Effect of H2O on Diamond Crystal Growth in Metal–Carbon Systems

Abstract

In this paper, we report on the influence of the H2O additive in the metal melt on the growth processes, morphology, and defect-and-impurity structure of diamond crystals. As a source of water, a mixture of Mg(OH)2 and SiO2 was used, which reacted under the experimental conditions following the reaction 2Mg(OH)2 + SiO2 = Mg2SiO4 + 2H2O. The main experiments were performed in the Ni0.7Fe0.3–C system at a pressure of 6 GPa, a temperature of 1370 °C, a duration of 15 h, and a H2O concentration (CH2O) ranging from 0 to 0.5 wt %. It is found that, as the H2O content increases from 0 to 0.2 wt %, the diamond morphology changes from bulky octahedra to {111} platelike crystals and then to {111} dendrites. Further increases in CH2O in the range of 0.22–0.4 wt % result in crystallization of bulky rhombic dodecahedra and then {110} dendrites. At a CH2O higher than approximately 0.43 wt %, nucleation and growth of diamond are completely terminated and only graphite crystallizes. The systematic changes in diamond growth and morphology are accompanied by a decrease in nitrogen impurity concentration in diamond from 220–230 to 40–50 ppm, an increase in the density of microinclusions, and the appearance in the IR spectra of a peak at 2840 cm–1, related to hydrocarbons.