A new crystalline allotrope of carbon, "C8", which is denser than diamond, has recently been claimed, following research on the plasma deposition of thin carbon films. The reported structure consists of a body-centered cubic array of C8 cubes, forming a lattice previously postulated by Burdett and termed "supercubane". Discrepancies in the crystallographic analysis and the unusual bond length distribution in the reported structure lead us, however, to doubt the validity of the proposed structure of C8. We have found, by means of a structural analysis combined with extended Hückel band calculations, that a likely alternative structure for C8 is the BC-8 structure adopted by the high-pressure γ-Si allotrope. Our calculations indicate that this allotrope of carbon should have a small direct band gap at point H in the Brillouin zone, though a slight structural distortion leads to an increase in the gap. The small band gap is associated with a relatively short (2.18 Å) nonbonded C-C contact in the BC-8 structure. Finally, the BC-8 structure has been compared to a polymer of [1.1.1]propellane.

http://dx.doi.org/10.1021/ja00185a004