Symmetry breaking and structural changes at the neutral-to-ionic transition in tetrathiafulvalene-p-chloranil

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Abstract
The structural aspect of the temperature-induced neutral-to-ionic transition in the mixed-stack charge-transfer complex TTF-p-chloranil is directly analyzed using results obtained from neutron-scattering experiments. The symmetry breaking of the structural order parameter is determined together with the complete structural change. It is shown that the ionic phase is characterized by a dimerization along the stacking direction into weakly bond donor-acceptor dimers, which adopt a ferroelectric arrangement. Values of both dipole moment and dipolar energy in the unit cell are evaluated and commented. The evolution of the three-dimensional contacts which interconnect the chains, and in particular of the hydrogen interaction network, is discussed. Moreover, the associated intramolecular deformations are related in a satisfying way with the change of the ionicity and with the loss of centrosymmetry at the transition. Phenomena of metastability are also briefly discussed.