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aThe configurations are given for icosahedral symmetry. The emptying of the HOMO (h„) level and the filling of the LUMO (/,„) level are indicated.

''Pauli-allowed states are labeled by their spin degeneracy and irreducible representations of group fh (see §4.1).

CJ values for Hund's rule ground state.

dIn specifying the hyperfine structure, the symmetry of the Hund's rule ground state is explicitly identified by the symbol d.

aThe configurations are given for icosahedral symmetry. The emptying of the HOMO (h„) level and the filling of the LUMO (/,„) level are indicated.

''Pauli-allowed states are labeled by their spin degeneracy and irreducible representations of group fh (see §4.1).

CJ values for Hund's rule ground state.

dIn specifying the hyperfine structure, the symmetry of the Hund's rule ground state is explicitly identified by the symbol d.

designations, the J values, and the hyperfine structures for C"(f molecules (-6 < n < 10) that is expected on the basis of Hund's rule. We see that J = 3/2 for a Cg0 molecular ion, which has three additional electrons on the C60 shell. Because of the Jahn-Teller effect, the resulting degenerate ground state would be expected to give rise to a lattice distortion, leading to a nondegenerate ground state, and a carbon cage of lower symmetry surrounding the endohedral dopant. The various entries in Table 18.3 correspond to the removal of electrons from the HOMO level (1 < n < 10) and to the addition of electrons to the LUMO level (-6 < n < —1).

In the above discussion the fullerene ion was considered as a large charged shell with icosahedral symmetry and localized electrons. If instead, for some doping stoichiometries, the transferred charge is delocalized and forms a free electron gas, then Pauli paramagnetism could result, as discussed in §18.4 [18.18-20],

18.4. Pauli Paramagnetism in Doped Fullerenes

Whereas undoped C60 is characterized by a weak temperature-independent diamagnetic susceptibility (see §18.1), a greater variety of magnetic behavior can occur in doped C60. Thus far, only a few systems of this kind have been studied with regard to their magnetic properties. One system that has been studied is the CsxC60 system, where the magnetic susceptibility can be described by a sum of contributions, including a temperature-independent term x<:> discussed in the present section, a Curie-Weiss contribution xcw(T,H) from localized moments (see §18.5), and a ferromagnetic contribution Xf{T,H) described in §18.5. For the doped CsC60 solid, the C60 molecules couple to each other through the dopant species and show no ring currents.

In this section we focus on the observation of Pauli paramagnetism associated with conduction electrons. Pauli paramagnetism is observed in materials with conduction electrons which give rise to positive, temperature-independent contributions to the susceptibility, and the corresponding ESR signals have intensities that are temperature-independent and linewidths that have a linear T dependence.

For the CsxC60 system, the Pauli paramagnetism contribution to x is strongly dependent on the Cs concentration x. Shown in Fig. 18.3(b) is a plot of the temperature-independent susceptibility term vs. Cs concentration x in Cs,C60 [18.18] over a range of x for which is paramagnetic, with a maximum value of ~ 5 x 10~7 emu/g occurring for x ~ 1. Also shown in the figure (dashed line) is the core diamagnetism, which is the only contribution to for undoped C60 and is almost independent of Cs concentration. On the basis of susceptibility, ESR, and temperature-dependent

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