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 $\ensuremath{\pi}$-electron donor (D) and acceptor (A) molecules. We were able grow tetrathiafulvalene-fluoranil (TTF-FA), show that it undergoes TI-NIT like the prototype CT crystal TTF-chloranil. characterized both room low-T phases through IR Raman spectroscopy x-ray diffraction, demonstrating while TTF-FA quasineutral at temperature, its ionicity jumps from 0.15 0.7 low T, therefore crossing neutral-ionic borderline. transition, around 150 K, first order, with large thermal hysteresis accompanied by cracking. In high-T D A molecules lie on inversion center, i.e., stacks are regular, whereas loss symmetry along as strongly dimerized doubling unit cell.