Structural evidence of ferrielectric neutral-ionic layered ordering in 2,6-dimethyltetrathiafulvalene-p-chloranil - art. no. 054105

Direct structural evidence of the periodic ordering of neutral and ionic pl anes is reported for 2,6-dimethyltetrathiafulvalene-p-chloranil, detected b y high-resolution x-ray diffraction experiments. Simultaneous to the neutra l-ionic layered ordering, theoretically predicted a long time ago to occur in stages, an antiparallel dimerization ordering takes place, leading to a "ferrielectric" structure. Both intramolecular deformations and intermolecu lar contacts, related to the charge-transfer ordering and to the dimerizati on process, are discussed and compared with results obtained for the protot ype compound of the neutral to ionic transition, tetrathiafulvalene-p-chlor anil. Thus essential insights are provided for an understanding of the mech anism of this unusual electronic-structural instability, and important phys ical features are discussed such as the interstack interactions, the couple d order parameters, and the interplay between the staging and condensation mechanisms.