Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline

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Ferroelectricity and Piezoelectricity in FreeStanding Polycrystalline Films of Plastic Crystals Jun Harada, Naho Yoneyama, Seiya Yokokura, Yukihiro Takahashi, Atsushi Miura, Noboru Kitamura, and Tamotsu Inabe J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10539 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 11, 2017

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Journal of the American Chemical Society

Jun Harada,*,†,‡ Naho Yoneyama,‡ Seiya Yokokura,† Yukihiro Takahashi,†,‡ Atsushi Miura,†,‡ Noboru Kitamura,†,‡ Tamotsu Inabe†,‡ †

Department of Chemistry, Faculty of Science, and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan

ABSTRACT: Plastic crystals represent a unique compound class that is often encountered in molecules with globular structures. The highly symmetric cubic crystal structure of plastic crystals endows these materials with multiaxial ferroelectricity that allows a three-dimensional realignment of the polarization axes of the crystals, which cannot be achieved using conventional molecular ferroelectric crystals with low crystal symmetry. In this work, we focused our attention on malleability as another characteristic feature of plastic crystals. We have synthesized the new plastic/ferroelectric ionic crystals tetramethylammonium tetrachloroferrate(III) and tetramethylammonium bromotrichloroferrate(III), and discovered that free-standing translucent films can be easily prepared by pressing powdered samples of these compounds. The thus obtained polycrystalline films exhibit ferroelectric polarization switching and a relatively large piezoelectric response at room temperature. The ready availability of functional films demonstrates the practical utility of such plastic/ferroelectric crystals, and considering the vast variety of possible constituent cations and anions, a wide range of applications should be expected for these unique and attractive functional materials.

INTRODUCTION On account of their intriguing intrinsic properties, plastic crystals have attracted continued attention in various areas of chemistry.1,2 The plastic crystal phase is a mesophase between the solid and liquid states, often found in compounds with globular molecular structures, such as adamantane and tetrachloromethane. In the plastic crystal phase, the molecules undergo rapid isotropic rotator motions and lose their order of orientation similar to the liquid state,3–5 while the center of gravity of each molecule retains three-dimensional long-range order as in normal crystals. In contrast to the characteristic rigidity and brittleness of ordinary crystals, plastic crystals exhibit mechanical deformability, which can be attributed to the migration of lattice defects such as vacancies and dislocations in the crystals. In plastic crystals, orientationally disordered globular molecules occupy approximately spherical volumes, which results in highly symmetric crystal structures that usually belong to the cubic crystal system, similarly to the crystals of metals. The spherical volumes occupied by the disordered molecules are larger than those expected for orientationally ordered molecules, and the intermolecular interactions in plastic crystals are therefore weaker and more isotropic than those in the crystals of orientationally ordered molecules. These features should be responsible for the plasticity of plastic crystals, wherein a large number of low-energy dislocation slip planes are accessible, which stands in sharp contrast to conventional crystals, wherein dislocation slips are highly anisotropic and require high energies. Molecules in plastic crystals also undergo self-diffusion, which has been exploited in addition to the rotator motion for the development of organic ionic plastic crystals (OIPC) that show high ionic conductivity.6,7 Apart from conventional plastic crystals, mo-

lecular crystals that are susceptible to anisotropic plastic deformation and the bending of single crystals have attracted increasing attention.8–11 Plastic crystals represent a new group of ferroelectric crystals that possess properties different from those of other molecular ferroelectric crystals. Ferroelectrics are materials that exhibit spontaneous electric polarization, the direction of which can be reversed by application of an electric field. 12 Ferroelectrics also show pyroelectricity, piezoelectricity, and nonlinear optical effects, the utility of which has been demonstrated in many technological applications. Molecular ferroelectric crystals have recently attracted growing interest as potential alternatives to or complements of ferroelectric perovskite oxides, which are currently widely used in industry. 13–16 We have recently discovered that plastic/ferroelectric molecular crystals, which contain a plastic crystal phase as a hightemperature phase and a ferroelectric phase as a lowertemperature phase, are attractive functional materials that exhibit multiaxial ferroelectricity.17 Such multiaxial ferroelectrics allow an alteration of the direction of the polar axis of the crystal and show effective ferroelectric polarization switching even in polycrystalline form. The multiaxial nature of the plastic/ferroelectric crystals is due to the cubic crystal structure in the high-temperature phase, which is one of the unique features of plastic crystals. Several studies have since reported plastic/ferroelectric crystals and confirmed the importance of multiaxial molecular ferroelectrics by demonstrating that thin crystalline films (a few micrometers or thinner) grown on substrates exhibit ferroelectric switching.18–23 These studies have also shown that exposing such thin films to strong electric fields (a.c.) leads to a high frequency performance, which is another advantage of thin crystalline films of multiaxial mo-

