Article Cite This: J. Am. Chem. Soc. 2019, 141, 9349−9357
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Plastic/Ferroelectric Crystals with Easily Switchable Polarization: Low-Voltage Operation, Unprecedentedly High Pyroelectric Performance, and Large Piezoelectric Effect in Polycrystalline Forms Jun Harada,*,†,‡ Yuto Kawamura,‡ Yukihiro Takahashi,†,‡ Yohei Uemura,§ Tatsuo Hasegawa,§ Hiroki Taniguchi,∥ and Koji Maruyama∥
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†
Department of Chemistry, Faculty of Science, and ‡Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-0810, Japan § Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan ∥ Department of Physics, Nagoya University, Nagoya 464-8602, Japan S Supporting Information *
ABSTRACT: Molecular ferroelectric crystals have attracted growing interest as potential alternatives to conventional leadbased ceramic ferroelectrics. We have recently discovered that a class of compounds known as plastic crystals can show multiaxial ferroelectricity, which allows ferroelectric performance even in polycrystalline forms. Here, we report new plastic/ ferroelectric ionic molecular crystals that exhibit remarkably small coercive electric fields at room temperature. The easily switchable ferroelectric polarization enables low-voltage switching operations and high-frequency performance. Such ferroelectric crystals can be readily processed into bulk polycrystalline forms with desired shapes that are characterized by unprecedentedly high pyroelectric figures of merit and large piezoelectricity. These multifunctional molecular crystals represent highly attractive prospects for device elements with a diverse range of applications, which will significantly boost the development of molecular ferroelectric crystals.
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methane.4 In the plastic crystal phase, the constituent molecules rotate isotropically, and their orientations are fully disordered.5 The rapid rotator motions of ionic molecules and their self-diffusion in their crystals have been exploited for the development of organic ionic plastic crystals (OIPC) with high ionic conductivity.6 In addition to conventional plastic crystals composed of molecules with globular shapes, the directionally dependent plasticity of organic crystals has also recently attracted considerable attention.7 When plastic crystals exhibit ferroelectricity, another unique feature of plastic crystals becomes apparent. The cubic crystal symmetry of plastic crystals endows ferroelectric plastic crystals with multiaxial ferroelectricity. Due to the highly symmetric cubic crystal structures in the plastic crystal phase, the molecular crystals show multiaxial ferroelectricity, which enables changing the polarization axis of the crystal in three dimensions and thus ferroelectric performance in polycrystalline forms.3 This feature is shared with ferroelectric perovskite oxides that are widely used as polycrystalline ceramics and stands in sharp contrast to conventional uniaxial molecular ferroelectric
INTRODUCTION Ferroelectrics are materials that exhibit spontaneous electric polarization, the direction of which can be reversed upon application of an external electric field.1 In addition to the switchable polarization, ferroelectrics exhibit pyroelectricity, piezoelectricity, and optical second harmonic generation, all of which have found a variety of technological applications. Remarkable progress has recently been achieved in the development of ferroelectric crystals of small molecules.2 Such molecular ferroelectric crystals are expected to serve as potential alternatives to or complements for widely used ferroelectric inorganic perovskite oxides, most of which are toxic lead-based compounds, such as lead zirconate titanate (PZT). In addition to the lack of toxicity, molecular ferroelectrics have various advantages, which include tunable chemical properties, solution processability, facile film synthesis, and mechanical flexibility. We have recently discovered that plastic/ferroelectric ionic molecular crystals, which have a plastic crystal phase as a hightemperature paraelectric phase and a ferroelectric phase as a lower-temperature phase, represent promising molecular ferroelectrics.3 The plastic crystal is a mesophase between the solid and liquid phases that is often found in molecules with globular structures, such as adamantane and tetrachloro© 2019 American Chemical Society
Received: March 28, 2019 Published: June 4, 2019 9349
DOI: 10.1021/jacs.9b03369 J. Am. Chem. Soc. 2019, 141, 9349−9357
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Journal of the American Chemical Society
remarkably large pyroelectricity, i.e., some of the pyroelectric figures of merit (FOMs) at room temperature showed record values that substantially surpass those of PZT, which is a widely used infrared-sensor material. Pellets of 1 also exhibited relatively large piezoelectricity compared to hitherto reported molecular ferroelectric crystals.
