Molecular structure, orientational order, and Jahn-Teller distortion of the C604- anion in Cs4C60

Abstract

The molecular structure of the C604- anion in orientationally ordered orthorhombic Cs4C60 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 C604- 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 A3C60 cubic superconductors.