Irreversible high pressure phase transformation of onion-like carbon due to shell confinement

Abstract Onion-like carbon (OLC) was investigated at high pressure (up to 48 GPa) in a shear diamond anvil cell (SDAC) equipped with in-situ Raman spectroscopy control. The OLC was obtained from natural gas by a highly productive and efficient partial oxidation method. Vast bulk modulus B0 = 486 ± 15 GPa and outstanding stability under high pressure were registered in the experiment. A phase transition was observed at 20 GPa under conditions of shear deformation. The transition is characterized by an appearance of a new Raman band at 1560 cm−1 accompanied with disappearance of G band of OLC. We attribute these features to sp2-sp3 transition in the carbon nanoclusters. It was found however that the concentric multi-shell onion-like structure remains intact after pressure release, which may witness that the outer shell of the OLC serves as a confinement and prevents structural collapse. X-ray photoelectron spectroscopy revealed however that the phase transition was not completely reversible because the OLC after pressure release showed the presence of sp3-hybridized carbon in significant amounts.