Lattice instability coupled with charge and/or spin degrees of freedom has become an interesting area of research in solid state science as the electric, magnetic and optical properties exhibit high susceptibility to external stimuli. One intriguing characteristic is ferroelectricity susceptible to changes in temperature, pressure and external electric field. Typical examples among π-conjugated molecular solids are charge-transfer (CT) complexes1,2,3,4,5,6,7,8 that have one-dimensional stacks of alternating π-electron donor (D) and acceptor (A) molecules, as in a series of complexes of p-benzoquinones with tetrathiafulvalene (TTF) or its 4,4′-dimethyl-substituted version (DMTTF). The bistable polarity along the molecular stack can be constructed by a pairwise molecular displacement (dimerization denoted by underlines such as DA DA.. or AD AD …).

There are at least two different classes of ferroelectrics in TTF complexes. The emergence of ferroelectricity in TTF–QCl4 (ref. 9; QCl4 or CA=p-chloranil) and antiferroelectricity in DMTTF–QCl4 (ref. 10) accompanies a phase transition from a neutral to ionic state under a temperature or pressure change. The charge degree of freedom is thus closely relevant to the mechanism of the neutral–ionic phase transition (NIT). The crystal volume is an effective parameter to characterize the NIT, because it can change the energy balance between the electrostatic Madelung gain and the cost of ionization during increasing the degree of CT ρ (ionicity defined by D+ρAρ)9. The recent discovery of large spontaneous polarization on TTF–QCl4 has highlighted a new mechanism of ferroelectric induction, ‘electronic ferroelectricity’, that is governed by the electronic overlap integral over the intermolecular space rather than the static molecular charge3,11,12,13. Note that the recent studies on organic CT compounds also demonstrated gigantic permittivity from the charge-order mechanism as the similar class of electronic ferroelectricity14.

The contrastive ferroelectric, the bromine-substituted analogue, TTF–QBr4 (QBr4=p-bromanil)15 exhibits a similar DA dimerization but preserves the monovalent ionic state during the ferroelectric transition. The antiferromagnetically coupled S=1/2 spins residing on D+ and A radical ions turn to spin-singlet D+A pairs on cooling, changing the chains from paramagnetic to nonmangetic. This ferroelectricity, thus, has a magnetic origin corresponding to the spin-Peierls (SP)-type phase transition that is manifested by the magnetoelectric phenomena such as magnetic control of electric polarization16.

For both NIT and SP systems, one challenging issue is the quantum phase transition17 achieved by fine tuning the continuous phase transition down to zero kelvin. New states of matter may materialize when the electronic, structural and/or magnetic long-range order is perturbed by quantum fluctuations such as the ‘zero-point motion’ between the degenerate ground states as opposed to thermal fluctuations. The ‘quantum ferroelectric’ and ‘quantum paraelectric’ states represent those in proximity to the zero-kelvin transition point, that is, the quantum critical point (QCP)18,19,20,21,22. For instance, in the typical quantum paraelectric SrTiO3, the temperature-dependent dielectric permittivity exhibits a saturation behaviour at a gigantic value (20,000) as the manifestation of the proximity to the QCP, and quantum ferroelectricity can be induced with subtle structural perturbations such as oxygen isotope exchange21. Although a quantum critical behaviour has been found in the NIT system by pressurizing DMTTF–QBr4 crystals, the inter-stack antiferroelectric coupling, which leads to the antiferroelectrically ordered state, is responsible for the much smaller permittivity (180) at the QCP.

To search for the QCP accompanied by quantum ferroelectricity, tetrahalo-p-benzoquinones of variable molecular volume are ideal for inducing structural changes with least perturbations to the electron affinities. In fact, the NIT has been demonstrated through optical spectra of two neutral TTF–QI4 complexes of different stoichiometries (1:1 and 2:1) under a modest pressure of 2–3 GPa23,24,25. In this study, we prepared tetrahalo-p-benzoquinones with n iodine and 4–n bromine substituents (QBr4–nIn, n=0–4; Fig. 1a) to examine the stepwise change in the lattice volume and to modify the NIT of TTF–QI4 and the ferroelectric SP of TTF–QBr4. In this family, we have realized the ideal NIT type quantum ferroelectricity (TTF–QBr2I2) by tuning the applied hydrostatic pressure

Figure 1: Chemical and crystal structures of tetrathiafulvalene (TTF) complexes of tetrahalo-p-benzoquinones (QBr4–nIn).
figure 1

(a) Chemical form of QBr4–nIn. (b-e) Molecular packings of the TTF–QBr4–nIn complexes. (b) Neutral (TTF)2(QBr2I2) projected along the c direction. (c) Ionic 1:1 TTF–QBr3I projected along the a direction. (d,e) Neutral 1:1 TTF–QBr2I2 projected along the crystallographic a and b directions. For clarity, the halogen positions of the QBr2I2 molecules are shown only for preferable occupations.