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lecular ferroelectrics. The utility and the ease of handling of plastic/ferroelectric crystals stand in sharp contrast to conventional uniaxial molecular ferroelectric crystals: their polarization switching is limited to 180° flipping, which is accessible only when single crystals are available in orientations and shapes that are favorable for the application of an electric field. Herein, we report a new series of plastic/ferroelectric molecular crystals and demonstrate that their ferroelectric polarization switching and piezoelectric response can be achieved at room temperature in free-standing polycrystalline films that were fabricated capitalizing on the malleability that is characteristic for plastic crystals. Previous studies on plastic/ferroelectric crystals have exploited the isotropic cubic crystal symmetry in the plastic crystal phase and solid-solid phase transitions to the ferroelectric phase, where the orientationally ordered polar structure generates the spontaneous polarization. However, another important feature of plastic crystals, i.e., plasticity, has not been used for the development of ferroelectric materials. In the plastic crystal phase, crystals show permanent deformation without fracture upon application of a mechanical stress that can be used to fabricate films from agglomerates of microcrystals. Although materialsprocessing concepts based on plasticity have not yet been employed for crystals of small molecules, it is quite common for metals to be deformed into desired shapes by taking advantage of their plasticity. In the present study, we have found that organic ionic crystals of tetramethylammonium tetrachloroferrate(III) (1) and tetramethylammonium bromotrichloroferrate(III) (2) show plastic behavior at high temperature and ferroelectric properties in their lower-temperature phases. We have used the unique properties of these plastic crystals to fabricate free-standing polycrystalline films that show ferroelectric polarization switching at and even below room temperature. We have also discovered that the corresponding ferroelectric films exhibit a piezoelectric d33 coefficient of up to 110 pC N–1, which is higher than that of the widely used vinylidene fluoride polymers.

EXPERIMENTAL SECTION Materials. [(CH3)4N][FeCl4] (1) was prepared from combining equimolar amounts of tetramethylammonium chloride and iron(III) chloride hexahydrate in water that was slightly acidified with hydrochloric acid to avoid hydrolysis.24 The yellow crystalline powder of 1 that was obtained after removing the water was purified by recrystallization from ethanol. [(CH3)4N][FeBrCl3] (2) was prepared in a similar fashion from tetramethylammonium bromide and iron(III) chloride hexahydrate dissolved in neutral water, which afforded 2 as an orange powder. Yellow single crystals of 1 suitable for a singlecrystal X-ray diffraction analysis were obtained from the slow evaporation of an acidic water solution of 1 at room temperature. Measurements. Differential scanning calorimetry (DSC) measurements were performed on a Rigaku Thermo Plus DSC8230 apparatus using heating/cooling rates of 5 K min –1. Microcrystalline samples were weighed and sealed in aluminum pans. Optical second harmonic generation (SHG) meas-

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urements were conducted by using microcrystalline powders of 1 and 2. Femtosecond pulses ( = 780 nm) from a modelocked Ti:sapphire laser (Coherent, Mira 900F) pumped by a diode laser (Coherent, Verdi) were used as the light source, with a pulse duration and a repetition rate of 150 fs and 78 MHz, respectively. The linearly polarized laser beam was loosely focused (0.5-1.0 mm in diameter) with an incident laser power of about 230 mW. The output SHG signals were collected by a plano-convex lens with backward configuration, and detected by a multichannel photodetector (Hamamatsu, PMA-12) through a bandpass filter. For the electric measurements, a homemade cryostat was used to control the temperature of the samples (240-390 K) under a helium atmosphere. The temperature was measured using a Si-diode thermometer mounted on the sample holder. The dielectric constant was measured with an Agilent E4980A precision LCR meter (1 kHz – 1 MHz). The polarization– electric field (P–E) diagrams were measured using a SawyerTower circuit25 with a high-voltage triangular wave field at a frequency of 10 Hz. Small contributions of electric conductivity were eliminated by adjusting a variable resistor, which afforded compensated hysteresis loops with straight lines in the high-electric field corners. For the P–E measurements, polycrystalline films were fabricated by applying a pressure of ~60 MPa to powdered samples. Carbon paste was applied to both faces of the films to form electrodes. The piezoelectric d33 coefficients were measured with a d33 meter system (Lead Techno Piezo reader) using polycrystalline films, whose polarization had been confirmed by the P–E hysteresis measurements. Powder X-ray diffraction patterns of microcrystalline powder samples were recorded using a Bruker D8 ADVANCE diffractometer. Variable-temperature measurements were performed on an Anton Paar TTK 450 apparatus. The temperatures of the samples were calibrated on the basis of the previously reported thermal expansion of Al2O3.26 The measured patterns of the samples used for the DSC, SHG, and electric measurements confirmed their purity and identity of the phases, being compared with those simulated on the basis of the crystal structures of 1 obtained from the single-crystal X-ray diffraction analysis. Single-Crystal X-ray Diffraction Analysis. Single-crystal X-ray diffraction analyses were performed on a Bruker APEX II Ultra diffractometer (Mo K radiation,  = 0.71073 Å). The temperature of the samples was regulated using a nitrogen gas flow, and calibrated using a thermocouple. Structures were solved by intrinsic phasing (SHELXT–2014)27 and refined by full-matrix least-squares on F2 using SHELXL–2014.28 For the crystal structures of 1 in phases II-V, all hydrogen atoms were refined using riding models, while all non-hydrogen atoms were refined anisotropically. Accurate molecular structures in the plastic crystal phase I could not be obtained on account of severe rotational disorder and the very weak diffraction in the high-diffraction-angle region, and hydrogen atoms were not incorporated in the refinement model. Refinement details have been deposited in the Supporting Information as Crystallographic Information Files embedding the SHELXL–2014 res files. The crystal and experimental data are summarized in Table 1. CCDC 1574208 (1 at 250 K, phase V), 1574206 (1 at 300 K, phase IV), 1574204 (1 at 330 K, phase III), 1574207 (1 at 360 K, phase II), and 1574205 (1 at 400 K, phase I) contain the supplementary crystallographic data for this paper, which


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can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures. Table 1. Crystal Data and Structure Refinement Information for 1 compound