crystals, where the ferroelectric performance is usually restricted to single crystals. Although plastic/ferroelectric crystals have overcome a significant drawback of molecular ferroelectrics, i.e., the low dimensionality by introducing multiaxial ferroelectricity, further improvements are still necessary to approach industrial applications and further development of the class of materials, especially with regard to their usability in the bulk polycrystalline form. One of the strengths of ferroelectric ceramics is that they can be used in any shape that can be manufactured by sintering ceramics from powdered ferroelectric oxides. On account of their malleability and multiaxial ferroelectricity, plastic/ferroelectric crystals can also be processed into bulk polycrystalline forms, such as free-standing films and pellets with a desired thickness.8 The utility of such polycrystalline materials, however, has not been systematically explored for the hitherto reported multiaxial ferroelectric molecular crystals. Since the first discovery of plastic/ferroelectric crystals, a variety of reports on plastic/ferroelectric and related molecular ferroelectric crystals have confirmed the versatility of multiaxial molecular ferroelectrics composed of globular ionic molecules.9 The reported high-frequency performance, the high Curie temperatures (high Tc), and the large polarizations have amply demonstrated the importance and potential utility of these materials. However, their macroscopic ferroelectric performance at room temperature, which should manifest in polarization−electric field (P−E) hysteresis loops, has mostly been limited to quasi-single-crystal thin films (thickness: up to a few μm) carefully grown on substrates.10 This is mainly due to their high coercive field (Ec), which represents the threshold electric field required for inversion of the ferroelectric polarization. Ferroelectrics with large Ec values need a large electric field for the polarization switching, and their performance is, therefore, restricted to thin films, where a higher electric field is available under a certain applied electric voltage. P−E hysteresis loops in the bulk polycrystalline forms at room temperature have only been reported for a few molecular ferroelectrics with moderate Ec values (40−120 kV cm−1).3,8,11 This feature stands in stark contrast to the currently prevalent ferroelectric perovskite oxides, and severely limits applications and the development of advanced materials. Herein, we report the new plastic/ferroelectric ionic molecular crystals, 1-azabicyclo[2.2.1]heptanium perrhenate ([AH][ReO4]; 1) and 1-azabicyclo[2.2.1]heptanium periodate ([AH][IO4]; 2), which exhibit ferroelectric performance in bulk polycrystalline forms with exceptionally small Ec values. The [AH]+ cation adopts a cage-shaped structure, and its dipole moment (μ = 2.6 D calculated at the MP2/6-31+G* level of theory) is oriented in parallel to the N−H bond. The perrhenate and periodate anions adopt nonpolar tetrahedral structures. These crystals exhibit ferroelectricity in the polycrystalline forms, due to the reorientation of the polar [AH]+ cations, which is reminiscent of ferroelectric crystals composed of similarly structured molecules such as quinuclidinium perrhenate (3)3 and quinuclidinium periodate (4).12 While the ferroelectric performance of 2 was limited to below room temperature, the low Ec values of 1 in the roomtemperature phase enabled low-voltage switching ( 2σ(F2)] wR(F2) (all data)
[AH][ReO4] (1)
[AH][IO4] (2)
330 HTP C6H12NO4Re 348.37 cubic Pm3̅m 6.2087(8) 90.0 239.33(9) 1 3418 80 0.0204 80/2/9
300 ITP 348.37 trigonal R3m 6.1776(7) 89.846(1) 235.75(8) 1 3410 421 0.0341 421/18/32
270 HTP C6H12NO4I 289.07 cubic Pm3̅m 6.218(10) 90.0 240.4(12) 1 2620 69 0.0703 69/2/9
1.223 0.0237 0.0660
1.171 0.0168 0.0415
1.311 0.0480 0.1153
RESULTS AND DISCUSSION
Phase Transitions. DSC measurements showed that 1 and 2 exhibit two solid−solid phase transitions (Figure 1) similar to the related plastic/ferroelectric crystal 3.3 The corresponding transition temperatures (1: 199/322 K; 2: 244/258 K) were estimated during the heating run. Upon further heating, both crystals decomposed (1: ∼540 K; 2: ∼440 K) prior to melting. Hereafter, we will refer to the three solid phases separated by the two transitions as the low-temperature phase (LTP), the intermediate-temperature phase (ITP), and the high-temperature phase (HTP). The large transition entropy values at the LTP/ITP (1: 18.1 J K−1 mol−1; 2: 30.3 J K−1 mol−1) and ITP/HTP (1: 10.5 J K−1 mol−1; 2: 4.2 J K−1 mol−1) transitions are consistent with plastic/ferroelectric crystals, where substantial increases in orientational degrees of freedom are expected at each transition. As described in later sections (cf. Crystal Structures and Ferroelectric Performance in Bulk Polycrystalline Forms), the
