The number of observed or guessed atomic architectures formed by elemental
carbon has unexpectedly increased in the last decades. In addition to graphite and
diamond, a multiplicity of other structures – such as well ordered fullerenes and
nanotubes, or less ordered structures like nanoporous and amorphous carbon – have
been synthesized and studied. Similarly to what happens for diamond with respect to
graphite, the new metastable phases (although basically sp2-bonded) are definitely
other than graphite (still to consider as the most stable phase) as for their physical
and chemical features in a wide range of temperatures and pressures. Interestingly,
the mechanical, structural, and electronic properties of these new forms of carbon
are extremely different, ranging from soft to hard, from compact to open, from insulating to metallic