Diamond crystallization from an Mg-C system under high pressure, high temperature conditions

Diamond nucleation and growth in the magnesium-carbon system were studied at a pressure of 7 GPa and temperatures in the range of 1500-1900 degrees C. To explore the effects of kinetics in diamond crystallization processes the duration of experiments was varied from 5 min to 20 h. It was established that the induction period preceding diamond nucleation decreased with increasing temperature from about 17.5 h at 1500 degrees C to almost zero at 1900 degrees C, while the rate of diamond growth increased by almost three orders of magnitude, from 10 mu m h(-1) (1500 degrees C) to 8.5 mm h(-1) (1900 degrees C). The cubic morphology was found to be the stable growth form of diamond over the entire range of temperatures used in the study. Based on the data obtained it was suggested that diamond growth in the Mg-C system took place in the kinetically controlled regime. Spectroscopic characterization revealed that the synthesized diamond crystals contained boron and silicon impurities. A specific continuous absorption, giving rise to the abundant brown coloration of the produced crystals, and a band at about 1480 cm(-1) found in the Raman spectra were tentatively assigned to defects involving p-bonded carbon atoms.