Revisited phase diagram on charge instability and lattice symmetry breaking in the organic ferroelectric TTF-QCl4

Abstract

We investigate the charge and lattice states in a quasi-one-dimensional organic ferroelectric material, TTF-QCl4, under pressures of up to 35 kbar by nuclear quadrupole resonance experiments. The results reveal a global pressure-temperature phase diagram, which spans the electronic and ionic regimes of ferroelectric transitions, which have so far been studied separately, in a single material. The revealed phase diagram clearly shows that the charge-transfer instability and the lattice symmetry breaking, which coincide in the electronic ferroelectric regime at low pressures, bifurcate at a certain pressure, leading to the conventional ferroelectric regime. The present results reveal that the crossover from electronic to ionic ferroelectricity occurs through the separation of charge and lattice instabilities.

Microscopic Modeling of Correlated Systems Under Pressure: Representative Examples

“…It remains to be seen whether this kind of magnetic order is, indeed, realized in this organic system. In the context of the recently discussed temperature‐pressure phase diagram, these findings suggest that a strong charge transfer and possibly antiferromagnetism do not necessarily rely on the presence of ferroelectricity, so that their evolution under pressure can be decoupled.…”

“…This contradicts a natural expectation that the intermolecular electron hopping should be enhanced under pressure. Finally, a more recent study of the temperature‐pressure phase diagram suggests that above a critical pressure the neutral‐ionic and ferroelectric transitions split and occur at different temperatures.…”

“…As a matter of fact, this technique has proven to be utterly useful to uncover the nature of quantum phases, such as a Mott insulator, spin liquid, correlated metal, multiferroic, or unconventional superconductor . Materials where such a tuning parameter is very effective are, for instance, organic charge transfer salts, where, due to the softness of the organic components, small pressures induce large changes in the electronic properties of the materials giving rise to complex phase diagrams . Also inorganic materials can show extreme sensitivity to pressure due to formation and breaking of certain covalent bonds in their crystal structures upon pressurizing.…”

Single‐Electron Effects for Probing Local Electrical Polarization Changes and Charge Hopping

“…TTF‐QCl 4 shows a first order paraelectric to ferroelectric phase transition at T C = 81 K in the bulk phase which is associated with an increase of the charge transfer from about 0.3 e (neutral phase, in short) to 0.6 e (ionic phase, in short) between donor and acceptor pairs and a dimerization along the donor–acceptor stacking axis . Along this stacking axis TTF‐QCl 4 is quasi one‐dimensional with weak electrostatic interactions between neighboring stacks.…”

“…We exemplarily compare our simulation results with experimental results on the threshold voltage in current–voltage curves of two‐dimensional nano‐island arrays of Co 3 Fe. Next, we simulate the expected conductance response of such an array embedded into TTF‐QCl 4 as an electronic ferroelectric in which charged domain walls occur close to the ferroelectric transition . From this we suggest to use conductance fluctuation spectroscopy of such nano‐island arrays for studying the dynamics of charged domain walls in highly insulating TTF‐QCl 4 and similar systems.…”