Materials preparation

Considering that even small chemical inhomogeneity will mask the genuine properties around the QCP, we pursued QBr4–nIn molecules with sufficient chemical purity for fine and systematic chemical tuning. However, previous synthetic protocols did not include the regioselective iodination step. This work established an effective purification method using gel permeation chromatography and regioselective iodination procedures, the details of which are described in the Supplementary Fig. 1. Gel permeation chromatography isolated 2-iodo-3,5,6-tribromo-p-benzoquinone (QBr3I) and 2-bromo-3,5,6-triiodo-p-benzoquinone (QBrI3) as well as pure QI4, but the dibromodiiodo-p-benzoquinones could not be separated into three isomers. The symmetrically diiodo-substituted quinone (QBr2I2) was synthesized through the new regioselective iodination process using symmetric dibromodimethoxybenzene.

CT complex formation

Without seeding specific crystal forms, the mixed acetonitrile solution of each QBr4–nIn and TTF afforded neutral CT complexes of a 2:1 stoichiometry as the main products. The crystal structures of (TTF)2(QBr4–nIn) are isomorphous and belong to the monoclinic system with a centric space group of P21/n. The molecules assemble into a DAD trimer with inversion symmetry, as shown in Fig. 1b. Previous spectroscopic studies of the pressure-induced NIT of (TTF)2(QI4) suggested an inhomogeneous charge distribution in the ionic phase at high pressures beyond 3.1 GPa26. High-pressure structural studies, albeit beyond the focus of this study, would be interesting to confirm the asymmetric charge distribution suggested as a polar D+AD0 and to examine the possible emergence of electronic ferroelectricity.

The 1:1 TTF complexes of QBr4–nIn with an infinitely alternating DA sequence are usually obtained as minor products. They crystallized as either fully ionic or neutral CT complex depending on the degrees of bromine substitution between the neutral TTF–QI4 and ionic TTF–QBr4. The overall CT complexes of TTF are listed in terms of the isomorphous crystal forms in Supplementary Table 1.

Ionic 1:1 TTF complexes

The 1:1 complex of 2-iodo-3,5,6-tribromo-p-benzoquinone (QBr3I) crystallizes as an isomorphous form of the fully ionic TTF–QBr4 (ref. 15), the SP-type ferroelectric. Crystal structural analysis determined an analogous triclinic lattice with a space group of P-1 and two formula units (Z=2; Fig. 1c). The D and A molecules, occupying their respective inversion centres, are alternately stacked with a regular separation. The lattice constants of the a and b axes, which are both parallel to the DA stack, are elongated by 1.2% and 1.1%, respectively, compared with those of the TTF–QBr4 crystal. Figure 2a,b depicts the temperature-dependent magnetic susceptibility and dielectric constant of these two isomorphous compounds. The magnetic susceptibility in the high-temperature region indicates the analogous paramagnetism arising from the spin-1/2 residing on each D+ and A radical, but its monotonous increase on cooling in the case of TTF–QBr3I represents the absence of the SP transition. In accord with this, the dielectric permittivity has no peak anomaly characteristic of the ferroelectric transition down to the lowest temperature. Nevertheless, the gradual increase in the permittivity with decreasing temperature may reflect the quantum fluctuations of the polar dimerization.

Figure 2: Temperature dependence of dielectric and magnetic properties of 1:1 TTF–QBr4–nIn complexes.
figure 2

(a) The spin susceptibility χs for ionic TTF–QBr4 and TTF–QBr3I crystals. The inset shows the corresponding χsTT plot. (b) The relative permittivity measured with an a.c. electric field applied along the crystallographic b axis parallel to the DA stack of ionic TTF–QBr4 and TTF–QBr3I crystals. (c) The relative permittivity measured with an a.c. electric field applied along the DA stack (parallel to the crystallographic b axis) for neutral 1:1 TTF–QBr2I2, TTF–QBrI3 and TTF–QI4 crystals at ambient pressure.

