Structural differences in two polymorphs of tetra-kis-(dimethylamino)- ethylene-C60: An x-ray diffraction study

A type of low-temperature structure for ferromagnetic α -tetra-kis (dimethylamino)-ethylene (TDAE)- C60 is proposed on the basis of low-temperature x-ray analysis. We observed that intense superlattice reflections with odd indices successively appeared below Ts =170 K. The space group symmetry of the low-temperature phase is determined to be P 21 n. Two inequivalent C60 sites exist in the low-temperature phase, which are indispensable to the orbital ordering model of C60. The contact configuration for the neighboring C60 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 (∼0.07) and that these shifts correlate perfectly to the alignment of C60. This result indicates that the steric effect between C60 and TDAE molecules plays an important role in the orientational ordering of C60. On the other hand, in the α′ phase, no structural phase transition was observed below 30 K. This indicates that all the C60 s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE- C60.