Nanocrystals of face-centred cubic carbon, i-carbon and diamond obtained by direct conversion of graphite at high temperatures and static ultra-high pressures

Abstract

Until recently graphite, diamond and amorphous carbon were considered to be the only three carbon modifications of a terrestrial origin present in nature. Recently numerous publications indicated the existence of two other uncommon metastable carbon allotropes - face-centred cubic carbon and i‑carbon – on Earth. Nanoparticles of these carbon allotropes were found in carbon spherules of samples taken from soils and rocks in different countries of Europe and North America, crude oil, ice sheets from Greenland, etc. The existence of these metastable carbon allotropes in nature appeared to confirm the Younger Dryas extraterrestrial impact hypothesis proposing such an impact over North America, that caused a global catastrophe and climate change. This hypothesis suggests an extraterrestrial impact origin of these carbon allotropes due to dynamic shock compression, since to date they have only been simultaneously produced in ultra-high pressures dynamic shock experiments. We report the first synthesis of nanocrystals of the both metastable carbon allotropes along with diamond at elevated temperatures and static ultra-high pressures. This provides evidence that nanocrystals of face-centred cubic carbon and i‑carbon present in nature are likely to form in Earth's mantle, so that they have a terrestrial origin. Results of electron diffraction and high-resolution transmission electron microscopy clearly indicate that face-centred cubic carbon and i‑carbon are characterized by unique crystal lattices, which have nothing common with the diamond crystal lattice. These two carbon allotropes are thought to be characterized by unconventional chemical bonds due to unusual combinations of their crystal lattice types and lattice parameters. The nature of chemical bonds in face-centred cubic carbon and i‑carbon is supposed to be different from that of conventional basic chemical bonds (covalent, ionic and metallic bonding as well as the van der Waals forces). These carbon allotropes can therefore possess unique and potentially valuable properties suitable for different applications.