Figure 1. DSC traces of (a) [AH][ReO4] (1) and (b) [AH][IO4] (2), where the solid-state phases are labeled HTP, ITP, and LTP.
HTP of 1 and 2 is the paraelectric plastic crystal phase with a cubic crystal structure, the ITP is the ferroelectric phase with low Ec values, and the LTP is also a ferroelectric phase, albeit with high Ec values. The transition temperatures for 1 indicate that the crystal exhibits a wide temperature window (123 K) of 9351
DOI: 10.1021/jacs.9b03369 J. Am. Chem. Soc. 2019, 141, 9349−9357
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Journal of the American Chemical Society low-Ec ferroelectricity, including room temperature, while that of 2 is narrow (14 K) and below room temperature (Figure 2).
Figure 2. Phase structures of 1−4. LTP: Ferroelectric phase with large Ec values; ITP: Ferroelectric phase with small Ec values; HTP: Paraelectric plastic crystal phase.
According to previous reports on the ferroelectric behavior of the related compounds 3 and 4, crystals of 3 also presented low Ec values (2−5 kV/cm),3 even though their performance was limited to above room temperature (ITP: 345−367 K). In contrast, crystals of 3 in the LTP and isomorphous crystals of 4, the latter of which do not exhibit a low-Ec ferroelectric phase, show relatively large Ec values at room temperature (3: 340 kV cm−1; 4: 225 kV cm−1).12,17 The phase structure of 1 thus indicates that the ferroelectric performance at around room temperature and the usability of these materials have been significantly improved compared to previously reported related plastic/ferroelectric crystals. Crystal Structures. Single-crystal X-ray diffraction analyses revealed that 1 adopts a polar crystal structure at room temperature (Table 1) that is isostructural with crystals of 3 in the ITP.3 Crystals of 1 in the ITP belong to the trigonal crystal system with a pseudocubic rhombohedral unit cell (a = 6.1776(7) Å and α = 89.846(1)° at 300 K), where one of the four body diagonals of the pseudocube is slightly longer than the others (Figure 3a). The longer body diagonal represents the crystallographic 3-fold axis. The polar point group 3m (C3v) with the noncentrosymmetric space group R3m is consistent with the observed ferroelectricity (cf. Figure 6), and the crystal exhibits spontaneous polarization along the 3-fold axis, i.e., the ⟨111⟩ direction. While the tetrahedral anion [ReO4]− can be refined to take only one orientation at each site, the large atomic displacement parameters suggest the presence of dynamic molecular reorientation. The polar [AH]+ cation, which does not exhibit 3-fold molecular symmetry, lies on the crystallographic 3-fold axis and is, therefore, orientationally disordered. Although the precise molecular structures were not available due to the level of disorder, the nitrogen atom of the [AH]+ cation and the bound hydrogen atom were refined to reside on the 3-fold axis and to form intermolecular hydrogen bonds with the oxygen atom of the adjacent [ReO4]− anion (N···O: 2.66(4) Å). In the HTP, crystals of 1 belong to the cubic crystal system (a = 6.2087(8) Å at 330 K), and the Pm3̅m space group was assigned. While the overall arrangement of the [AH]+ cations and [ReO4]− anions are not significantly different from that in the ITP, the orientations of both ions are fully disordered, which was attributed to their isotropic rotator motion in the crystal (Figure 3b). The seemingly octahedral structures obtained for both ions are an artificial result of the averaged
Figure 3. Crystal structures of [AH][ReO4] (1) and [AH][IO4] (2) with thermal ellipsoids drawn at 20% probability. (a) 1 at 300 K (ITP). The direction of the spontaneous polarization is indicated by the red arrow. Only one of the three equivalent orientations is shown for the disordered [AH]+ cation. (b) CsCl-type structure of 1 at 330 K (HTP). (c) CsCl-type structure of 2 at 270 K (HTP).
