Diamonds really are forever

Even with the weight of worlds on its shoulders, diamond resolutely refuses to buckle under the pressure. Thanks to the highest-pressure X-ray diffraction experiments ever reported, researchers have revealed that the structure of diamond remains unchanged at 2 terapascals (TPa)—more than five times the pressure at Earth’s core (Nature 2021, DOI: 10.1038/s41586-020-03140-4).

“It’s a fantastic technical achievement to be able to take a material and compress it up to 20 million times atmospheric pressure, if only for billionths of a second, and then use X-ray diffraction to determine how its atoms are arranged,” says Malcolm I. McMahon, a high-pressure physicist at the University of Edinburgh who was not involved in the work.

Diamond’s atoms are normally arranged in a repeating pattern that is a variant of a common crystal structure known as face-centered cubic. But density functional theory calculations suggest that at about 1 TPa, a more complex structure, based on a different arrangement called body-centered cubic, would be more stable. At even higher pressures, the material should prefer a form known as a simple cubic structure. However, the strong carbon-carbon bonds in diamond create a huge energy barrier to transitioning to these other structures, and researchers want to know how much extra pressure would be needed to force diamond to make the switch.

For these kinds of high-pressure experiments, researchers often use a diamond anvil cell to squeeze samples between the tips of two diamonds. But diamond anvils themselves cannot survive more than about 1 TPa. An intense laser blast can reach higher pressures, but the sudden shock also generates a lot of heat, and would start to melt diamond above 0.6 TPa.

“So instead of having an instantaneous rise in pressure, we tried to increase the pressure slowly,” says Amy Lazicki of Lawrence Livermore National Laboratory (LLNL), who led the experimental work for the new study. “And by slowly, I mean over the course of 20–30 nanoseconds.”

To achieve this, Lazicki’s team used the world’s highest-energy laser system, at LLNL’s National Ignition Facility. They ramped up the intensity of 16 laser beams focused on samples of diamond, creating a tiny explosion that launched a compression wave into the crystals. At the same time, two dozen separate laser beams fired into a metal foil to generate a 2 ns long burst of intense X-rays, which scattered through the compressed diamond to reveal its structure. Lazicki says that it took about 5 years of work to get this finely tuned system up and running, reaching almost twice the pressure achieved in any previous X-ray diffraction experiments.

“The hope was that you would only need to nudge diamond to 1.2–1.3 TPa, and then it would transform,” McMahon says. “But no: 2 TPa and it still won’t do it.”

The persistence of diamond’s face-centered cubic structure is very unusual, Lazicki adds, and no other known material can remain unchanged so far outside its comfort zone. “This bonding in diamond is really uniquely stable and strong,” she says.

The result may prove useful in modeling the interiors of carbon-rich exoplanets, where the properties of materials under enormous pressure shape the geology of these distant worlds. “There are even papers suggesting there are planet-sized diamonds out there,” McMahon says.

Lazicki’s team now hopes to try a different route to create body-centered-cubic carbon. Rather than firing lasers at conventional diamonds, the researchers aim to start with hexagonal diamond—another crystalline arrangement that is itself formed from shock-compressed graphite—which should have a lower energy barrier to transition between structures.