Formation of pure polycrystalline diamond by direct conversion of graphite at high pressure and high temperature

Polycrystalline diamond aggregates have been synthesized by direct conversion of graphite at pressures of 12–25 GPa and temperatures 1800–2500 °C using a multianvil apparatus. The synthesized diamond was optically transparent and colorless, and was of cubic symmetry as determined by micro-focus X-ray diffraction. TEM analyses revealed that the sample consists of minute crystals of typically 10–20 nm, and only a very weak and broad band at ∼1332 cm−1 was observed by Raman spectroscopy. An indentation test demonstrated that the present polycrystalline diamond possesses a Knoop hardness of up to 140 GPa, which is equivalent to or even higher than those of natural and synthetic single-crystal diamonds (∼60–130 GPa) and nearly twice as high as those of synthetic polycrystalline diamonds containing binders (∼50–70 GPa). The present results imply that natural polycrystalline diamonds may have been formed by a rapid transformation from metastable graphite present in cold subducted crust, when encountering in warmer regions, such as rising plumes in the mantle transition region.