Synthesis of pure sintered body of hexagonal diamond and its characterization

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We performed a series of high pressure and high temperature experiments using a large-volume press to synthesize a pure, single-phase hexagonal diamond sintered body. Our study demonstrated that single-phase hexagonal diamond, which includes neither traces of unreacted graphite starting material nor cubic diamond, can be obtained at 25 GPa and 1100-1500C through direct conversion of highly-oriented, highly-crystalline graphite. The sample consists of extremely thin layered units (a few to several tens of nm) and is as dense as cubic diamond. Since our preliminary hardness measurements provided very high indentation hardness values comparable to or exceeding that of cubic diamond, this nano-sintered body of pure hexagonal diamond would have potential application as a novel ultra-hard material.