[(CH3)4N][FeCl4] (1)

temperature (K)

250

300

330

360

400

phase

V

IV

III

II

I

empirical formula

C4H12NCl4Fe

formula weight

271.80

crystal system

orthorhombic

orthorhombic

orthorhombic

orthorhombic

space group

Pbcm

Pma2

Amm2

Cmcm

cubic Pm3̅ m

a (Å)

6.4415(15)

14.268(5)

7.218(8)

8.964(9)

6.81(3)

b (Å)

13.071(3)

6.442(2)

9.019(9)

9.618(9)

6.81(3)

c (Å)

14.001(3)

6.451(2)

9.362(10)

14.158(13)

6.81(3)

1178.8(5)

592.9(4)

609.4(11)

1221(2)

316(4)

Z

4

2

2

4

1

reflections collected

15789

8298

4233

9245

3464

independent reflections

1411

1419

803

785

86

Rint

0.0422

0.0154

0.0326

0.0350

0.0562

1411/0/53

1419/49/74

803/31/43

785/45/57

86/1/9

V

(Å3)

data/restraints/parameters goodness-of-fit on

F2

1.038

1.089

1.127

1.148

2.166

R[F2 > 2(F2)]

0.0445

0.0246

0.0355

0.0636

0.1740

wR(F2)

0.1271

0.0720

0.1008

0.2068

0.5132

(all data)

RESULTS AND DISCUSSION Phase Transitions. DSC measurements on powdered crystalline samples of 1 and 2 revealed a series of solid-solid phase transitions for both crystals. Successive phase transitions have also been reported for tetramethylammonium tetrahalometalates(II) of the type [(CH3)4N]2[MX4] (M = Zn, Co, Fe, Cu, X = Cl, Br, I). While some of these compounds exhibit weak ferroelectricity in one of the low-temperature phases, they do not exhibit plastic crystal phases.29-33 The DSC traces of 1 showed three phase transitions above room temperature, and one below room temperature (Figure 1a). The transition temperatures in the heating run of 1 (295, 309, 344, and 384 K) are consistent with previously reported values. 34,35 Hereafter, we refer to the five solid phases separated by the four transitions as phases I-V, where the highest-temperature solid phase was denoted as phase I and subsequent lower temperature phases as phases II-V. Crystals of 2, which have been reported to be isostructural with crystals of 1,36 exhibited a series of phase transitions similar to 1 (Figure 1b). In the heating run of 2, transition temperatures of 263 285, 346, and 384 K were observed for the transitions V/IV, IV/III, III/II, and II/I, respectively. For both 1 and 2, the phases III and IV are ferroelectric phases, while the phases I, II, and V are paraelectric phases, and phase I is the plastic crystal phase with cubic crystal symmetry. The large entropy-change values at the II/I transition (1: 13.5 J K-1 mol-1; 2: 15.6 J K-1 mol-1) are consistent with this notion. It should be noted that both compounds show a tendency to supercooling or superheating, which is often observed for plastic crystals,2 and the transition temperatures vary by several degrees kelvin, depending on the thermal history of the

samples (cf. Figures S1 and S2). Especially notable was the substantial lowering of the transition temperatures observed in the cooling run, except for the highest-temperature transition between the phases I and II, which is comparable to the reported behavior of 1.34 Owing to the supercooling, the asgrown crystals of 1 are in the ferroelectric phase (phase IV) at room temperature, which is stable down to 267 K despite the presence of the V/IV transition at 295 K in the heating run.37



Figure 1. DSC traces for (a) [(CH3)4N][FeCl4] (1) and (b) [(CH3)4N][FeBrCl3] (2) in the heating run, whereby the solid-state phases are labeled I-V.

Dielectric Properties. The dielectric constants of 1 and 2 were measured as a function of temperature at several frequencies. Figure 2 depicts the real part ( ′) of the complex dielectric constant of compaction pellets of polycrystalline samples. In addition to the drastic increase of ′ at the II/I transition, which has been reported for 1,34 dielectric anomalies were observed at the phase-transition points. The III/II transition represents a ferroelectric-to-paraelectric transition for both compounds (1: 344 K; 2: 346 K). However, the dielectric constants do not obey the Curie-Weiss law, and hence sharp shaped peak characteristic of ordinary ferroelectrics were not observed. This behavior indicates that both crystals are improper ferroelectrics, whereby the paraelectric-to-ferroelectric phase transition should be driven by the ordering of physical quantities other than the polarization.38 Significantly increased dielectric constants in phase I can be attributed to ionic selfdiffusion in the plastic crystal phase.

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The crystal of 1 in phase IV belongs to the noncentrosymmetric space group Pma2 (orthorhombic crystal system). The structure observed at 300 K is essentially identical to that previously reported (Figure 3c).36 The lattice in phase IV is approximately related to that in phase V by the transformations aIV = cV, bIV = 1/2 bV, and cIV = aV. The overall arrangement of TMA and [FeCl4]– did not change significantly at transition V/IV, and the crystal adopts a pseudo-cubic CsCl-type structure. The TMA cations are located on the twofold axis parallel to the c-axis, while the [FeCl4]– anions are situated on the mirror plane perpendicular to the a-axis. Each TMA cation can be refined as the overlap of two different orientations that are related to each other by the twofold axis. The crystal belongs to the polar point group mm2 (C2v), and the polarization is directed along the c-axis, which is parallel to one side of the pseudo-cubic lattice. As shown in Figure 6a, 1 exhibits ferroelectricity in phase IV.

