fullerite (rus. фуллерит) — molecular crystals of fullerenes molecules.

Description

Fullerites are the solid phases of fullerenes. They are molecular crystals with van der Waals intermolecular interactions. They can be obtained, for example, via the evaporation of fullerene extracts in toluene that are being used to isolate fullerenes from arc-discharge soot. W. Kratschmer and D. Huffman, inventors of the arc-discharge technique, were the first to observe a solid fullerite in 1990 at the Institute of Nuclear Physics in Heidelberg. Below the most widely studied C60 fullerite will be discussed, the properties of other fullerites being (with some apparent reservations) rather similar.

C60 fullerene is a semiconductor with a band gap of 1.5 eV (see the band theory). Since the C60 molecules are spheroidal, the most favourable packings thereof are close-packed lattices, which can be viewed as stacks of close-packed flat layers where each sphere is surrounded by six others. The spheres of each layer fit into the triangular interstices formed by the previous layer. As each layer gives rise to two equivalent subsets of interstitial cavities on each side, there is a potentially infinite family of close-packed forms that differ in relative layout of the layers, the so-called polytypes. In C60 fullerite, the molecules form a three-layer close-packed lattice ... ABCABC ..., which means that the molecules are exactly above each other every three layers. This lattice is also known as a face-centred cubic (FCC). However, there can be other, slightly less stable fullerite modifications, such as hexagonal close-packed (HCP), a two-layer close-packed lattice ... ABAB ... The polytypes differ also in the topology of the system of interstices.

The C60 molecules in a fullerite are capable of reorientation (rotation), the rotational degrees of freedom gradually freeing with the increase of temperature. This results in orientational phase transition at around 261 K. At higher temperatures, the molecules rotate in quite an isotropic way and the dynamic symmetry of the crystal matches the symmetry of the fcc lattice. At lower temperatures, only certain types of reorientations are possible, so the true symmetry of the crystal becomes lower, though the positions of the molecules do not change.

Fullerites are quite chemically and thermally stable, although they are thermodynamically metastable with respect to graphite. They remain stable in an inert gas atmosphere up to 1200 K, when they start to graphitize. No liquid phase is observed up to this temperature. In presence of oxygen, significant oxidation is reported at as low as 500 K and CO and CO2 are formed. Traces of solvents also facilitate the chemical degradation of fullerites. Fullerites are fairly readily soluble in nonpolar aromatic solvents and in carbon disulphide CS2.

The fullerene molecules in fullerites being closely packed, the fullerites can be used to produce various oligomers and polymer phases via exposure to light, electron bombardment or pressure. The orthorhombic phase built of linear chains of linked C60 molecules, as well as the tetragonal and rhombohedral phases, consisting of covalently bound tetragonal and hexagonal layers of C60, respectively, were obtained under pressures up to 10 GPa.

Also reported were ferromagnetic polymerised phases (the so-called magnetic carbon) obtained from fullerites under increased pressure and temperature, although the nature of these magnetic phenomena and the experimental data themselves are not entirely clear. Such effects can be due to the formation of defects, the presence of impurities, and partially destructed fullerene molecules. At pressures above 10 GPa and temperatures above 1800 K, the diamond phase is obtained, and under certain conditions nanodiamonds are formed. It is noteworthy that diamonds form from fullerites at lower temperatures than from graphite.

Fullerites feature relatively large intermolecular interstices, which can encapsulate atoms and small molecules. Filling them with alkali metal atoms leads to fullerides that can exhibit superconducting properties at temperatures of up to 20-40 K.

Illustrations

Fullerite structure.
Fullerite structure.

Authors

  • Zaitsev Dmitry D.
  • Ioffe Ilya N.

Sources

  1. Sidorov L.N., Jurovskaja M. A. Borshhevskijj A. Ja. et al. Fullerenes. (in Russian) — Moscow: Ehkzamen, 2005. — 687 pp.
  2. Zolotukhin I. V. Fullerenes - a new form of carbon (in Russian)// Sorosovskijj obrazovatel'nyjj zhurnal. 1996. №2. P. 51–56. — http://window.edu.ru/window/catalog?p_rid=21293

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