Discovery of Gradia between Graphite and Diamond

ConspectusGraphite and diamond, two naturally occurring carbon allotropes, have been extensively exploited for millennia. However, it was not until the mid-20th century that scientists successfully synthesized diamonds using the phase transformation of graphite under high pressure and high temperature. Understanding the mechanism of direct phase transformation from graphite to diamond is of great scientific and practical significance and has stimulated extensive interest and efforts. Although the concerted transformation mechanism and nucleation-and-growth mechanism proposed theoretically are helpful for the understanding of phase transformations, they fail to account for the diffraction peak of so-called "compressed graphite" observed at ∼3.1 Å in partially transformed samples. Recently, we proposed a new mechanism called motif propagation mechanism by combining electron microscopy observation and theoretical simulation. It solves the long-standing puzzle of how graphite transforms into diamond. Within the motif propagation mechanism, the formation of diamond motifs initially begins with the connection of two carbon six-membered rings belonging to adjacent graphite layers; then, the motifs gradually advance toward the graphite region along the graphite–diamond coherent interface, thereby achieving the transformation from graphite to diamond. Our results demonstrate that graphite is not completely transformed into diamond at once but first transforms into an intermediate structure, namely a graphite–diamond hybrid (called Gradia). Then, Gradia gradually transforms into diamond. Gradia, a metastable carbon form, can be quenched to ambient conditions and exists stably. The diffraction peaks of Gradia include those of both diamond and so-called compressed graphite, which match well with the diffraction peaks of intermediate products determined in experiments. In this Account, we provide a comprehensive overview of the concerted transformation mechanism, the nucleation-and-growth mechanism, and the motif propagation mechanism as well as the discovery of Gradia. Gradia represents a new form of carbon with excellent mechanical properties, such as superhigh hardness and toughness, and adjustable electrical properties. In Gradia, the proportion of graphite and diamond domains as well as interface types can be tailored, which opens up promising opportunities for nanostructure engineering to achieve the desired properties that are inaccessible to diamond and graphite alone. The discovery of Gradia also holds significant implications for previously unresolved scientific mysteries, such as the formation mechanism of "cold-compressed graphite", diamond graphitization, and the phase transformation of boron nitride (BN), known as the twin brother of carbon. Gradia-BN is expected to be synthesized, which may possess excellent thermal stability and chemical inertness, superior hardness and toughness, and unique electronic properties.