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We have studied the crystal structure of ((phenyl) 4 As) 2 C 60 Cl at room temperature and below the phase transition at  T c   = 125 K by single-crystal x-ray diffraction. At room temperature tetragonal lattice parameters were found as  a  =  b  = 12.588(1)Å and  c  = 20.345(1)Å. At  T  = 120 K the lattice parameters were determined as  a  =  b  = 12.5060(1)Å and  c  = 20.4420(1)Å. The room temperature and low-temperature structures were found to be isostructural, with space group  I 4/ m . Structure refinements were performed with restrictions on the C 60  molecules according to the noncrystallographic isohedral point symmetry  I h  . Derivations from this symmetry could not be found, suggesting that any possible Jahn-Teller distortion of the C 60  radical anions will be smaller than 0.01Å. The phase transition observed in ESR at  T c   = 125 K is proposed to be a glass transition. At room temperature there is dynamic disorder between two orientations of the C 60  molecules as they are related by the 4-fold rotation. Below  T c   the disorder becomes static. Two types of twinning have been observed in different crystals. The first type is represented by a rotation over 180 about [1,1,1] axis. It results in different orientations of the 4-fold unique axes with angles of 82.4 degrees between them. Secondly, merohedral twinning was observed corresponding to the two orientations of the structure with 4/ m  symmetry on the tetragonal lattice. Both twinnings result in extra orientations of the C 60  molecules, and they should be taken into account, when analysing the anisotropy of the physical properties of crystals of this compound.

The glass transition in twinned ((phenyl) 4 As) 2 C 60 Cl crystals