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).
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.
On the opposite end of the density spectrum is a recently theorized tetrahedral structure called T-carbon.