8-4	Exploration of Diamond Formation Process -Real-Time Observation of This Reaction under High Pressure and High Temperature-

Fig. 8-8 In situ X-ray diffraction experiment at high pressure and high temperature The sample is placed in a high-pressure cell assembly and compressed from six directions with a large hydraulic press. White X-rays collimated by incident slits irradiate the sample. A detector measures X-ray diffraction profiles of the sample.

Fig. 8-9 Variation of X-ray diffraction profiles with time during the graphite-diamond conversion process X-ray diffraction profiles of the sample were recorded consecutively every 10 seconds; this profile was taken at 8.8 GPa and 1450degree Celsius. The intensity of the graphite peak clearly was reduced, while the diamond peak became larger, reflecting the amount of diamond present.

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It is well known that graphite is transformed to a diamond under high pressure-temperature conditions. Natural diamonds were formed in the interior of the earth, while large quantities of diamonds are now industrially synthesized by high pressure-temperature technology. When using a large volume press, however, the real-time observation of the diamond conversion process has been very difficult because the reaction occurs in a heavy high-pressure vessel. Synchrotron radiation allows direct observation of such reactions in a high-pressure vessel. We have been successful in making, for the first time, an in situ observation of a graphite-diamond transition with the time-resolving X-ray diffraction method by using a large high pressure-temperature apparatus installed on the JAERI beamline at SPring-8. Graphite peaks clearly become smaller while the diamond peaks become larger (Fig. 8-9). By analyzing the experimental data obtained under various pressure-temperature conditions, we can determine the kinetics of this transition, the mechanism of nuclei formation, and crystal growth of the diamond. This study provides important information about diamond synthesis technology and significant insight into the formation mechanism of natural diamonds in the earth.

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Reference T. Okada et al., In Situ X-ray Observations of the Decomposition of Brucite and Graphite-Diamond Conversion in Aqueous Fluid at High Pressure and Temperature, Phys. Chem. Minerals., 29, 439 (2002).

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