Figure 2. Temperature dependence of the dielectric constants of (a) [(CH3)4N][FeCl4] (1) and (b) [(CH3)4N][FeBrCl3] (2), measured at various frequencies.

Crystal Structures and Their Changes at the Phase Transitions. The crystal structures of 1 and their changes at the four phase transitions were fully disclosed by X-ray diffraction analysis (Table 1). At 250 K in phase V, 1 exhibits an orthorhombic crystal system with the centrosymmetric space group Pbcm (Figure 3a). The orientations of both the tetramethylammonium (TMA) cation and the tetrachloroferrate ([FeCl4]–) anion are ordered. The TMA cations are located on the twofold rotational axis parallel to the crystallographic a-axis. The [FeCl4]– anions are situated on the mirror plane perpendicular to the caxis. The crystal belongs to a non-polar point group mmm (D2h) and is, therefore, not ferroelectric in phase V. The crystal exhibits a pseudo-cubic CsCl-type structure, where each [FeCl4]– is surrounded by eight TMAs (Figure 3b).

Figure 3. Crystal structures of [(CH3)4N][FeCl4] (1) with thermal ellipsoids drawn at 50% probability, whereby hydrogen atoms are omitted for clarity. The direction of the spontaneous polarization in the ferroelectric crystal in (c) and (d) is indicated by red arrows. (a) Projection along the c-axis at 250 K (phase V). (b) The pseudo-cubic CsCl-type arrangement of ions at 250 K, whereby the cube is outlined by dashed red lines. (c) Projection along the aaxis at 300 K (phase IV). (d) Projection along the a-axis at 330 K (phase III). (e) Projection along the c-axis at 360 K (phase II). (f) The CsCl-type structure at 400 K (phase I).

In phase III at 300 K, 1 also adopts a polar crystal structure. The crystal belongs to the orthorhombic crystal system and the space group Amm2 (Figure 3d). The lattice in phase III is ap-


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proximately related to that in phase IV by the transformations aIII = 1/2 aIV, as well as by bIII and cIII, which are equal to either bIV + cIV or cIV – bIV. The site symmetry of both TMA and [FeCl4]– is mm2, i.e., both ions are situated on the twofold axis and on the two mirror planes including the twofold axis, respectively. The TMA cations are orientationally disordered over two positions, while the [FeCl4]– anions are not disordered. As shown in Figure 6a, 1 exhibits ferroelectricity in phase III. It should be noted here that while both phase III and IV belong to the same point group mm2 (C2v), the corresponding crystals are polarized in different directions with respect to the pseudo-cubic lattice, i.e., the polar axis in phase III is the c-axis, which is oriented in parallel to a face diagonal of the cubic lattice, while that in phase IV is located along a side. The crystal 1 in phase II at 360 K belongs to the nonpolar point group mmm (D2h) (Figure 3e). The crystal belongs to the orthorhombic crystal system with the space group Cmcm. The lattice in phase II can be related to that of phase III by the transformations aII = bIII, bII = cIII, and cII = 2 aIII. The site symmetry of the TMA cations is 2/m, and the fourfold orientationally disordered cations are located on the inversion center. The [FeCl4]– anions are not disordered and subject to an mm2 site symmetry. In the highest-temperature phase (phase I) at 400 K, the Xray diffraction images of crystals of 1 exhibit strong diffuse scattering and a drastic reduction in Bragg reflection intensity in the high-angle region, which is characteristic for plastic crystals composed of severely disordered molecules. The crystal belongs to the cubic crystal system and the m3̅ m Laue class, while the space group was assigned as Pm3̅ m (Figure 3f). The three equivalent cubic axes in phase I are related to the three vectors 1/2 (aII – bII), 1/2 (aII + bII), and 1/2 cII of the lattice in phase II. The cubic crystal structure is consistent with the reported optical isotropy of crystals of 1 in the highesttemperature solid phase.39 However, only limited structural information was obtained from the severely disordered molecular structures, and the seemingly octahedral structures obtained for both ions can be interpreted in terms of the averaged structures of the totally disordered tetrahedral molecules with a site symmetry of m3̅ m (Oh). The totally disordered constituent molecules and the CsCl-type cubic structure indicate that phase I is indeed a plastic crystal phase, where the molecules execute an isotropic rotator motion. The crystal-structure changes and molecular orientations for both ions of 1 involved in the phase transitions are summarized in Table 2. Phases III and IV belong to a polar noncentrosymmetric point group and can be ferroelectric. The other phases belong to non-polar centrosymmetric point groups and are therefore not ferroelectric. Non-zero values of the intensity of the optical SHG of 1 observed in phases III and IV, which fell to zero above the III/II transition temperature, are consistent with the observed crystal-structure changes (Figure S3). The polar axis of phase IV corresponds to the directions of the cubic lattice in phase I, which consist of six equivalent directions that include positive and negative orientations. The polar axis of phase III, on the other hand, corresponds to the directions of the cubic crystal, which consist of twelve equivalent directions. The ferroelectricity in phases III and IV is, therefore, multiaxial and polarization switching is expected even in polycrystalline samples. As for the molecular structures, the molecular orientation of the TMA cations is disordered except in phase V, while that of the

