Effects of additive Al on the HPHT diamond synthesis in an Fe–Mn–C system

In this paper, diamond crystals with perfect octahedron shape were synthesized in an Fe–Mn–C system with and without additive Al at 5.6–5.8 GPa and 1600–1800 K. The color, morphology, nitrogen impurity concentration and inclusions of synthetic diamond crystals were characterized by optical microscope, scanning electron microscope, microinfra-red spectrometer, and Mössbauer spectrometer, respectively. The results have shown that the additive Al has a significant influence on the color of crystals, growth rate, and inclusions etc. The color of diamond crystals synthesized without additive Al is yellow, while that with additive Al becomes light and nearly disappears. The growth rate of synthetic diamond crystals without additive Al is higher than that with additive Al. The concentration of nitrogen impurity in diamond crystals without additive Al is higher than that with additive Al. The inclusions in diamond crystals synthesized without additive Al is mainly Fe and Fe3C, while that with additive Al is Fe–Al alloy.