structures of the fully disordered tetrahedral ions on the site symmetry of m3̅m (Oh). The CsCl-type structure with the nonpolar point group m3̅m is isostructural with the HTP of 2 (vide infra), 3,3 and 4.12 The cubic crystal symmetry and the fully disordered molecular orientations strongly suggest that the HTP is the plastic crystal phase and that the crystals of 1 are plastic/ferroelectric crystals. All the four body diagonals of the cubic crystal lattice are equivalent 3-fold axes, each of which corresponds to the polarization axis of the trigonal ITP lattice. The plastic/ferroelectric crystals of 1 should, therefore, exhibit multiaxial ferroelectricity, and ferroelectric performance is expected not to be limited to single crystals but also to occur in polycrystalline samples. The crystal structure of 1 in the LTP could not be determined due to the inevitable twinning induced by the ITP/LTP phase transition. The crystallization of 1 at room temperature affords crystals in the ITP, which are transformed into the LTP upon cooling. The ITP/LTP transition induces twinning of the crystal, probably due to the associated reduction of lattice symmetry, which prevented determining the structure using single-crystal diffraction techniques. 9352
DOI: 10.1021/jacs.9b03369 J. Am. Chem. Soc. 2019, 141, 9349−9357
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Journal of the American Chemical Society Compound 2 crystallizes in the plastic crystal phase (HTP) at room temperature. The CsCl-type cubic crystal structure (space group: Pm3̅m) at 270 K is isostructural to that of 1, 3, and 4 in the HTP.3,12 Both the cation and the anion of 2 are fully disordered, which suggests that crystals of 2 in this phase are also in a plastic crystal phase (Figure 3c). Crystals of 2 in the lower-temperature ITP and LTP suffered from twining, similar to the case of the LTP of 1, which prevented determining the crystal structures in the ferroelectric phases of 2. Plastic Behavior. Crystals of 1 and 2 exhibit plastic deformation, and application of pressure easily allows processing their powders into bulk polycrystalline forms, such as free-standing polycrystalline films and pellets (Figure 4). The transparency of the films and pellets may be useful for
Figure 4. Photograph of a transparent polycrystalline film and a pellet of [AH][ReO4] (1): a large film (thickness: 80 μm; width: ∼1 cm) and a small pellet (thickness: 380 μm; diameter: 3 mm).
