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Fig. 18.2. Electron ring currents in Qo for a magnetic field oriented perpendicular to a plane containing (a) a pentagon and (b) a hexagon, respectively. The electron ring currents in the pentagons are paramagnetic, while the currents in the hexagons are diamagnetic. The ring current strength is given with respect to that in benzene [18.6].

magnetic and of about the same magnitude as that for benzene, while the adjacent pentagons make a paramagnetic contribution of similar magnitude. Thus for C60, the diamagnetic and paramagnetic contributions nearly cancel, yielding a very small diamagnetic value for x■ Since C70 has five additional diamagnetic hexagons, x for C70 is thereby somewhat more diamagnetic than for C60, but still the diamagnetic behavior is weak and roughly comparable to that of diamond. As the number of hexagons in a fullerene is further increased, as for example to form a carbon nanotube, the magnitude of x becomes large and highly diamagnetic (see Table 18.1), approaching about half the value of graphite for H || c-axis, when the magnetic field is directed normal to the tubule axis (see § 19.6). The value of x f°r Qo >s> however, roughly comparable to that for graphite with H _L c-axis [18.12], as is also seen in Table 18.1.

18.2. Magnetic Endohedral and Exohedral Dopants

The introduction of endohedral or exohedral dopants with unfilled d or / shells into fullerene host materials (see §5.4) is expected to result in paramagnetic materials. For example, in Table 18.2 we present a list of rare earth ions that might be doped endohedrally into C6(l. For each rare earth ion, the electronic configuration and basic level designation for the S, L, and J quantum numbers are given on the basis of Hund's rule. Also given in this table are the corresponding magnetic moments (xm calculated

Table 18.2

Level configurations and magnetic moments expected for various rare earth endohedral species in .

Table 18.2

Level configurations and magnetic moments expected for various rare earth endohedral species in .

Ion

Configuration

Basic level"

Moment6

Ce3+

4/'5i2p6

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