Reactivities of transition metals with carbon: Implications to the mechanism of diamond synthesis under high pressure

The ability for a transition metal to react with carbon increases with its number of electron vacancies in d-orbitals. Elements (e.g. Cu, Zn) with no d-vacancies are inert relative to carbon. Elements (e.g. Fe, Co) with few d-vacancies are effective carbon solvents. Elements (e.g. Ti, V) with many d-vacancies are carbide formers. Transition metals with intermediate reactivities can attract carbon atoms in graphite without forming a carbide. Such a moderation of interaction may catalyze the graphite-->diamond transition in the diamond stability field and its back conversion in graphite stability field. The catalytic conversion of graphite to diamond, under high pressure in a molten metal, proceeds by nucleation and growth. Graphite will first be disintegrated into flakes by the invasion of liquid metal. These flakes are then puckered by the catalytic action of liquid metal that pulls every other carbon atom away from the basal planes of graphite. The puckering converts a graphite flake into a diamond nucleus that grows by feeding on either carbon atoms dissolved, or graphite flakes suspended in the molten catalyst. The capability of a catalyst to nucleate and grow diamonds under high pressure may be modeled by its atomic size and electronic configuration. This model predicts that the most powerful elemental catalysts are Co, Fe, Mn, Ni and Cr. These transition metals are the most commonly used catalyst components for the commercial production of synthetic diamond under high pressure. The catalytic power, as already determined is based on a microscopic mechanism described in this research, also correlates well with the activation energies calculated from macroscopic kinetics data available in literature. (C) 1997 Published by Elsevier Science Limited.