Intermediate carbon phase. New experimental data and atomic model

Abstract

We investigated phase transitions in diamond under cyclic loading near the graphite-diamond equilibrium line at temperatures sufficiently below the Debye temperature (maximum stresses and temperature in the diamond do not exceed 6 GPa and 420 K accordingly). Using Raman and transmission electron microscopy, we reveal the following stages of the transition of diamond to graphite-type phase (onions). The first stage is the plastic deformation by the mechanical twinning along with martensitic phase transition of diamond to intermediate carbon phase (ICP). The next stage is the transition of ICP to onions with a radius below ~5 nm. The proposed theoretical ICP model based on the crosslinking of graphene layers by Frenkel defects displays not only good agreement with the available structural experimental data but also specific properties like enhanced out-of-plane stiffness and metallicity.