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20 (degrees)

Fig. 8.15. A powder x-ray diffractogram of the TDAE-Cm compound. The constituents of the TDAE molecule, which has the basic ethylene structure, are shown in the inset [8.165].

Fig. 8.16. Crystal structure of

HI CM Q TBAE

TDAE-C^i at room temperature determined by x-ray diffraction, showing the location of the Cm molecules and the orientation of the anisotropic TDAE molecules in the unit cell [8.165].

lines for unreacted fee C60 (a0 = 14.18 A). Since the TDAE molecule lacks inversion symmetry, the logical choice of the space group is P2, with the C60 molecules centered at the 2a sites of the space group. In this structure shown in Fig. 8.16, the molecules in the monoclinic unit cell are located at (0,0,0) and (¿,§,0), and the organic molecules are in the large interstices centered around the 2d sites at and (0,|,|). The long axis of the molecule (a = 15.87 A) is believed to be along the c-axis, as indicated in Fig. 8.16. The C-C bond lengths given for the C60 constituents in this compound are 1.37 A and 1.45 A, with one of the twofold axes of C60 along the ¿-axis of the crystal. The nearest-neighbor separation between C60 molecules in TDAE is 9.98 A along the c-axis and 10.26 A within the a-b plane, thereby giving further evidence for anisotropy in the crystal structure. If the van der Waals radius for C60 is taken as 5.0 A, then 47% of the volume of the unit cell is available to accommodate the guest TDAE molecule. The nearest approach between the C60 and TDAE molecules in the solid phase is 2.58 A [8.165]. The anisotropic crystal properties are expected to give rise to anisotropic magnetic properties as discussed in §18.5.2.

8.7.4. Metal-C60 Multilayer Structures

Reports on the structure and properties of synthetic metal-C60 multilayer structures are now becoming available [8.166]. In these synthetic materials, multilayers of alternating metal/C60 sequences are deposited under high-vacuum conditions on a substrate such as quartz. For example, up to 20 multilayers of Al/C60 on an insulating substrate [8.166] and C60 overlayers on thin films of Sn, Ba, and Ga have been studied [8.167].

Based on characterization of the Al/C60 multilayers using x-ray diffraction, in situ resistivity, and Raman scattering measurements, it is concluded that when depositing A1 on C60 the aluminum penetrates the C60 layer because of the different sizes of the A1 atoms and C60 molecules, so that a conducting A1^C60 phase forms at the interface. When C60 is now deposited on the A1 layer, a doped monolayer of C60 is formed by charge transfer from the A1 to the C60 of up to six electrons per C60 [8.166] (see §14.1.5). The strong binding of C60 to metal substrates and the charge transfer that occurs at such interfaces have been studied in some detail (see §17.9).

8.8. The Doping of C70 and Higher-Mass Fullerenes

To date, most studies of doped fullerenes have been confined to C60, although a number of studies of doped C70 have also been carried out (see §11.7.3, §12.7.5, and §14.1.4) and a very few have been made of the higher fullerenes (see §16.2 and §18.5). Almost all of these studies have involved alkali metal exohedral dopants and a few have involved the organic compound TDAE-C„c. The doping techniques that have been employed for C70 are similar to those for C60, and the charge transfer effect is believed to be similar to that for C60, or basically one electron transferred per alkali metal dopant, at least for low dopant concentrations. Because of the lower symmetry of C70, the number of electrons required to fill electronic levels in the case of C70 is different from that in C60 (see §12.6.1). Correspondingly, half-filled and completely filled electronic bands occur for higher-mass fullerenes at different numbers of electrons relative to doped C60.

Structural information is now available for MXC70 (M = K, Rb, Cs), and stable phases have been reported for x = 0,1,3,4,6,9 [8.168-170]. The various phases and lattice constants that have been reported are listed in

Table 8.6

Structure and lattice constants for the M^C70 alkali metal compounds" [8.168-171],

Table 8.6

Structure and lattice constants for the M^C70 alkali metal compounds" [8.168-171],

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