“Compressed Graphite” Formed DuringC60to Diamond Transformation as Revealed by Scattering Computed Tomography

The collapsing of C₆₀ into polycrystalline diamond has been studied after nonhydrostatic pressurization at ambient temperature using x-ray scattering computed tomography. Using this selective structural probe we provide evidence of concentric coexistence of ``compressed graphite'' (d₀₀ₗ~3.09--3.11), sp²-graphitelike phase (d₀₀ₗ~3.35--3.42), and sp³-like amorphous carbon surrounding polycrystalline diamond (a~3.56--3.59). The so-called ``compressed graphite'' exhibits a collapsed c axis and is textured with disordered layers. This latter phase is better described as a short interlayered carbon phase with buckled sp²-sp³ layers with possible interlayer bonding. Additionally, our 3D maps of phase distribution and of the residual stress retained in the polycrystalline diamond phase support the importance of stressed synthesis conditions for diamond formation.