Structural evidence of ferrielectric neutral-ionic layered ordering in 2,6-dimethyltetrathiafulvalene-p-chloranil

Direct structural evidence of the periodic ordering of neutral and ionic planes is reported for 2,6-dimethyltetrathiafulvalene-p-chloranil, detected by high-resolution x-ray diffraction experiments. Simultaneous to the neutral-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 intermolecular contacts, related to the charge-transfer ordering and to the dimerization process, are discussed and compared with results obtained for the prototype compound of the neutral to ionic transition, tetrathiafulvalene-p-chloranil. Thus essential insights are provided for an understanding of the mechanism of this unusual electronic-structural instability, and important physical features are discussed such as the interstack interactions, the coupled order parameters, and the interplay between the staging and condensation mechanisms.