Revisited phase diagram on charge instability and lattice symmetry breaking in the organic ferroelectric TTF-QCl 4

There has been increasing interest in electronic ferroelectricity, which stems from deformation of the electronic wave function coupled with lattice symmetry breaking, invoking a Berry-phase description of electronic polarization beyond the conventional notion of ionic displacement. How does a ferroelectric transition of an electronic nature cross over to a conventional one of an ionic nature? This is a fundamental issue of ferroelectricity that has yet to be answered. Here, investigating a quasi-one-dimensional organic ferroelectric material, TTF-${\mathrm{QCl}}_{4}$, under pressures of up to 35 kbar, we reveal a pressure-temperature phase diagram that spans the electronic and ionic regimes of ferroelectric transitions in a single material. The global phase diagram revealed by nuclear quadrupole resonance experiments demonstrates that electronic ferroelectricity crosses over to conventional ionic ferroelectricity through the separation of charge-transfer instability and lattice symmetry breaking (dimerization), and that the previously reported dimer liquid phase is largely extended to high pressures.