[FeCl4]– anions is ordered, except in the plastic crystal phase. Although each [FeCl4]– anion in phases II–V adopts only one orientation, the orientation and the position of the ions relative to the surrounding eight TMA cations varies with the phases, which results in a variation of the crystal symmetry and spontaneous polarization (vide infra). Although we have not yet obtained single crystals of 2 that are of sufficient quality to conduct a single-crystal X-ray diffraction analysis, powder X-ray diffraction measurements revealed that the crystal structures of 2 in phases I-IV are identical to those of 1.40 Figure 4 shows the temperature-dependent powder X-ray diffraction patterns of 2 together with the simulated ones for 1. The diffraction pattern for each phase of 2 is very similar to the simulated pattern of 1 for the corresponding phase calculated on the basis of the crystal structure determined by the single-crystal diffraction analysis. These results clearly demonstrate that 1 and 2 are isostructural in phases IIV, and indicate that phases III and IV of 2 could also be ferroelectric.41 Table 2. Selected Structural Features of 1 in Phases I–V. phase

I

II

III

IV

V

measured temp. (K)

400

360

330

300

250

point group

m3̅ m

mmm

mm2

mm2

mmm

polarization

none

none

//c

//c

none

no. of equiv. polarization directions in the cubic crystal

–

–

12

6

–

no. of orientations for the TMA cations

many

4

2

2

1

no. of orientations for the [FeCl4]– anions

many

1

1

1

1

Figure 4. Temperature-dependent powder X-ray diffraction patterns for (a) [(CH3)4N][FeBrCl3] (2) (experimental) and (b) [(CH3)4N][FeCl4] (1) (simulated).



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Free-Standing Polycrystalline Films of 1 and 2. Freestanding polycrystalline films of plastic crystals of 1 and 2 were easily prepared by application of a uniaxial stress to microcrystalline powders in phase I (Figure 5). The powder diffraction patterns of the thus obtained films showed that the crystals maintain crystallinity after the plastic deformation at high temperature, and that some degree of preferred orientation was induced to the polycrystals during deformation (Figure S5). The translucency of the free-standing films can be advantageous for applications in e.g. optoelectronic devices. The crystals of 1 and 2 show some degree of plasticity even at room temperature, and films with similar quality can also be prepared by pressing powders at room temperature, though this often requires repeated application of stress due to the reduced plasticity at room temperature.

Figure 5. Photographs of free-standing translucent polycrystalline films of (a) [(CH3)4N][FeCl4] (1; 110 m thickness), and (b) [(CH3)4N][FeBrCl3] (2; 130 m thickness).

Ferroelectric Properties of Polycrystalline Films of 1 and 2. Crystals of 1 and 2 clearly exhibit ferroelectricity in phases III and IV. The polarization–electric field (P–E) diagrams of 1 in polycrystalline films of 45 m thickness showed welldefined rectangular hysteresis loops between 290 K and 340 K (Figure 6a). The remnant polarization (Pr) and coercive field (Ec) were determined from the intercepts in the P–E hysteresis loops (Pr at E = 0 and Ec at P = 0). The Pr values showed significant variation depending on the ferroelectric phases, i.e., the Pr value in phase III (3.3 C cm–2 at 310 K) is larger than that in phase IV (2.0 C cm–2 at 298 K). These values are comparable to those of typical molecular ferroelectrics, such as single-crystal triglycine sulfate (TGS; 2.8 C cm–2).12 The Ec value, which represents the minimum electric field required for an inversion of the polarization, was 67 kV cm–1 at 298 K and showed a significant dependence on phase and temperature. These values were also much smaller than those of poly(vinylidene)difluoride (PVDF) copolymers (~ 500 kV cm– 1 13 ). Crystals of 2 exhibited ferroelectric behavior similar to that of 1. The P–E loops of a polycrystalline film of 2 (thickness: 30 m) exhibited well-defined hysteresis loops between 270 K and 320 K (Figure 6b). The Pr value of 2 also depends on the phase, i.e., the value in phase III (4.5 C cm–2 at 300 K) is significantly larger than that in phase IV (2.1 C cm–2 at 280 K). Moreover, the polarizations of 2 are higher than those of 1 in the same phases, especially in phase III. We did not observed any significant variations of the Pr values of polycrystalline films of 1 and 2 upon changing the conditions of preparation, which include variations of the grain size of the polycrystals (powders or single crystals), the applied pressure, and the pressing temperature.

Figure 6. Hysteresis loops of the electric polarization upon application of an a.c. electric field (10 Hz) to polycrystalline films of (a) [(CH3)4N][FeCl4] (1) and (b) [(CH3)4N][FeBrCl3] (2).

The reasonably large Pr values and well-behaved polarization switching observed for the polycrystalline samples of 1 and 2 are due to the cubic crystal structure in the hightemperature plastic crystal phase (phase I) and the resulting multiaxial ferroelectricity. This feature of plastic/ferroelectric molecular crystals stands in stark contrast to those of conventional uniaxial molecular ferroelectric crystals, for which polarization switching can usually only be achieved for singlecrystal samples. The direction of the polarization axis of a multiaxial ferroelectric crystal can be changed to one of several equivalent directions in the highly symmetric crystal structure of the paraelectric phase that corresponds to the original polarization axis in the ferroelectric phase. In the polycrystalline films of the plastic/ferroelectric crystals of 1 and 2, the randomly oriented polarization axis of each microcrystalline grain can be aligned to one of the directions that is equivalent in the cubic crystal lattice. Among these equivalent directions, the crystal is polarized along the direction that is most closely parallel to the applied electric field, i.e., perpendicular to the film surface (Figure 7). The polarization of the entire polycrystalline film is, therefore, much larger than that expected for uniaxial ferroelectric crystals in the polycrystalline form and is close to that of single crystals. The present results also show that a realignment of polarization axes can be achieved despite the presence of a paraelectric phase (here: phase II) between the ferroelectric phases (phases III and IV) and the plastic crystal phase with cubic crystal symmetry (phase I).


