Diamond is a high-pressure crystalline form of carbon. Each carbon atom is bonded by four covalent sp3 bonds, which is a distinctive difference from low-pressure carbon forms, whose carbon atoms have three covalent sp2 bonds. The strong sp3 bonds are the origin of the high hardness of diamond. Because of the long-range crystal structure, the physical properties of diamond are anisotropic. Therefore, it is desirable to synthesize a non-crystalline, fully sp3-bonded form of carbon. We successfully synthesized non-crystalline fully sp3-bonded carbon using advanced high-pressure technology. Detailed structural analysis clarified that this material is neither crystalline nor amorphous, but paracrystalline, having no long-range but medium-range order. Thus, this material is a novel form of carbon and is referred to as ‘paracrystalline diamond’.
Figure: Structural model of paracrystalline diamond from MD simulation. Colors represent different types of atomic packing. The turquoise, gold, and red atoms represent cubic-,
hexagonal-like, and disordered atomic packing within two atomic shells, respectively.
Inset: Optical microphotograph of ‘paracrystalline diamond’. Its appearance is indistinguishable from crystalline diamond.
Tang, H., Yuan, X., Cheng, Y., Fei, H., Liu, F., Liang, T., Zeng, Z., Ishii, T., Wang, M.-S., Katsura, T., Sheng, S., Gou, H., Synthesis of paracrystalline diamond, Nature, 599, 605–610, 2021.