courtesy of Lucara. Diamond planet illustration courtesy of Shim/ASU/Vecteezy.