Figure 5. Dielectric constant, ε′, measured as a function of the temperature at various frequencies. The insets show a linear fit to the Curie−Weiss law using ε′ at 1 kHz: (a) [AH][ReO4] (1) (thickness: 340 μm) and (b) [AH][IO4] (2) (thickness: 230 μm).
applications in optoelectronic devices. Depending on the shapes desired for each measurement, e.g., thin films for highelectric-field applications or thick and robust pellets for piezoelectric d33 coefficient measurements, polycrystalline samples with a variety of shapes were prepared and used for the specific measurements. In addition to the plastic deformation observed for both crystals in the HTP, crystals of 1 also exhibited some degree of plasticity in the ferroelectric ITP, and films with similar quality can be prepared even at room temperature. Dielectric Properties. The dielectric constants of polycrystalline pellets of 1 and 2 were measured as a function of temperature at several frequencies (Figure 5). For both crystals, the real part (ε′) of the complex dielectric constant exhibited a sharp peak that is characteristic of ferroelectric/ paraelectric phase transitions at the ITP/HTP transition temperatures, which indicates that the HTP and ITP are the paraelectric and ferroelectric phases, respectively. The ε′ values obeyed the Curie−Weiss law, ε′ = C/(T − θ), above the Tc, and the Curie constants C (1: 4.3 × 103 K; 2: 2.5 × 103 K) and Curie−Weiss temperatures θ (1: 319 K; 2: 258 K) were deduced from the linear fitting of the 1/ε′ − T relationship. Both crystals also exhibited dielectric anomalies at the LTP/ ITP transition temperatures (1: 199 K; 2: 244 K), i.e., a steplike change of the ε′ values. The increase in the ε′ values in the ITP compared to those in the LTP are essentially identical with that reported for crystals of 3, which can be attributed to the onset of molecular reorientation of the polar cations and the resulting orientational polarization.3 Ferroelectric Performance in Bulk Polycrystalline Forms. Crystals of 1 exhibit ferroelectricity in the ITP and LTP. Figure 6a shows P−E diagrams of 1 in polycrystalline pellets (thickness: 180 μm). Well-defined hysteresis loops in
the ITP (300−320 K) provided decisive proof of ferroelectricity, while the linear P−E relationship in the HTP at 325 K is consistent with paraelectricity in the plastic crystal phase. The spontaneous polarization (Ps) and coercive field (Ec) were estimated from the intercepts in the P−E hysteresis loops (Ps at E = 0; Ec at P = 0). The Ps value at room temperature (4.1 μC cm−2 at 300 K) is comparable to those of typical molecular ferroelectrics, such as single-crystal triglycine sulfate (TGS; 2.8 μC cm−2)1a and polycrystalline forms of plastic/ferroelectric crystals such as 3 (3.4 μC cm−2 at 350 K; Figure S3). Crystals of 1 show a high fatigue resistance, and the P−E hysteresis loop did not exhibit any signs of deterioration after 105 cycles of polarization switching (Figure S4). The Ps values of 1 showed sizable changes with temperature in the ITP, i.e., large pyroelectricity, which is further discussed in a later section (cf. Pyroelectricity of 1 in Polycrystalline Forms). The large spontaneous polarization and well-defined hysteresis loops even in the polycrystalline films were attributed to the cubic crystal symmetry in the HTP and the resulting multiaxial ferroelectricity, which is one of the most valuable features of plastic/ferroelectric crystals. Another notable feature of 1 are the low Ec values (2−4 kV cm−1) in the ITP, which are by more than 2 orders of magnitude lower than those of poly(vinylidene)difluoride (PVDF) and its copolymers (∼500 kV cm−1).2a The Ec value at room temperature (4 kV cm−1) is the smallest hitherto reported value for multiaxial molecular ferroelectric crystals at room temperature, which are typically >100 kV cm−1.9 The small Ec values for 1 are comparable to those of 3 in the ITP, although the latter’s are limited to hightemperature operation (2−5 kV cm−1 at 345−367 K).3 The 9353
DOI: 10.1021/jacs.9b03369 J. Am. Chem. Soc. 2019, 141, 9349−9357
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of which are comparable to those of 1 and 3 in the ITP. While the Ps values significantly decrease to ∼0.2 μC cm−2 in the LTP, the hysteresis loops showed that the LTP of 2 is a ferroelectric phase with Ec values (>40 kV cm−1) that are significantly larger than those in the ITP. Low-Voltage Ferroelectric Operation of Thin Films of 1. The small Ec values of 1 at room temperature and the wellbehaved performance in polycrystalline forms enables lowvoltage ferroelectric switching of thin-film crystals of 1. A simple solution process readily afforded continuous thin crystalline films without visible pinholes (Figure S6). Ferroelectric hysteresis loops of such a thin film (thickness: 1.1 μm) were measured at room temperature under low-voltage operation (Figure 7a). The thin film exhibited well-defined
Figure 6. Polarization−electric field (P−E) diagrams of polycrystalline films of [AH][ReO4] (1) under an a.c. electric field (10 Hz): (a) HTP and ITP, as well as (b) ITP and LTP.
