Editing Buckminsterfullerene (section)

===Solid===
[[Image:C60-Fulleren-kristallin.JPG|thumb|Micrograph of C<sub>60</sub>.]]
[[File:Fullerite structure.jpg|thumb|Packing of {{chem|C|60}} in crystal.]]
In solid buckminsterfullerene, the C<sub>60</sub> molecules adopt the fcc ([[face-centered cubic]]) motif. They start rotating at about −20&nbsp;°C. This change is associated with a first-order phase transition to an fcc structure and a small, yet abrupt increase in the lattice constant from 1.411 to 1.4154&nbsp;nm.<ref name=k372>[[#Katz|Katz]], 372</ref>

{{chem|C|60}} solid is as soft as [[graphite]], but when compressed to less than 70% of its volume it transforms into a [[superhard materials|superhard]] form of [[diamond]] (see [[aggregated diamond nanorod]]). {{chem|C|60}} films and solution have strong non-linear optical properties; in particular, their optical absorption increases with light intensity (saturable absorption).

{{chem|C|60}} forms a brownish solid with an optical absorption threshold at ≈1.6&nbsp;eV.<ref>[[#Katz|Katz]], 361</ref> It is an n-type [[semiconductor]] with a low activation energy of 0.1–0.3&nbsp;eV; this conductivity is attributed to intrinsic or oxygen-related defects.<ref name=k379>[[#Katz|Katz]], 379</ref> Fcc C<sub>60</sub> contains voids at its octahedral and tetrahedral sites which are sufficiently large (0.6 and 0.2&nbsp;nm respectively) to accommodate impurity atoms. When alkali metals are [[Doping (semiconductor)|doped]] into these voids, C<sub>60</sub> converts from a semiconductor into a conductor or even superconductor.<ref name=k372/><ref name=k381>[[#Katz|Katz]], 381</ref>