Diamond Spontaneous Crystallization From Mg-Ni-C System at HPHT

In this work, different Mg-Ni alloys were used as carbon solvents in the synthesization of diamonds at a pressure of 7.7 GPa and temperatures of 1473 to 2073 K. All the alloys were melted by a special treatment at high pressure. The temperature ranges were determined for spontaneous diamond crystallization, using Mg2Ni, MgNi2 compounds and two eutectic alloys, according to the Mg-Ni phase diagram. Diamonds were observed to form at temperatures exceeding 1873 K when Mg2Ni and 0.89at.%Mg+0.11at.%Ni alloys were used as carbon solvents. X-ray diffraction patterns revealed the presence of MgO. Spontaneous diamond crystallization was found to occur above 1573 K, using the MgNi2 alloy. Cubo-octahedral morphology of diamond crystals were detected at temperatures of 1723-1823 K. Intense spontaneous diamond crystallization was observed at temperatures of 1573 K and upward when the eutectic 0.23at.%Mg+0.77at.%Ni alloy was employed. Unlike the behavior with the MgNi2 alloy, the rate of spontaneous crystallization increased, as did the number of crystallization centers. The MgNi2 carbon solvent alloy was found to be preferable for the synthesis of high-quality diamond crystals.