small Ec values offer a wide range of choices regarding the shape of the ferroelectric polycrystalline materials of 1. For example, robust pellets with a thickness of up to 1 mm can be easily subjected to ferroelectric switching under electric fields of a few hundred volts. Together with the malleability of the crystals, the small Ec values of 1 are expected to significantly enhance the utility of 1 in materials processes. Although the origin of the small Ec values in the ITP of 1 and 3 remains so far unclear, it might be related to the arrangements of the constituent ions with almost cubic symmetry as well as the virtually spherical structures of the polar cations: It is hardly likely that the former imposes the extra energy barriers for polarization inversion, which involves the inversion of the entire crystal structure, while the latter allows almost frictionless molecular reorientations of globular molecules during the ferroelectric switching. Crystals of 1 also show ferroelectricity in the LTP. Figure 6b shows the P−E diagrams of 1 in polycrystalline films of 50 μm thickness. Although the Ec values increased significantly in the LTP (≤198 K) to >40 kV cm−1, the well-defined hysteresis loops clearly show that the crystals of 1 are also ferroelectric in this phase. Large Ec values in the LTP were also reported for the related compounds 3 and 4 (3: 340 kV cm−1; 4: 225 kV cm−1; both at room temperature).12,17 Although the lack of information on the crystal structures of 2 in the ITP and LTP did not permit any conclusive interpretation on the basis of the crystal structures, crystals of 2 in these phases also show ferroelectricity in polycrystalline films (Figure S5). P−E hysteresis loops exhibited ferroelectricity in the ITP (246−260 K) with Ps values of ∼2 μC cm−2 (at 246 K) and relatively small Ec values ( several hundred) and their Tc values (322 K for both) not far above room temperature. It should be noted here that in contrast to the uniaxial ferroelectricity of TGS and LiTaO3, the multiaxial ferroelectricity of 1 allows using the high-performance pyroelectric materials in polycrystalline form. Together with the possibility of enhancing the sensitivity by fabrication of thin films and the corresponding multilayer-structured devices, crystals of 1 represent highly promising prospectives for pyroelectric materials that will in all likelihood find a number of applications, including in high-sensitivity infrared human body sensors. Piezoelectricity of Polycrystalline Pellets of 1. Polycrystalline pellets of 1 also exhibit relatively large piezoelectricity at room temperature. Piezoelectricity is the ability of a material to develop electric charge in response to applied mechanical stress (direct piezoelectric effect) and to generate mechanical strain resulting from an applied electric field (converse piezoelectric effect). Ferroelectric materials exhibit piezoelectricity and have hence found a wide range of applications, including in sensors, actuators, and other electromechanical device elements. In contrast to the recent
The temperature dependence of the Ps and p values was determined by pyroelectric-current measurements of polycrystalline pellets, which yielded p = 0.015 μC cm−2 K−1 at 298 K (Figure 8a). The large p value combined with a relatively small dielectric constant of 1 leads to high pyroelectric FOMs; particularly notable are the voltage responsivity, Fv, which is useful for infrared sensor elements, and the electrothermal coupling factor, k2, which estimates the effectiveness of thermal-energy harvesting. Fv and k2 are defined by eqs 2 and 3: p Fv = ′ ρc pε0ε33 (2) k2 =
p2 Thot ′ ρc pε0ε33
(3)
where ρ is the density, cp the specific heat capacity, ε0 the vacuum permittivity, ε33′ the dielectric constant measured parallel to the polarization direction of the poled sample, and Thot the maximum working temperature (for details, see Figure S9).20b The Fv value (0.45 m2 C−1 at 298 K) that was obtained for polycrystalline pellets of 1 is comparable to that of single crystals of TGS (0.36 m2 C−1) and larger than that of single crystals of LiTaO3 (0.14 m2 C−1), both of which are used as device elements in high-performance infrared sensors (Figure 8b).19a Compared with other polycrystalline materials, the Fv value of 1 is much higher than those of widely used PZT ceramics (0.059 m2 C−1). The k2 (Thot = 300 K) value of 9355
