Evolution of ferroelectricity in tetrathiafulvalene-p-chloranil as a function of pressure and temperature

Abstract

The neutral-to-ionic phase transition in the mixed-stack charge-transfer complex tetrathiafulvalene-p-chloranil (TTF-CA) has been studied by pressure-dependent infrared spectroscopy up to p = 11 kbar and down to low temperatures, T = 10 K. By tracking the C=O antisymmetric stretching mode of CA molecules, we accurately determine the ionicity of TTF-CA in the pressure-temperature phase diagram. At any point the TTF-CA crystal bears only a single ionicity; there is no coexistence region or an exotic high-pressure phase. Our findings shed new light on the role of electron-phonon interaction in the neutral-ionic transition.

Infrared Investigations of the Neutral-Ionic Phase Transition in TTF-CA and Its Dynamics

“…Infrared data by Masino et al [33] on single crystals of TTF-CA under hydrostatic pressure suggest that in the paraelectric phase two distinct ionic states are present. In contrast, most recent infrared investigations by Dengl et al [21] on TTF-CA powder, examining the whole P-T-space as depicted in Figure 4, give clear evidence that in this specific phase, there is no coexistence of two different ionic configurations. To finally make a conclusion on this "exotic" state, it will nevertheless be necessary to perform polarized infrared studies on single crystals under high pressure.…”

“…In Figure 3, the p-T-phase diagram is depicted that is composed of several studies of different properties, i.e., electric transport [11], optical measurements [14,21,[28][29][30][31][32][33], neutron scattering experiments and 35 Cl NQR [34,35]. The phase diagram reveals similarities to a solid-liquid-gas phase diagram with a triple point at 210 K and 500 MPa where all three phases coexist.…”

“…Thus, varying the distance by thermal contraction upon cooling or by hydrostatic pressure, the Madelung energy increases and becomes larger than I − A. This leads to a transition from the neutral to the ionic phase occurring at T NI = 81.5 K or at approximately 9 kbar for room temperature [21].…”

“…The phase diagram resembles the characteristic of a solid-liquid-gas phase diagram. The red circles refer to neutron scattering and the blue plus signs to NQR results [34]; the violet triangle is obtained by vibrational spectroscopy [21,28,29]; the green square is extracted from transport measurements [11,30]. The black dot around T c = 260 K and a pressure slightly above 700 MPa indicates the critical point labelled by C. Adapted from [6].…”

“…While there is a clear jump of ρ(T) at T NI = 81 K at ambient pressure, the step becomes reduced and the transition smoother as pressure increases. Above 9 kbar, the ionicity basically reaches the value of the ionic phase already at room temperature [21]. Table 1.…”