In situ observation of transformation of neutron-irradiated highly oriented pyrolytic graphite (HOPG) by X-ray diffraction under high-pressure and high-temperature treatment

In situ X-ray diffraction observation was done for neutron-irradiated and un-irradiated highly oriented pyrolytic graphite (HOPG) samples with synchrotron radiation to clarify the effect of irradiation-induced defects on the transformation to diamond under high-pressure and high-temperature treatment. At 16 GPa, no remarkable change appeared for the irradiated HOPG with increasing the temperature up to 800 °C. At temperatures of 1200 °C and 1400 °C, hexagonal diamond was formed, along with the formation of cubic diamond. This is probably due to annealing of the irradiation defects that led to partial restoration of the structure to the original HOPG and then enables the formation. On the other hand, in un-irradiated HOPG, hexagonal diamond was formed at 400 °C, which changed to cubic diamond at 1200 °C or higher. We guess that irradiation defects promote the nucleation of cubic diamond in the irradiated sample and then contribute to the formation of isotropic polycrystalline diamond or amorphous diamond.