Influence of rotational disorder in C60 on electrical conductivity

The rotational order and disorder of C60 molecules in a lattice strongly modulates the density of states at the Fermi level and exerts a large influence on electrical transport. We report herein a study of this influence at high temperatures by employing microwave conductivity and static magnetic susceptibility as the observation probes, focusing on Na2CsC60. Microwave conductivity becomes 30% higher on going from a low-temperature (LT) orientationally ordered phase (space group: Pa-3) to a freely rotational high-temperature (HT) disordered phase (space group: Fm-3m). The density of states at the Fermi level, DEF, evaluated by static magnetic susceptibility measurements, increases from 15.1 states eV−1 C60−1 for the LT ordered phase to 17.4 states eV−1 C60−1 for the HT disordered phase. Both microwave conductivity and magnetic susceptibility show that Na2CsC60 maintains its macroscopic metallicity at least up to 673 K, although local probe NMR studies suggest electron localization in the disordered state at high temperatures. The rotational disorder greatly increases the scattering of conduction electrons. Different temperature evolutions of DEF(Na2CsC60) implied by NMR spectrometry and SQUID magnetometry are discussed.