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Figure 7. Realignment of the polarization axes (red arrows) in polycrystalline films (microcrystalline grains are enclosed by solid lines). The film before poling (left) is depicted as an agglomerate of crystalline grains with one domain, which actually consist of multiple domains with differently oriented polarizations.

It should be noted that plastic/ferroelectric crystals of 1 and 2 show ferroelectric polarization switching even after plastic deformation in the plastic crystal phase, which might increase crystal defects and non-switchable polarization around the defects. The results indicate that film fabrication by application of a stress can be a useful technique to form free-standing ferroelectric films, and to modify the morphology of thin crystalline films after they have been grown on substrates using solution processes. Another notable point is that free-standing polycrystalline films of 1 and 2 show ferroelectric polarization switching at and even below room temperature. Although recently reported multiaxial ferroelectric molecular crystals exhibited polarization switching in the form of polycrystals (compaction pellets of powders or thin films grown on substrates), P–E hysteresis loops at room temperature were only observed for thin films (thickness: a few m or less) on substrates.17-23, 42-44 This is mainly due to the high Ec values required for switching in the ferroelectric phases, which exhibit orientationally ordered molecular structures. The relatively small Ec values for 1 and 2 at room temperature allow ferroelectric switching at and below room temperature, which should probably be ascribed to the disordered molecular structures in the ferroelectric phase. Ferroelectric molecular crystals undergo polarization switching through an inversion of the entire molecular and crystal structure. Orientationally disordered molecules in the ferroelectric phase undergo thermally activated molecular reorientation, which may facilitate the orientational switching involved in the polarization switching under an electric field and thus reduce the coercive electric field.45 A small coercive field is, in general, advantageous for the development of low-voltagedriven electronic devices. In addition, malleability of the materials allows the facile fabrication of free-standing films that are both sufficiently thin to make a strong electric field available and sufficiently robust to be handled easily. Origin of the Polarization. The polarizations of plastic/ferroelectric crystals of 1 and 2 and their phase dependence can be interpreted in terms of the off-center displacement of their constituent ions. Both the TMA cations and the [FeCl 4]– anions of 1 possess non-polar tetrahedral structures, and the polarization of the crystals in the ferroelectric phases should originate from the displacement of both ions from non-polar symmetric structures. The spontaneous polarization in each ferroelectric crystal phase was calculated using a simple pointcharge model, where the positive and negative charges of the cation and anion are represented by point charges at their center of gravity, i.e., the nitrogen and iron atoms (for details, see Supporting Information). The polarizations calculated for a single crystal of 1 in phases III (4.3 C cm–2) and IV (3.0 C cm–2) reasonably reproduced the observed values (phase III: 3.4 C cm–2; phase IV: 2.0 C cm–2) for the polycrystalline films, which confirms that the ionic displacement is the factor that dominates the ferroelectric polarization.

The smaller polarization in phase IV compared to that in phase III should be attributed to a partial cancellation of the polarization vectors in the two adjacent pseudo-cubes in phase IV. As shown in Figure 8a, the two adjacent [FeCl4]– anions in the unit cell of phase IV are not aligned along the b-axis. The polarization of one half of the cell (a pseudo-cube), which can be estimated to be 4.1 C cm–2, is as high as that in phase III, and directed approximately along the [01̅ 1] direction (Figure 8b). However, the polarization of the other half of the cell is directed along the [011] direction (Figure 8c). The polarizations of these half-cells thus cancel each other partially, resulting in a smaller polarization along the [001] direction. These observations also indicate that even though these ferroelectric ionic molecular crystals are composed of identical cations and anions, the polarization of the crystals can significantly change depending on the arrangements of both ions in the crystals. Moreover, in contrast to molecular ferroelectric crystals whose polarization arises from the dipole moments of polar molecules, orientationally disordered structures do not reduce the polarization of ferroelectric crystals where the polarization is dominated by the displacement of ions with opposite charges.

Figure 8. Crystal structure of [(CH3)4N][FeCl4] (1) in phase IV with thermal ellipsoids drawn at 50% probability and hydrogen atoms omitted for clarity. The direction of the polarization of each half of the unit cell is indicated by red arrows in (b) and (c). (a) Projection along the c-axis. (b) Projection along the a-axis, whereby only one half of the unit cell (0.5 < x < 1.0) is depicted. (c) Projection along the a-axis, whereby only one half of the unit cell (0 < x < 0.5) is depicted.

The lack of experimental information on crystal structures of 2 prevented a convincing interpretation of the origin of its polarization, which is higher than that of 1 in phase III. This can possibly be attributed to a larger displacement of the [FeBrCl3]– anion from the center of a pseudo-cubic TMA arrangement, or to contributions of the dipole moment of the polar [FeBrCl3]– anion ( = 1.55 D; calculated at the MP2/631+G* level of theory) in addition to the polarization that originates from the displacement. However, it should also be noted that the small modification of the constituent molecules (in this case: from [FeCl4]– to [FeBrCl3]–), can significantly enhance the Pr values at room temperature (1: 2.0 C cm–2 in phase IV; 2: 4.5 C cm–2 in phase III). The present results thus imply substantial potential for improvement and tuning of the performance of plastic/ferroelectric ionic crystals due to the rich diversity and availability of their constituent ionic components.



