Structural differences in two polymorphs of tetra-kis-(dimethylamino)-ethylene-C-60: An x-ray diffraction study

A type of low-temperature structure for ferromagnetic alpha-tetra-kis (dimethylamino) -ethylene (TDAE)-C-60 is proposed on the basis of low-temperature x-ray analysis. We observed that intense superlattice reflections with odd indices successively appeared below T-s = 170 K. The space group symmetry of the low-temperature phase is determined to be P2(1)/n. Two inequivalent C-60 sites exist in the low-temperature phase, which are indispensable to the orbital ordering model Of C-60. The contact configuration for the neighboring C(60)s along the stacking c direction is uniquely determined. The double bond between the hexagons faces the neighboring pentagon. We found that the surrounding TDAE molecules shift along the c axis (similar to 0.07 angstrom) and that these shifts correlate perfectly to the alignment of C-60. This result indicates that the steric effect betwee n C-60 and TDAE molecules plays an important role in the orientational ordering Of C-60, On the other hand, in the alpha' phase, no structural phase transition was observed below 30 K. This indicates that all the C(60)s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE-C-60.