Highly boron-doped graphite and diamond synthesized from adamantane and ortho-carborane under high pressure

Abstract

The high-pressure synthesis of boron doped diamonds from boron-containing organics has proven effective. This work is devoted to the study of the synthesis under high pressure of diamond and graphite doped with boron, both materials being in demand in high-tech applications. At 8 GPa and 1700 °C, graphite is obtained from adamantane C10H16, whereas microcrystals of boron-doped diamond (2 ÷ 2.5 at.% of boron) are synthesized from a mixture of adamantane and ortho-carborane C2B10H12 (atomic ratio B:C = 5:95). This result indicates the promotional activity of boron in the synthesis of diamond under high pressure. At pressures lower than 7 GPa, only graphite is synthesized from the adamantane and carborane mixture. Graphitization starts at quite low temperatures (below 1400 °C) and an increase in temperature simultaneously increases boron content and the quality of the graphite crystal lattice. Extensive study of the material structure allows us to assume that the substitutional boron atoms are distributed periodically and equidistantly from each other in the graphite layers at boron concentrations above ∼1 at.%. The theoretical arguments and model ab initio calculations confirm this assumption and explain the experimentally observed boron concentrations.