Diamond Melting in Shock Compression Experiments at 1 TPa Pressures

Carbon’s abundance, strong covalent bonding and relevance to inertial confinement fusion and planetary science have motivated extensive investigation of its phase diagram at terapascal pressures. However, the melting curve and the existence of phases beyond diamond remain uncertain. Here we resolve the discrepancy between experiments and theoretical simulations of the melting temperature of diamond and show that the diamond structure persists up to 1 TPa. This contradicts a previous report of a transition to the BC8 phase, which density functional theory predicts to be the thermodynamically stable phase of carbon above pressures around 1 TPa. We combine optical velocimetry, pyrometry and X-ray diffraction to probe microcrystalline diamond under nanosecond shock compression. Shock temperature and reflectivity measurements reveal changes in thermodynamic and optical properties, along with a decrease in X-ray diffraction intensity. These results provide evidence for shock-induced melting with a slight decrease in melting temperature with increasing pressure near 7,300 K. Our work delivers atomic-scale benchmarks for quantum simulations of condensed matter at extreme conditions, with implications for planetary interiors. Our improved understanding of diamond melting might also be relevant for achieving higher energy gain in laser-driven nuclear fusion. The location of the melting curve of diamond is unclear. Now, this is illuminated with shock compression experiments that provide strong evidence for shock-induced melting.