Neutral 1:1 TTF complexes

The 1:1 TTF complexes of QBr2I2 and QBrI3 with ρ0.11 at ambient pressure (as estimated from the C=O stretch mode frequency; see Supplementary Discussion) are isomorphous to TTF–QI4 (ref. 24) in crystal structure. According to previous studies of high-pressure optical spectra at room temperature, the ionicity of TTF–QI4 steeply increases from 0.3 to 0.5 and the DA stack starts to dimerize at pressures beyond 1.9 GPa23. Because previous studies of TTF–QI4 were not accompanied by a full report of the atomic coordinates, we have repeated the structural analysis. The triclinic lattice with a space group of P-1 comprises simply one formula unit (Z=1). The D and A molecules, occupying their respective inversion centres, are alternately stacked with a regular separation along the crystal b direction (Fig. 1d,e). The molecular arrangement is very similar to that of DMTTF–QCl4 (ref. 10) in the triclinic cell (Z=1) and distinct from that of TTF–QCl4 in the monoclinic cell (Z=2)27.

Bromine substitution of TTF–QI4 shortens the D–A distance along the stack, namely, the half of b axis lattice constant, and would stabilize the ionic state by chemical pressure effect, or equivalently by increasing the attractive Coulomb interaction between D and A. We have examined the compressibility of the lattice parameters to relate the lattice shrinkage to the effective pressure Δpeff that would contribute to promotion of NIT. Using the volume compressibility of TTF–QBr2I2 crystal (ΔV=–28 Å3 at Δp=1 GPa), the volume change from that of TTF–QI4 corresponds to the application of Δpeff=0.29 GPa for TTF–QBrI3V=–8.2 Å3) and 0.60 GPa for TTF–QBr2I2V=–16.7 Å3). From the b axis compressibility (Δb=–0.23 Å at Δp=1 GPa), we obtained similar pressures of 0.29 GPa (Δb=–0.067 Å) and 0.68 GPa (Δb=–0.157 Å), respectively.

Dielectric properties of neutral 1:1 TTF complexes

Figure 2c depicts the temperature T dependence of relative permittivity ɛr measured at 300 kHz along the DA stacking direction of neutral 1:1 compounds. In the displayed temperature range, the permittivity exhibited no frequency dispersion, at least up to 1 MHz. The increase in permittivity with lowering temperature starts to deviate from the Curie–Weiss law for ferroelectrics, ɛr=C/(Tθ), where θ is the Weiss temperature and C is the Curie constant. The temperature dependence including the saturation behaviour around the lowest temperature has been often fitted with the theoretical (Barrett) formula for quantum paraelectricity28:

Here T0 represents the Curie–Weiss temperature in the classical (high T) limit, and T1 is the characteristic crossover temperature dividing the quantum-mechanical and classical regions. The fitting parameters for T<140 K are T0=+3(1) K and T1=71(1) K; and C=6.3(2) × 103 K for TTF–QBr2I2; and T0=–62(10) K, T1=70(4) K and C=5.4(7) × 103 K for TTF–QBrI3. In the case of TTF–QI4, the values are T0=–165(12) K, T1=86(2) K and C=8.0(7) × 103 K (see the Supplementary Fig. 4 for the fittings). It should be noted that the saturated value of the permittivity as well as T0 increase with the effective (chemical) pressure noted above, whereas C is almost unchanged.

This behaviour is quite analogous to the case of the antiferroelectric NIT of DMTTF–QBrnCl4–n (ref. 29). Lattice compression under chlorine substitution or hydrostatic pressure increases the low-temperature permittivity until the temperature-induced phase transition appears together with a peak anomaly of the permittivity. The temperature–pressure phase diagram marks the zero-kelvin transition point, that is, the QCP. One may think that decreasing the D–A separation with further bromine substitution beyond TTF–QBr2I2 would surpass the QCP and realize the temperature-induced ferroelectric NIT at ambient pressure. Unfortunately, this expectation has not yet materialized solely by the chemical modifications because of formation of the ionic CT complexes as noted above. To search for the QCP, we applied the hydrostatic pressure on the TTF–QBr2I2 crystal.

