Synthesis of Diamonds and Their Identification

Abstract

Since 1797, when Tennant demonstrated that diamond consists solely of elemental carbon by comparing the volume of carbon dioxide formed by burning identical weights of charcoal and diamond (Tennant 1797), scientists have been trying to synthesize diamond by converting various carbon-containing substances. Several attempts followed, but it would take over 150 years of continued research before the first successful report of diamond growth was published by scientists at General Electric (Bundy et al. 1955). Diamond synthesis by Union Carbide in 1952 and Allmänna Svenska Elektriska Aktiebolaget (ASEA) in 1953 predate the work at General Electric, but were not reported until later (Liander and Lundblad 1960; Eversole 1962; Angus 2014). The most familiar carbon allotropes are the crystalline phases of graphite and diamond. The hexagonal form of graphite consists of sheets of sp2 hybridized carbon atoms in a hexagonal array, with each atom bonded to three equidistant nearest neighbor atoms. The layers are attracted to each other by weak van der Waals forces and may be arranged in a hexagonal, or rarely rhombohedral, stacking sequence. Meanwhile, the carbon orbitals in diamond are sp3 hybridized, with each atom covalently bonded to four nearest neighbors in a tetrahedral arrangement. The prevalent diamond structure is cubic, though the hexagonal lonsdaleite form also exists. Figure 1 illustrates the pressure and temperature (P, T) phase and transition diagram for pure carbon. Theoretically and experimentally determined conditional phase boundary lines separate the diamond and graphite stability fields. The high cohesive and activation energies associated with the different carbon phases mean that other metastable forms can occur under conditions at which they are not thermodynamically stable. For instance, diamond exists at room temperatures and pressures, whereas graphite can survive pressures well into the diamond stability field (Bundy 1980; Bundy et al. 1996).