Molecular Structure, Orientational Order, and Jahn−Teller Distortion of the C₆₀⁴⁻ Anion in Cs₄C₆₀

The molecular structure of the C 60 4 - anion in orientationally ordered orthorhombic Cs 4 C 60 is refined from powder neutron diffraction data at 5 and 293 K. The anion is observed to be more strongly distorted from spherical symmetry than the 3 -, 5 -, and 6 - anions previously studied in extended close-packed fulleride arrays. The orthorhombic orientationally ordered structure becomes orientationally disordered at 623 K due to jump interconversion of the C 60 4 - anions between two mmm symmetry anion orientations. This order−disorder transition is driven by the removal of the steric constraint which the large cesium cations exert on fulleride orientation through thermal expansion of the cation−anion distance and demonstrates that it is possible to avoid the quenched orientational disorder which characterizes the fulleride anion orientations in the A 3 C 60 cubic superconductors.