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{sub 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{sub s}=170 K. The space group symmetry of the low-temperature phase is determined to be P2{sub 1}/n. Two inequivalent C{sub 60} sites exist in the low-temperature phase, which are indispensable to the orbital ordering model of C{sub 60}. The contact configuration for the neighboring C{sub 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 ({approx}0.07 A) and that these shifts correlate perfectly to the alignment of C{sub 60}. This result indicates that the steric effect between C{sub 60} and TDAE molecules plays an important role in the orientational ordering of C{sub 60}. On the other hand, in the {alpha}{sup '} phase, no structural phase transition was observed below 30 K. This indicates that all the C{sub 60}s are crystallographically equivalent. Structural differences separate the magnetic peculiarities of the two polymorphs in TDAE-C{sub 60}.