Possible 3D Carbon Structures as Progressive Intermediates in Graphite to Diamond Phase Transition

Modeling successive contractions of conventional graphite structure in different directions, together with some chemical reactions (consisting of Csp2 to Csp3 hybridization changes, followed by new bonding between closing carbons), simulates a graphite to diamond phase transition. The stable intermediate structures were optimized by a molecular mechanic approach, giving rise to new hypothetical 3D crystal structures which progress along the transition in the order of increasing density from 2.22 to 3.30 gcm-3. There are graphite bilayers with planar and pyramidal Csp2, graphite bilayers with Csp2 and Csp3, diamonds with Csp2 and Csp3, and diamonds with only Csp3. We present eight different models: seven orthorhombic and one cubic Ia-3, the latter close to one structure proposed for supercubane C8. Additionally, some other noncrystalline models, made by optimizing unconstrained clusters from the above crystal models, are proposed. Although these structures are speculative, some of them present synthetic polycrystal X-ray diffraction with their highest peaks among those observed in reported experimental phases between graphite and diamond, which gives more reality to the proposed models.