Characterization of the nitrogen state in HPHT diamonds grown in an Fe–C melt with a low sulfur addition

Abstract

This paper reports the results of high-pressure high-temperature (HPHT) diamonds growing in an Fe–C melt with introduction of 1 wt% sulfur. Experiments were carried out at a pressure of 6.0 GPa and a temperature of 1450 °C. It was found that adding 1 wt% sulfur leads to a significant decrease in the concentration of nitrogen defects in diamonds. As a result, the grown diamonds are near-colorless containing nitrogen in the range of 20–40 ppm. The dominant state of nitrogen impurities in the form of single substitutional atoms was confirmed by intense photoluminescence arising from the nitrogen-vacancy complexes in different charge states (NV⁰ and NV⁻). Only a very small number of nitrogen atoms are present as pairs (A and H3 centers). It is suggested that the pronounced effect of sulfur is attributed to the role of supercritical fluid in crystallization medium. Interaction between components of the H–O–C–S–N fluid leads to an increase in the proportion of complex hydrocarbons, including N-containing hydrocarbons. As a result, nitrogen atoms, which are present in the metal melt, are bound within the supercritical fluid. This nitrogen binding keeps dissociated nitrogen from entering the diamond structure.