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.
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 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]
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]