A quenchable superhard carbon phase synthesized by cold compression of carbon nanotubes

A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of ≈100 GPa. A hexagonal carbon phase was formed at ≈75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp³-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a₀ = 2.496(4) Å, c₀ = 4.123(8) Å, and V₀ = 22.24(7) ų. There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 ± 0.2 g/cm³, which is at least compatible to that of diamond (3.52 g/cm³). The bulk modulus was determined to be 447 GPa at fixed K′=4, slightly greater than the reported value for diamond of ≈440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.