Diamond powders synthesized at high pressure and high temperature from graphite with nickel in the presence of aluminum. Applicability of methods for analyzing nitrogen concentration in diamonds

Two samples of microcrystalline diamond powders (<200 ​μm) were synthesized with high growth rates at high pressure (5 ​GPa) and temperature (1650 ​°C) in a graphite–nickel medium (reference sample #D) and in the same mixture with the addition of aluminum powder (#DAl). Commercial diamond powder SDB1100 was used as a reference. The content of nitrogen in various forms (substitutional, charged substitutional, pairs) in diamond crystals was calculated from the FTIR, EPR, and Raman scattering data using different approaches. It is shown that some equations widely used to determine the concentration of nitrogen in diamond crystals give overestimated values and should be used with caution. The size fractionation of the synthesized diamond powders makes it possible to reveal the features of crystallization and the effect of aluminum on the growth patterns. The nitrogen concentration in sample #DAl is 100–190 ​ppm, which is about 2–4 times less than in sample #D. Aluminum significantly reduces spontaneous nucleation, which leads to a change in the crystal size distribution with the predominance of large crystals.