▎ 摘　　要

We show that two graphene layers stacked directly on top of each other (AA stacking) form strong chemical bonds when the distance between planes is 0.156 nm. Simultaneously, C-C in-plane bonds are considerably weakened from partial double bond (0.141 nm) to single bond (0.154 nm). This polymorphic form of graphene bilayer is metastable with an activation energy of 0.16 eV/cell with respect to the standard configuration bound by van der Waals forces at a larger separation between planes (0.335 nm). Carbon atoms form four single bonds in a geometry mixing 90 degrees and 120 degrees angles, intermediate between the usual sp(2) and sp(3), but similar to the one found in molecules like the cubane, pentaprismane, or hexaprismane. Under an in-plane stress of 9 GPa, this carbon allotrope becomes the global energy minimum. As a function of the separation between layers, the electronic band structure goes through different regimes: It is a semimetal at van der Waals-like distances, a wide gap semiconductor at covalentlike distances, and in between it displays metallic behavior.