TTF–QBr2I2 under hydrostatic pressure

At ambient and low pressures, the relative permittivity around the lowest temperature (4 K) exhibits saturation behaviour and increases from 200 to 700 with pressure (Fig. 3a). According to the fit with the Barrett formula, T0 increases to +18.2(6) K at 0.23 GPa with T1=68.7(7) K and C=1.22(2) × 105 K. Although the phase transition is expected near this positive T0 in the classical picture, it is suppressed by the quantum fluctuations. Beyond the critical pressure of pc=0.25 GPa, a sharp peak indicative of the phase transition appears and shifts towards a higher temperature with further increasing pressure. The phase transition is accompanied by a small peak anomaly in the temperature dependence of dielectric loss (that is, imaginary part of dielectric constant).

Figure 3: Temperature- and pressure-dependent properties of TTF–QBr2I2 crystal.
figure 3

(a) Temperature dependence of the relative permittivity under various hydrostatic pressures. The applied pressure value, corrected considering its thermal change in the medium for each measurement, is represented by the value at the transition point or lowest temperature when the phase transition is absent. The inset depicts the data in the low-pressure range. (b) Temperature–pressure phase diagram; linear extrapolation of the phase boundary at high-temperature region points to 1.64 GPa at room temperature. The inset represents the quantum critical behaviour of ferroelectrics obeying the relation Tc(ppc)1/2 in the low-critical temperature region. The open square represents the pressure at which the room temperature conductivity exhibited a sharp peak caused by the pressure-induced NIT. (c) Inverse relative permittivity as the function of the square of the temperature. The solid line represents the quantum critical behaviour ɛr–1T2.

The Barrett formula has a drawback in its theoretical accuracy, although it is a prevailingfitting equation for quantum paraelectricity and convenient for estimating Curie–Weiss temperature in the classical limit as above: it cannot describe the behaviour around the QCP22. One of the reasons is that the fluctuation spectrum is assumed to be independent of wave vector and temperature, in contradiction to the observed softening of optical phonon mode in the displacive ferroelectrics. The mode softening has been observed near the ferro- and antiferroelectric transitions also in the family of electronic ferroelectric TTF–CA3,4,30. The fluctuating and softening polar mode in the latter ferroelectric is considered to be the so-called Peierls mode, which is accompanied by the intermolecular electron transfer oscillation coupled with the dimeric molecular displacement. According to the theory of quantum criticality22, the inverse dielectric susceptibility should be proportional to the square of temperature near the QCP for the typical displacive ferroelectrics such as SrTiO3 and KTaO3. In fact, this critical behaviour is clearly seen in the linear T2-1/ɛr relation (Fig. 3c) at temperatures below 50 K and pressures near the QCP for the TTF–QBr2I2 crystal.

Figure 3b shows the pressure–temperature phase diagram obtained from this permittivity measurement. Here the hydrostatic pressure values at the phase transition temperatures are corrected considering the effect of the thermal contraction of the pressure-transmitting oil in the clamp-type high-pressure cell (see Methods). The NIT critical temperature (Tc) tends to increase linearly with pressure in the high-pressure region. Its extrapolation to 1.63 GPa at room temperature (295 K) agrees well with the pressure of 1.55 GPa (marked by an open square in the figure), at which a sharp peak of conductivity (Supplementary Fig. 5) can be related to the pressure-induced NIT; the 0.35 GPa reduction from the pressure required for the NIT of TTF–QI4 (1.9 GPa)23 is roughly explained by the effective pressure of Δpeff=0.60 GPa noted above. The large slope (dTc/dp=0.166 K MPa–1), which is half that of TTF–QCl4 (0.32 K MPa–1)31, also reflects the pressure-sensitive nature of the NIT.

In contrast, the phase boundary (Fig. 3b) in the low-pressure region exhibits a critical drop, which can be approximated as Tc(ppc)1/2. The QCP is well defined at pc=0.246 GPa through the linear pTc2 relation (see inset in Fig. 3b).

The permittivity at the lowest temperature ɛr (T=5 K) as a function of pressure reveals a divergent-like sharp maximum at pc (Fig. 4a). There are some similarities and dissimilarities in the quantum critical behaviour near the QCP between the present case and the pressure-induced antiferroelectric NIT of DMTTF–QBr4 (ref. 29). Despite the very similar crystal structures in the paraelectric state, the inverse-ɛr versus p plot (inset to Fig. 4a) indicates the different behaviour between the two crystals around the QCP; in TTF–QBr2I2, the permittivity increases much more rapidly, obeying a simple power law of ɛr (T=5 K)|ppc|–1. Because the maximum ɛr (T=5 K) of TTF–QBr2I2 (800) is four times as large as that of DMTTF–QBr4 (<200), these observations reflect the different nature, that is, ferroelectric versus antiferroelectric, of the pressure-induced ordered phases. This difference manifests itself in structural changes.

