HIGH-PRESSURE INSITU X-RAY-DIFFRACTION STUDY OF THE PHASE-TRANSFORMATION FROM GRAPHITE TO HEXAGONAL DIAMOND AT ROOM-TEMPERATURE

High-pressure in situ x-ray diffraction was carried out to clarify the nature of the pressure-induced phase transformation in graphite at room temperature. The combined use of a Drickamer-type high-pressure apparatus with sintered diamond as an anvil material and very intense x rays from synchrotron radiation made it possible to obtain high-quality x-ray-diffraction data, as well as information on the orientation relation, for this phase transformation. It was found that the transition starts at 14 GPa at room temperature, although this onset pressure is sensitive to the nature of the sample and of the applied pressure. X-ray-diffraction profiles obtained on the high-pressure phase are well explained by the hexagonal diamond structure, but the observed c/a ratio is slightly larger than that of ideal packing. The observed orientation relation satisfies the previously proposed martensitic transition mechanism from graphite to hexagonal diamond. But this hexagonal diamond formed at room temperature is unquenchable upon the release of pressure, and how it differs from the quenched phase formed under high pressure and temperature remains to be clarified.