Temperature induced Neutral to Ionic phase Transition of the charge transfer crystal Tetrathiafulvalene-Fluoranil

TL;DR

This study demonstrates a temperature-induced neutral to ionic phase transition in Tetrathiafulvalene-Fluoranil, characterized by spectroscopic and structural analysis, revealing a first-order transition with significant structural changes around 150K.

Contribution

First to show TI-NIT in Tetrathiafulvalene-Fluoranil, expanding understanding of charge transfer crystals and their phase transition behaviors.

Findings

Ionicity jumps from 0.15 to 0.7 at transition

Transition occurs around 150K with hysteresis

Structural changes include loss of inversion symmetry and dimerization

Abstract

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 $\pi$-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 XRD, demonstrating that while TTF-FA is quasi-neutral at room T, its ionicity jumps from 0.15 to 0.7 at low T, therefore crossing the Neutral-Ionic borderline. The transition, occuring around 150K, 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.