Superdense carbon allotropes are proposed configurations ofcarbon atoms that result in a stable material with a higher density thandiamond. 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.

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]

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.

All hypothetical superdense carbon allotropes have dissimilarband 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]

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 calledT-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]

• SACADA - Samara Carbon Allotrope Database

• Allotropes of carbon

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