Figure 4: High-pressure properties of TTF–QBr2I2 crystal at low temperatures.
figure 4

(a) Hydrostatic pressure dependence of the relative permittivity at 5 K, ɛr (T=5 K) on a TTF–QBr2I2 crystal (filled squares) in comparison with a quantum (anti)ferroelectric DMTTF–QBr4 crystal (filled circles, redrawn based on data in ref. 29). The inset shows the inverse permittivity obeying a simple power law, ɛr (T=5 K)|ppc|–1, as expected for quantum ferroelectricity. (b) Electric polarization (P) versus electric field (E) hysteresis loops with a triangular a.c. electric field at T=4 K and frequency f=1 kHz.

Figure 4b presents the electric polarization (P) versus electric field (E) diagram measured along the DA stacking direction at 4 K. The linear PE relation at ambient pressure is characteristic of the paraelectric state. The expected pressure-induced ferroelectric state is clearly demonstrated by the PE hysteresis loop at p=0.34 GPa. The magnitude of Pr (1.7 μC cm–2 at p=0.34 GPa) is >10 times as large as that of the ionic ferroelectric TTF–QBr4 (0.15 μC cm–2)16 but rather close to that of TTF–QCl4 (6.3 μC cm–2)12. This fact suggests that the microscopic origin of ferroelectricity, in particular its electronic nature as mentioned above, is similar in TTF–QCl4 and TTF–QBr2I2.

According to the X-ray diffraction experiments with TTF–QBr2I2 at 7 K under several different pressures (0.61, 0.67, 1.40, 1.80 and 2.27 GPa), the low-temperature, high-pressure phase is not accompanied by the appearance of any satellite reflections arising from superlattice. Because the triclinic unit cell involves simply one formula unit (Z=1), the symmetry-breaking DA dimerization therein confirms its ferroelectric nature. This structural change is in sharp contrast to the antiferroelectric change of unit-cell doubling in the DMTTF complexes29. The full structural analysis of the high-pressure phase is under examination and will be reported elsewhere.


QBr4 has been successfully substituted with iodine in a stepwise and regioselective manner. The TTF–QBr2I2 complex has enabled us to continuously control the ferroelectric NIT over a wide temperature range from zero kelvin to room temperature. In past studies, the TTF–QCl4 complex was partially replaced with tetraselenafulvalene (TSF), which has a larger molecular volume, to reduce the ferroelectric NIT temperature32. Quantum fluctuations are generally fragile against the inhomogeneity of the system, which cannot be removed in terms of the ionization energy in ternary compounds such as TTF1–xTSFx–QCl4. The binary compound DMTTF–QBr4 overcame this problem, and clear quantum critical behaviour of the NIT was exhibited, although the ionic DA lattice is dimerized into an antiferroelectric structure, cancelling out the polarization between neighbouring chains29. Renewed interest in the NIT system has also originated from the large electric polarization of TTF–QCl4 arising from the intermolecular CT process, namely, ‘electronic ferroelectricity’. The TTF–QBr2I2 complex is free from inhomogeneities such as the site energy variations and disordered molecular dipoles. TTF–QBr2I2 exhibits quantum critical behaviours relevant to the ferroelectric order, and hence can be considered as the first clean example of a ‘quantum electronic ferroelectric’.

While the temperature-induced NIT would be expected for a lattice-contracted isomorph of TTF–QBr2I2, the TTF–QBr3I complex forms a different type of crystal structure as another interesting class of quantum ferroelectrics. The monoiodination of the paramagnetic TTF–QBr4 salt is accompanied by a complete suppression of the ferroelectric SP transition. The lattice expansion and/or random orientation with dipolar QBr3I molecules may be plausible origins of the modified ground state. The ferroelectric SP state of TTF–QBr4 is suppressed under a strong magnetic field, exemplifying the ‘organic multiferroelectric’. Further chemical modifications or high-pressure studies of the TTF–QBr3I single crystal would provide promising phase controls for magnetoelectric phenomena.


