M. I. Heggie & C. D. Latham / Stability of the hypothetical superhard carbon metal H-6
Recently there has been speculation about the possible appearance in low pressure diamond growth of new phases of carbon comprising cross-linked sp² chains [1]. The zig-zag chains, like the backbones of polyethylene molecules, are bound by sp² sigma bonds and the unhybridised p orbitals form a delocalised pi system. In polyethylene there are single bonds from the backbone to hydrogen atoms, whereas in the new carbon phase there are single bonds to other backbones lying in different planes. The new phases have the important property of reduced mechanical anisotropy (compared with graphite) arising from the fact that the backbones in adjacent planes are bonded together by single covalent bonds. This fact, together with the substantial concentration of sp² sigma and pi bonds, indicates that the new phases should be harder than diamond. At the same time the delocalised pi systems are predicted to give rise to metallic behaviour.
One of these phases, referred to as H6 in ref. 1, is believed to grow very easily on the {111} surface of diamond and has been suggested as the cause of the electrically conducting surface layer found on freshly deposited homo-epitaxial diamond films[1].
However, the metallic system suggested, with a significant density of states at the Fermi level, should suffer from the Peierls instability and undergo a pairing distortion. Using an interatomic potential derived for graphite and diamond and studying unit cells of double the conventional size (pairing sometimes cannot occur in the conventional unit cells) we show that the erstwhile metallic phases are likely to give rise to insulating phases of lower energy.
[Abstract] [Introduction] [Computer Models] [Results] [Acknowledgement] [References] [Figure 1]
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