The phase transition from graphite to diamond has been extensively studied. In meteoritic samples, it comes intermingled within the diamond network showing features consistent with faults in the diamond network 4. Lonsdaleite is the less common of the two diamond polytypes and it is not detected free in nature. The investigation of the Cañón del Diablo meteorite uncovered another sp 3 allotrope, the lonsdaleite, where carbon atoms are bonded in a hexagonal crystalline structure 1, 2, 3. In diamond, carbon atoms are bonded through sp 3 bonds in a cubic network. Within each sheet the atoms are disposed in a honeycomb lattice, each atom linked to the three neighbours through strong covalent sp 2 bonds. In graphite, carbon atoms are arranged in sheets, weakly bound together by van der Waals forces with an interlayer separation of ~ 3.4 Å. The two most common allotropes of solid carbon are graphite and diamond. High sensitivity X-ray diffraction experiments and Raman spectroscopy confirm the formation of diamond within the islands. Electron energy loss spectroscopy of the islands show that the sp 2 to sp 3 hybridation transition is a surface effect. We show high resolution electron microscopy images of pyrolytic carbon evidencing the dislocation of the superficial graphitic layers after irradiation and the formation of crystallite islands within them. In this work, we report a third method consisting in the irradiation of graphite with ultraviolet photons of energies above 4.4 eV. They induce it by increasing either pressure or temperature on graphite. As today, two basic processes have been successfully tested to drive this transition: strong shocks and high energy femtolaser excitation. The out-of-plane distortions required for the transition are a good tool to understand the collective behaviour of layered materials (graphene, graphite) and the van der Waals forces.
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