Self-compression and diamond formation in carbon onions

Carbon onions-concentric spherical graphite layers stacked one inside the other-are stable only during electron irradiation. If the irradiation is continued at high temperature, self-compression is shown to occur, in which the individual layers are compressed until the onion cores finally transform into diamond crystals, which then grow under further radiation. Two scenarios are sketched for the mechanism of pressureless transformation of graphitic materials into diamond and the consequences of these observations for future work are highlighted.