Superdense carbon allotropes are proposed configurations of carbon atoms that result in a stable material with a higher density than diamond. Few hypothetical carbon allotropes denser than diamond are known. All these allotropes can be divided at two groups: the first are hypothetically stable at ambient conditions; the second are high-pressure carbon allotropes which become quasi-stable only at high pressure.

Ambient conditions

According to the SACADA ^[1] database, the first group comprises the structures, called hP3, ^[2] tI12, ^[2] st12, ^[3] r8, ^[4] I41/a, ^[4] P41212, ^[4] m32, ^[5] m32*, ^[5] t32, ^[5] t32*, ^[5] H-carbon ^[6] and uni. ^[7] Among them, st12 carbon was proposed as far as 1987 in the work of R. Biswas et al. ^[3]

High-pressure carbon

The following allotropes belong to the second group: MP8, ^[8] OP8, ^[8] SC4, ^[9] BC-8 ^[10] and (9,0). ^[11] These are hypothetically quasi-stable at the high pressure. BC-8 carbon is not only a superdense allotrope but also one of the oldest hypothetical carbon structures - initially it was proposed in 1984 in the work R. Biswas et al. ^[10] The MP8 structure proposed in the work J. Sun et al., ^[8] is almost two times denser than diamond - its density is as high as 7.06 g/cm³ and it is the highest value reported so far.

Band gaps

All hypothetical superdense carbon allotropes have dissimilar band gaps compared to the others. For example, SC4 ^[9] is supposed to be a metallic allotrope while st12, m32, m32*, t32, t32* have band gaps larger than 5.0 eV. ^{[5] [3]}

Carbon tetrahedra

These new materials would have structures based on carbon tetrahedra, and represent the densest of such structures. On the opposite end of the density spectrum is a recently theorized tetrahedral structure called T-carbon. This is obtained by replacing carbon atoms in diamond with carbon tetrahedra. In contrast to superdense allotropes, T-carbon would have very low density and hardness. ^{[12] [13]}

References

External links

• SACADA - Samara Carbon Allotrope Database