Temperature-induced neutral-to-ionic phase transition of the charge-transfer crystal tetrathiafulvalene-fluoranil

The temperature induced neutral to ionic phase transition (TI-NIT) is a rare phenomenon occurring in mixed stack charge transfer (CT) crystals made up of alternating π-electron donor (D) and acceptor (A) molecules. We were able to grow crystals of tetrathiafulvalene-fluoranil (TTF-FA), and to show that it undergoes TI-NIT like the prototype CT crystal TTF-chloranil. We characterized both room and low-T phases through IR and Raman spectroscopy and x-ray diffraction, demonstrating that while TTF-FA is quasineutral at room temperature, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the neutral-ionic borderline. The transition, occurring around 150 K, is first order, with large thermal hysteresis and accompanied by crystal cracking. In the high-T phase D and A molecules lie on inversion center, i.e., the stacks are regular, whereas the low-T phase is characterized by the loss of the inversion symmetry along the stack as the stacks are strongly dimerized and by the doubling of the unit cell.