Formation of the high pressure graphite and BC₈ phases in a cold compression experiment by Raman scattering

We use Raman scattering to study phase transition in the graphitic g-BC₈ phase and graphite at high pressure up to 84 GPa. The E _2g Raman active mode of graphite (G peak) can be detected up to 84 GPa. We demonstrate that there is (1) a phase transition in g-BC₈ and in graphite at 35 GPa and (2) that above 35 GPa, the g-BC₈ and graphite transform under high pressure to possibly fully sp³-bonded, disordered hp-BC₈, and hp-C phases. Below the phase transition, a polynomial fit to the G peak position versus pressure data yielded a quadratic relation; above the phase transition, it demonstrates linear behavior. The phase transition at high pressure in BC₈ system and graphite is reversible. Quenched hp-BC₈ and hp-C phases have the Raman spectrum typical to that of the graphitic phases.