LLNL Researchers Resolve Diamond Melting Discrepancies at Extreme Pressures

Researchers at Lawrence Livermore National Laboratory (LLNL) have published a groundbreaking study in Nature Physics that measures how diamond behaves under extreme pressures—three times greater than those found at Earth’s core. This work addresses long-standing questions about the melting behavior of diamond, a material of scientific interest due to its incredible hardness and potential applications in inertial confinement fusion.

For over 20 years, scientists have debated how diamonds respond to extreme conditions. Previous experiments by LLNL’s Jon Eggert and colleagues revealed that diamond densities increased upon melting, contradicting common material behavior. “Liquid water is denser than ice, which makes ice cubes float. Jon’s finding means that diamond would float in liquid carbon at high pressures,” explained LLNL scientist Marius Millot.

However, earlier melting temperature measurements differed by approximately 20% from theoretical predictions, leading to confusion. Even advanced simulations failed to reproduce the experimental results. Additional experiments at Sandia National Laboratories suggested that diamond might transition to another crystalline phase before fully melting, but this theory lacked direct atomic evidence.

Advancements through Laser-Driven Compression

To delve deeper into these discrepancies, the LLNL team conducted laser-driven dynamic compression experiments at the University of Rochester’s Laboratory for Laser Energetics (LLE). Using intense laser energy, they generated powerful shockwaves that compressed tiny diamond samples at conditions lasting just a billionth of a second. The team was able to measure properties like atomic structure and temperature in real-time.

Millot reported, “This was the first time that shock-compressed diamond was probed with X-ray diffraction all the way up to melting.” The researchers’ new diagnostics improved data quality significantly, leading to findings that aligned closely with quantum mechanical simulations, thus settling the long-standing temperature discrepancy.

Diamonds Retain Structure Under Pressure

The experiments revealed that diamonds maintain their structure up to the point of melting without transforming into an intermediate phase. Millot noted, “We think that is because the sample does not have time to change when it only experiences a single shock.” This insight may influence future experiments dealing with materials at high energy densities, suggesting that shock delivery methods can significantly affect material behavior.

Implications for Fusion Energy and Planetary Science

These findings have immediate implications for inertial confinement fusion, where precise diamond capsule implosions are critical for efficient fusion reactions. The new results indicate that the initial shock wave’s strength could be reduced while still ensuring complete melting of the diamond capsule. “Our work indicates that we could use slightly slower initial shocks and still achieve full melting of the diamond in our NIF implosions,” noted Millot. This adjustment could potentially triple energy gain in fusion reactions, enhancing the effectiveness of the lasers used.

Furthermore, the study’s insights into diamond behavior could aid scientists in modeling the interiors of ice giants like Neptune and Uranus, where extreme pressures may cause carbon to crystallize into diamonds that then sink, creating what is referred to as “diamond rain.” The new melting measurements facilitate a stronger foundation for understanding these distant planets.

The LLNL team intends to continue exploring diamond under even more extreme conditions, focusing on how diamond capsules perform during later stages of fusion implosions and the stability of their crystal structure subjected to multiple shock waves. Other contributors to the study include Federica Coppari, Amy Lazicki, Yong-Jae Kim, Otto Landen, Vladimir Smalyuk, and Peter Celliers, with support from LLNL’s Laboratory Directed Research and Development program.