Diamond formation inside carbon onions

Carbon onions are concentric near-spherical carbon shell structures that self-assemble during electron irradiation of graphitic and disordered carbon precursors. A possible structure for the onion is a hierarchical Russian doll structure with larger fullerene-like shells enclosing smaller ones, keeping graphite-like interlayer separation. Onions assembled at room temperature, however, contains large amounts of structural defects that lead them to disintegrate slowly when the irradiation is stopped. Nigh coherency between the shells can be obtained when onions are assembled at higher temperatures but the resulting structures show large compressions in the inter-layer spacings towards the cores. This compression in the layer spacing (from 0.34 nm down to 0.22 nm) towards the cores implies extremely high pressures that are able to nucleate diamond phase in the onion centers. Once formed the diamond remains stable and grows under continued irradiation, completely transforming the onions into diamond crystallites. A critical evaluation of this phenomenon is described here.