STRUCTURAL ASPECTS OF THE NEUTRAL-TO-IONIC TRANSITION IN MIXED-STACK CHARGE-TRANSFER COMPLEXES

Original and intriguing features associated with the so-called neutral -to-ionic (N-I) transition have been examined in some organic mixed-st ack charge-transfer (CT) complexes, involving simultaneously electroni c, structural and magnetic aspects. The origin of the N-I transition a nd the essential parameters governing it are still controversial. Stru ctural aspects of the N-I transition in the prototype compound tetrath iafulvalene- p-chloranil (TTF-CA) are reviewed. In the ionic phase, a ferroelectric arrangement is apparent between the dimerized chains. Ho wever, these long-range dipolar interactions are too weak to drive the phase transition. At atmospheric pressure, a significant volume varia tion occurs at T-N-I with an abrupt change in the cell parameters in d irections perpendicular to the stacking axis. This clear three-dimensi onal aspect of the compound is also evident from several close interch ain contacts. Intramolecular deformations observed at the transition h ave been directly related to the ionicity change and the symmetry brea king, in agreement with the evolution of the Cl-35 NQR spectrum. The p hase diagram has been determined and a clear change in the nature of t he transition has been observed with a tendency towards second-order c haracter at high pressures.