Monitoring diamond crystal growth, a combined experimental and SIMS study

Abstract

Detailed ion microprobe measurements have been made on two synthetic diamond crystals in order to investigate how variations in chemical and isotopic compositions are related to growth sectors and overall growth history. The crystals were grown by the metal catalyst technique under identical T - P conditions of 1450 °C and 5.5 GPa, but with different source nitrogen abundances. Measurements for carbon and nitrogen isotope compositions and nitrogen abundance were made in traverses across the crystal sectors, which included cubic sectors and octahedral sectors of both relatively rapid and relatively slow growth. In both crystals an early phase of growth dominated by falling δ 13 C and rising of N ppm is followed by an extensive phase of growth with moderately constant δ 13 C and gradually descending N ppm . The change from falling to stable δ 13 C ratios has been numerically modelled on the basis of the carbon isotope fractionation between the carbon solution in metal melt and the growing diamond in closed system; the stabilization of the diamond carbon isotope composition is achieved once a steady state is attained and diamond grows with the same carbon isotope composition as the graphite source. The decreasing N ppm values appear to be a product of Rayleigh fractionation. Carbon isotope compositions show little difference between different growth sectors, and δ 15 N values in any given sector show no change with time of growth. However, the nitrogen isotope compositions show major differences of ca. 30 ‰ between octahedral and cubic sectors. These differences are not related to growth rate or time of growth and appear to represent a consistent difference in N isotope adhesion between the two faces.