Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni₇₀Mn₂₅Co₅-C System at HPHT

High-quality type-IIa gem diamond crystals are successfully synthesized in a Ni₇₀Mn₂₅Co₅-C system by temperature gradient method (TGM) at about 5.5GPa and 1560K. Al and Ti/Cu are used as nitrogen getters respectively. While nitrogen getter Al or Ti/Cu is added into the synthesis system, some inclusions and caves tend to be introduced into the crystals. When Al is added into the solvent alloy, we would hardly gain high-quality type-IIa diamond crystals with nitrogen concentration N_c< 1ppm because of the reversible reaction of Al and N at high pressure and high temperature (HPHT). However, when Ti/Cu is added into the solvent alloy, high-quality type-IIa diamond crystals with N_c < 1ppm can be grown by decreasing the growth rate of diamonds.