Crystallization of TTF CT complexes

Neutral CT complexes of 2:1 stoichiometry, (TTF)2(QBr4–nIn) (n=0–4), were obtained by slow evaporation or cooling (0 °C) of mixed acetonitrile solution of QBr4–nIn and TTF as the main products. Occasionally, the 1:1 neutral complexes of TTF–QBr2I2 and TTF–QBrI3 crystallized as dark brown elongated plates, while the 1:1 ionic complexes of TTF–QBr3I and TTF–QBr4 crystallized as black rectangular plates. These minor crystal forms could be selectively crystallized with a size sufficient for electric measurements by prior seeding of several small single crystals. The 1:1 form of the TTF–QI4 complex was crystallized as elongated plates by slow evaporation or cooling of the toluene solution of the stoichiometric mixture of components.

Electric, magnetic and optical measurements

Infrared absorption spectra of the samples dispersed in KBr disks (Supplementary Figs 2 and 3) were recorded at room temperature with a Fourier transform infrared spectrometer (JASCO FTIR-200) in a frequency range of 4,000−400 cm−1 at a resolution of 4 cm−1. The magnetization of the polycrystalline TTF–QBr3I powder wrapped in a thin aluminium foil was measured at a constant magnetic field of 0.2 T by a superconducting quantum interference device magnetometer (MPMS XL, Quantum Design). The spin susceptibility data were obtained with correction for the magnetization of a blank foil and the core diamagnetism using the compilation of Pascal’s constant (2.2 × 10−4 e.m.u. mol−1). The temperature-dependent dielectric constant was measured with an LCR metre (HP 4284A) and at a cooling/heating rate of 1−2 K min−1. Hydrostatic pressure in the pressure-transmitting oil (Daphne 7,373, Idemitsu Kosan) was generated using a clamp-type high-pressure cell. The applied pressure at each transition point or T=5 K (Figs 2b and 3) was obtained by correcting the thermal change of the pressure (dp/dT=0.76 MPa K−1 for T>90 K and constant p for T<90 K) owing to the contraction of the medium33. The PE hysteresis curves were measured at 1 kHz with a commercial ferroelectric tester (Precision Premier II, Radiant Technologies) equipped with a voltage amplifier (Precision 4 kV HVI, Radiant Technologies).

Crystallographic studies

For the TTF–QBrI3 and (TTF)2(QBr2I2) crystals, both the X-ray diffraction data collection at room temperature and the assignment of the crystallographic axes of the bulk single crystals were completed using a four-circle diffractometer (Rigaku AFC7R; graphite-monochromated MoKα radiation) equipped with a charge-coupled device area detector. For the TTF–QBr2I2, TTF–QBrI3 and TTF–QI4 crystals, the X-ray diffraction experiments at room temperature were carried using a Rigaku DSC imaging plate diffractometer and synchrotron radiation (λ=0.6889 Å) at beamline BL-8A or BL-8B of the photon factory, High-Energy Accelerator Research Organization. The X-ray beam was monochromatized by a Si double-crystal monochromator and focused by a bent cylindrical mirror made of a Si crystal coated with Rh. The reflection intensity data were collected with the use of the Rapid-AUTO software package (Rigaku Corp.) and analysed with the Crystal Structure crystallographic software packages (Molecular Structure Corp. and Rigaku Corp.). The final refinements were done with anisotropic atomic displacement parameters for the non-hydrogen atoms and with calculated positions (with a fixed C–H distance of 0.95 Å) for the hydrogen atoms. Crystallographic data and experimental details of the CT complexes of TTF and QBr4–nIn are summarized in Supplementary Table 2 and Supplementary Data 1–5.

The change in the lattice parameters under hydrostatic pressure was examined at room temperature for a TTF–QBr2I2 single crystal using synchrotron radiation and a clamp-type Be-cylinder high-pressure cell filled with Daphne 7373 oil as the pressure-transmitting medium34. The diffraction study of a TTF–QBr2I2 single crystal under hydrostatic pressure was performed using synchrotron radiation and a diamond anvil cell filled with a 4:1 methanol–ethanol mixture. Diffraction spots were recorded on the imaging plate using a diffractometer equipped with a closed-cycle He-gas refrigerator, and the diamond anvil cell was mounted on the cold head of the refrigerator. The pressure was evaluated from the lattice constant of a NaCl internal pressure marker.

Additional information

Accession codes: The X-ray crystallographic coordinates for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre (CCDC), under deposition number CCDC-1059244-1059248. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via

How to cite this article: Horiuchi, S. et al. Quantum ferroelectricity in charge-transfer complex crystals. Nat. Commun. 6:7469 doi: 10.1038/ncomms8469 (2015).