Separation of Charge Instability and Lattice Symmetry Breaking in an Organic Ferroelectric

TL;DR

This study uses nuclear quadrupole resonance to map the phase diagram of an organic ferroelectric, revealing how charge transfer and lattice symmetry breaking separate under pressure, indicating a crossover from electronic to ionic ferroelectricity.

Contribution

It provides the first comprehensive phase diagram showing the separation of charge and lattice instabilities in an organic ferroelectric under pressure.

Findings

Charge-transfer instability and lattice symmetry breaking bifurcate at a certain pressure.

The phase diagram spans electronic and ionic ferroelectric regimes.

Crossover from electronic to ionic ferroelectricity involves separation of charge and lattice instabilities.

Abstract

We investigate the charge and lattice states in a quasi-one-dimensional organic ferroelectric material, TTF-QCl$_{4}$, 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.