Metastability of diamond ramp-compressed to 2 terapascals*

Carbon is the fourth most prevalent element in the universe and essential for all known life. In the elemental form it is found in multiple allotropes including graphite, diamond, and fullerenes, and it has long been predicted that even more structures can exist at greater than Earth-core pressures. Several new phases have been predicted in the multi-terapascal (TPa) regime, important for accurately modeling interiors of carbon-rich exoplanets. By compressing solid carbon to 2 TPa (20 million atmospheres; over 5 times the pressure at the Earth's core) using ramp-shaped laser pulses, and simultaneously measuring nanosecond-duration time resolved x-ray diffraction, we found that solid carbon retains the diamond structure far beyond its regime of predicted stability.  The results confirm predictions that the strength of the tetrahedral molecular orbital bonds in diamond persists under enormous pressure, resulting in large energy barriers that hinder conversion to the more stable high-pressure allotropes, just as graphite formation from metastable diamond is kinetically hindered at atmospheric pressure. This work nearly doubles the record high pressure at which x-ray diffraction has been recorded on any material. [Lazicki, A., McGonegle, D., Rygg, J.R. et al. Metastability of diamond ramp-compressed to 2 terapascals. Nature 589, 532–535 (2021)]