Scientists explain why graphite forms hexagonal diamond first under pressure

Nucleation favors hexagonal structure initially

Scientists discovered that a hexagonal form of diamond called lonsdaleite forms first when graphite is subjected to extreme pressure. This occurs even though the familiar cubic diamond has lower bulk enthalpy in the final crystal state. The study compared nuclei within a graphite matrix at pressures ranging from 18 to 22 gigapascals. At this scale, creating an interface with graphite and deforming surrounding layers costs more energy for cubic diamond.

Machine learning models reveal atomic rules

The team used interatomic potentials trained on quantum-mechanical calculations to model systems containing up to 700,000 atoms. Machine learning allowed them to describe atomic interactions large enough to account for graphite deformation around the nucleus. Professor Pavel Sorokin noted that the barrier to forming an optimal lonsdaleite nucleus is about half as high. Consequently, the critical number of atoms required is two to three times smaller than for cubic diamond.

Lonsdaleite adapts better to graphite

Models showed that lonsdaleite adapts better to the graphite environment during the initial nucleation stage. As the crystal grows and the boundary with graphite relaxes, it eventually transforms into cubic diamond. If subsequent restructuring is hindered, the metastable lonsdaleite structure may persist indefinitely. This mechanism helps scientists control synthesis of superhard materials and predict properties under extreme conditions.