Piezoelectricity of Polycrystalline Films of 1 and 2. Piezoelectricity is the ability of a material to develop an electric charge upon application of a mechanical stress, and ferroelectric crystals always exhibit piezoelectricity. The range of applications for piezoelectrics is wide and includes sensors, actuators, and other electronic device elements. In contrast to the recent rapid development of ferroelectric crystals of small molecules, the corresponding studies regarding their piezoelectric properties still are in their infancy. While an excellent piezoelectricity of an organic-inorganic hybrid perovskite crystal (d33 = 185 pC N–1) has recently been discovered, the reported piezoelectric performance is limited to single crystals.46 In contrast, plastic/ferroelectric crystals are expected to exhibit piezoelectricity even in the form of polycrystals. This is due to the multiaxial ferroelectricity of the crystals, which allows their polycrystalline aggregates to be polarized in any given direction upon application of a strong electric field, and thus avoids the reciprocal cancelling of the piezoelectric effect of individual crystal grains. This feature renders ferroelectrics of inorganic oxides useful as piezoelectrics in the form of polycrystalline ceramics. This concept has been demonstrated in a recent study on guanidinium perchlorate crystals, which show substantial piezoelectricity (d33 = 10 pC N–1) in the form of compaction pellets of polycrystals.23 We have discovered that plastic/ferroelectric crystals of 1 and 2 show remarkably large piezoelectric coefficients (d33) in free-standing polycrystalline films. The d33 coefficient is one of the most useful parameters for the evaluation of the piezoelectric performance of materials, and is expressed as induced charge per unit force applied in the direction of the polarization. Polycrystalline films of 1 showed significantly larger d33 values (~80 pC N–1) at room temperature after poling in phase III37 than the widely used polyvinylidenedifluoride and other copolymers (~30 pC N–1).47 Polycrystalline films of 2 also showed high d33 values (~110 pC N–1) at room temperature in phase III. The fact that the d33 value of 2 is higher than that of 1 may be related to the higher polarization observed for 2. Films of 1 in phase IV at room temperature are also available,37 and these exhibit significantly smaller d33 values (~20 pC N–1) than those in phase III, which may reflect the smaller electric polarization in phase IV. Although the origin of the high piezoelectric coefficients of the films of the plastic/ferroelectric crystals of 1 and 2 remains unclear, it might be related to the softness of the crystals even in the room temperature phase. The spontaneous polarization, due to the displacement of ionic molecules, can also be attributed to the high piezoelectric response, as the application of stress to the crystals can readily alter the electric polarization. The large d33 values of the polycrystalline films of these plastic/ferroelectric crystals should lead to future applications in e.g. sensors. CONCLUSIONS In summary, we have discovered [(CH3)4N][FeCl4] (1) and [(CH3)4N][FeBrCl3] (2) as a new series of plastic/ferroelectric molecular crystals. Owing to their intrinsic multiaxial ferroelectricity and plastic deformability, their crystals show ferroelectric polarization switching and piezoelectricity in the form of free-standing polycrystalline films. The crystal structures in five different solid phases, including two ferroelectric phases, were determined by single-crystal X-ray diffraction analysis, and the origin of the ferroelectricity was attributed to the displacement of the molecular ions with opposite charges. The

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variation of spontaneous polarization in different ferroelectric phases was rationalized on the basis of the crystal-structure changes in the different phases. The crystals maintained their crystallinity and ferroelectric performance after mechanical deformation in the plastic crystal phase. Translucent ferroelectric polycrystalline films also showed a large piezoelectric response. The results also showed that a substitution of the [FeCl4]– anions (1) with [FeBrCl3]– anions (2) changed the phase-transition temperatures and the magnitude of polarization, which induced a significant improvement of the ferroand piezoelectric performance at room temperature. The ready availability of functional films and their performance tunability should greatly enhance the utility of plastic/ferroelectric crystals and thus promote the development of this class of materials. Many molecular ionic crystals have been reported to belong to the OIPC class, and their physical and chemical properties have been studied, especially with respect to ionic conductivity and molecular motions. However, details on their crystal structures and changes at phase transitions are often scarce. In most cases, the cubic crystal symmetry in the highesttemperature phase inferred from powder X-ray diffraction patterns is the only available source of structural information. This is probably due to the difficulties associated with obtaining single crystals of OIPCs that are of sufficient quality for single-crystal X-ray diffraction analyses. The present study shows that any OIPC can potentially afford ferroelectric crystals if they exhibit polar crystal structures in a low-temperature phase. This is due to the fact that the polarization can arise from the displacement of ionic molecules, while a molecular dipole moment is not required. Moreover, the ferroelectricity can be tested in polycrystalline films, which avoids the sometimes-problematic preparation of large single crystals that is necessary for conventional uniaxial molecular ferroelectrics. Furthermore, it should be noted that even if OIPCs exhibit a non-polar paraelectric phase as the orientationally ordered low-temperature phase as e.g. in the case of 1, they still could be ferroelectric in the intermediate temperature phases. Considering the large number of reported OIPCs that show a number of phase transitions upon cooling, and the vast variety of possible constituent ions, we expect to find a wealth of new plastic/ferroelectric crystals by exploring the reported and newly synthesized OIPCs, which should lead to new applications of this class of unique functional materials.

Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1012/jacs.xxxxxxx. DSC traces of 1 and 2; SHG of 1; identification of phase V for 2; powder X-ray diffraction patterns for polycrystalline films of 1 and 2; calculation of the polarization of 1; (PDF) Crystallographic file (CIF)

* E-mail: [email protected] The authors declare no competing financial interest.


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This work was supported by JSPS KAKENHI grants JP16H04126 and JP16H06353, as well as by a Grant-in-Aid for Scientific Research on Innovative Areas “-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (JP17H05135) and by the Iketani Science and Technology Foundation. The authors would like to thank Dr. A. Kobayashi (Hokkaido University) for access to a Bruker D8 ADVANCE powder X-ray diffractometer. The authors would also like to thank Dr. N. Yamada (Bruker AXS K. K.) for his assistance with the temperature calibration of powder X-ray diffraction measurements.

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(30) Sawada, S.; Shiroishi, Y.; Yamamoto, A.; Takashige, M.; Matsuo, M. Phys. Lett. A 1978, 67, 56-58. (31) Shimizu, H.; Abe, N.; Kokubo, N.; Yasuda, N.; Fujimoto, S.; Yamaguchi, T.; Sawada, S. Solid State Commun. 1980, 34, 363-368. (32) Wada, M.; Suzuki, M.; Sawada, A.; Ishibashi, Y.; Gesi, K. J. Phys. Soc. Jpn. 1981, 50, 1813-1814. (33) Gesi, K.; Perret, R. J. Phys. Soc. Jpn. 1988, 57, 3698-3701. (34) Czapla, Z.; Czupiński, O.; Galewski, Z.; Sobczyk, L.; Waśkowska, A. Solid State Commun. 1985, 56, 741-742. (35) Ruiz-Larrea, I.; López-Echarri, A.; Tello, M. J. Solid State Commun. 1987, 64, 1099-1101. (36) Wyrzykowski, D.; Kruszyński, R.; Kłak, J.; Mroziński, J.; Warnke, Z. Inorg. Chim. Acta 2008, 361, 262-268. (37) Ground microcrystalline powders of 1 are often a mixture of crystals in phases III and IV. The diffraction patterns of 1, measured at room temperature, thus usually represent a superposition of the patterns corresponding to phases III and IV, even though single crystals at room temperature belong to phase IV. The contamination of the higher-temperature phase (phase III) can be interpreted in terms of the transition point close to room temperature (309 K; from IV to III) that can be readily reached by the heat generated from grinding the crystals. The resulting crystalline sample (or parts thereof) in phase III remains in this phase after cooling to room temperature due to supercooling. After the mixture of crystals in phases III and IV are transformed to phase III by heating to 340 K and subsequent cooling to room temperature, the crystals remain in the phase III for at least seven days. On the other hand, after the crystals are transformed to phase IV by cooling to 280 K, and then left at room temperature, they also stay in phase IV for at least seven days. (38) Dvořák, V. Ferroelectrics 1974, 7, 1-9. (39) Kosturek, B.; Podsiadła, D.; Czapla, Z. Ferroelectrics 1999, 223, 57-61. (40) Phase V of 2 can also be presumed to be isostructural with that of 1. For details, see Supporting Information. (41) Under the present experimental conditions, SHG measurements of crystals of 2 resulted in significant sample deterioration. Although non-zero values of SHG signals were observed at and above room temperature (phase III), the intensity gradually decreased to zero under continued photoirradiation, even when the temperature was fixed. (42) Pan, Q.; Liu, Z.-B.; Tang, Y.-Y.; Li, P.-F.; Ma, R.-W.; Wei, R.-Y.; Zhang, Y.; You, Y.-M.; Ye, H.-Y.; Xiong, R.-G. J. Am. Chem. Soc. 2017, 139, 3954-3957. (43) Xu, W.-J.; Li, P.-F.; Tang, Y.-Y.; Zhang, W.-X.; Xiong, R.G.; Chen, X.-M. J. Am. Chem. Soc. 2017, 139, 6369-6375. (44) Zhang, W.-Y.; Tang, Y.-Y.; Li, P.-F.; Shi, P.-P.; Liao, W.-Q.; Fu, D.-W.; Ye, H.-Y.; Zhang, Y.; Xiong, R.-G. J. Am. Chem. Soc. 2017, 139, 10897-10902. (45) Quinuclidinium perrhenate plastic/ferroelectric crystals represent an example that supports this supposition. These crystals exhibit small coercive fields (2-5 kV cm–1) in the intermediate temperature phase (345-367 K), where both ions are orientationally disordered, while the orientationally ordered room-temperature phase does not show ferroelectric switching in spite of the polar crystal structure. 17 The absence of switching at room temperature was explained in terms of a coercive field that is larger than the applied electric field. The large coercive field of quinuclidinium perrhenate in the room temperature phase is supported by ferroelectric switching of the isostructural quinuclidinium periodate crystal with a large coercive field (225 kV cm–1 at room temperature). See also reference 22. (46) You, Y.-M.; Liao, W.-Q.; Zhao, D.; Ye, H.-Y.; Zhang, Y.; Zhou, Q.; Niu, X.; Wang, J.; Li, P.-F.; Fu, D.-W.; Wang, Z.; Gao, S.; Yang, K.; Liu, J.-M.; Li, J.; Yan, Y.; Xiong, R.-G. Science 2017, 357, 306-309. (47) Ramadan, K. S.; Sameoto, D.; Evoy, S. Smart Mater. Struct. 2014, 23, 033001.



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Ferroelectricity and Piezoelectricity in Free-Standing Polycrystalline

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