Читать книгу Solid State Chemistry and its Applications - Anthony R. West - Страница 44

The simplest, best known and symmetrical fullerene is C₆₀ whose discovery led to the 1996 Nobel Prize in Chemistry awarded to Kroto, Curl and Smalley. It is a hollow, spherical cage of diameter 7.1 Å formed from 3‐coordinate (approximately sp ² hybridised) C atoms which link to form a network of 12 pentagons and 20 hexagons, Fig. 1.27(a), that match exactly the pattern on a soccer ball (hence the alternative, colloquial, American name ‘buckyball’; the name ‘fullerene’ is in honour of Buckminster Fuller, who designed the geodesic dome that has the same network structure as C₆₀).

Not surprisingly, in crystalline C₆₀, the C₆₀ molecules arrange themselves according to the principles of close packing. At room temperature, crystalline C₆₀ is face centred cubic, Z = 4 (C₆₀), a = 14.17 Å, and the C₆₀ molecules are orientationally disordered. The separation of adjacent C₆₀ molecules can be calculated with the aid of Fig. 1.22(a); a cube face diagonal corresponds to a cp direction from which we calculate that the centres of adjacent molecules are separated by 10.0 Å. The ‘hard diameter’ of a C₆₀ molecule is calculated to be 7.1 Å which leaves a gap of 2.9 Å between adjacent molecules for van der Waals bonding.

Figure 1.27 (a) The C60 molecule; one pentagon surrounded by five hexagons is shown in bold. (b) Parts of two C60 molecules in the region of closest contact showing a short C–C bond separating two hexagons in one molecule pointing towards the middle of a pentagon in an adjacent molecule.

Adapted from W. I. R. David et al., Nature 353, 147 (1991).

Because the C₆₀ molecules are not perfectly spherical, Fig. 1.27(a), they settle into an ordered arrangement below 249 K. The driving force for this ordering is optimisation of the bonding between adjacent C₆₀ molecules. In particular, in a given [110] cp direction, the short electron‐rich C–C bond that links two hexagons in one C₆₀ molecule points directly at the centre of an electron‐deficient pentagon, on an adjacent C₆₀ molecule, as shown for two ring fragments in Fig. 1.27(b).

This arrangement minimises direct C–C overlap but maximises donor–acceptor electronic interactions between adjacent molecules. The C–C bonds in fullerene fall into two groups, 1.40 Å for C–C bonds linking two hexagons and 1.45 Å for bonds linking one hexagon and one pentagon; pentagons are isolated in C₆₀. Comparing these with typical single and double bond lengths in organic compounds, 1.54 and 1.33 Å, respectively, we can see that the C–C bonds in C₆₀ are of intermediate bond order.

Some other fullerenes, such as C₇₀, also form cp structures, even though the molecules are not spherical. Thus, the C₇₀ molecule is an ellipsoid (shaped like a rugby ball with a long axis of 8.34 Å and a short axis of 7.66 Å), but in crystalline C₇₀ at room temperature the ellipsoids are able to rotate freely to give quasispherical molecules on a time average which form an fcc structure, a = 15.01 Å.

cp structures have, of course, tetrahedral and octahedral interstitial sites and in C₆₀ these may be occupied by a range of large alkali metal cations to give materials known as fullerides. The most studied C₆₀ fullerides have general formula A₃C₆₀ as in Rb₃C₆₀ or K₂RbC₆₀, in which all T₊, T_– and O sites are occupied. These materials are metallic since the alkali metals ionise and donate their electrons to the conduction band of the C₆₀ network, which is half full in A₃C₆₀. At low temperatures, many of these fullerides become superconducting. The highest T _c (the temperature at which the metal–superconductor transition occurs on cooling, Chapter 8) to date in the fullerides is 45 K in Tl₂RbC₆₀.

Other fullerides exist with different patterns of occupancy of interstitial sites, e.g. rock salt or zinc blende analogues in AC₆₀ and fluorite analogues in A₂C₆₀. It is also possible to increase the number of A ions per C₆₀ beyond 3; in A₄C₆₀ (A = Na), A₄ clusters form in the octahedral sites. In A₆C₆₀, the packing arrangement of the C₆₀ molecules changes from ccp to bcc; the bcc structure has a large number of interstitial sites, e.g. twelve distorted tetrahedral sites, distributed over the cube faces in the bcc array of anions (discussed in Chapter 8, for α‐AgI, Fig. 8.28). This wide range of formulae arises because the C₆₀ molecule can accept a number of electrons to give anions : n = 1, 2, 3, 4, … Part of the excitement in C₆₀ chemistry is that the electronic properties range from insulating to metallic/superconducting, depending on the value of n.