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relatively a large piezoelectric response. Combined with their advantageous low-temperature solution processability and the ease of fabrication of polycrystalline materials with desired shapes, these multifunctional crystals of 1 are expected to find a wide range of applications, including in sensor elements. The emergence of this new key player and its family of closely related compounds has established plastic/ferroelectric crystals in a position, from which a serious challenge could be launched on the current preeminence of perovskite oxides and vinylidene fluoride copolymers in this area. Further investigations into this class of ferroelectric crystals, including performance tuning by custom-tailoring the materials, will most likely help to elevate molecular ferroelectric crystals to the next development stage and hopefully soon yield widespread applications and rich materials science.
progress on the development of ferroelectric molecular crystals, the development of materials focusing on their piezoelectric properties is still in its infancy. Excellent piezoelectricity has recently been reported for organic−inorganic hybrid perovskites that are composed of tetraalkylammonium cations and infinite one-dimensional inorganic oxide chains.21 Although their piezoelectric coefficient, d33, which represents the induced charge per unit force applied in the direction of the polarization, ranges from 112 to 185 pC N−1, which reaches that of barium titanate (190 pC N−1), the reported macroscopic performance is limited to single crystals. This limitation is presumably due to the noncubic hexagonal structures of one-dimensional hybrid perovskites in the paraelectric phase. We have discovered that plastic/ferroelectric crystals of 1 show relatively large piezoelectricity in polycrystalline form.22 After poling, polycrystalline pellets of 1 exhibited d33 values (90 pC N−1) at room temperature that are significantly higher than that of the widely used piezoelectric PVDF and copolymers (∼30 pC N−1).23 The d33 values of 1 are comparable to those of recently reported plastic/ferroelectric crystals such as tetramethylammonium tetrachloroferrate(III) (80 pC N−1), tetramethylammonium bromotrichloroferrate(III) (110 pC N − 1 ), and tetramethylammonium tetrachlorogallate(III) (80 pC N−1).8,11 While the polarization of the latter three originated from the displacement of ionic molecules from nonpolar symmetrical arrangements in the crystals, the ferroelectricity of 1 is due to the reorientation of the polar cations. The large piezoelectricity commonly observed for the plastic/ferroelectric crystals regardless of the origin of their polarization might be interpreted in terms of the softness of the crystal lattices even in the ferroelectric phase at room temperature. It should also be noted here that the manifestation of large piezoelectricity in the form of polycrystals is due to the cubic crystal structures in the plastic crystal phases of these molecular crystals and the resulting multiaxial ferroelectricity. Three-dimensional realignment of the polarization axis during the poling process avoids the reciprocal canceling of the piezoelectric effect of individual crystal grains. Together with the ease of fabrication in bulk polycrystalline forms by the simple application of pressure, such plastic/ferroelectric crystals with large d33 values represent promising candidates for piezoelectric materials that may find a variety of applications in, e.g., sensors.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.9b03369. Fabrication of crystalline thin films; P−E hysteresis loops of 1−3; optical microscopy images of thin films of 1; temperature dependence of Ps of 1; temperature dependence of dielectric constants of a poled pellet of 1 and the specific heat capacity of 1 (PDF) Crystallographic information (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
[email protected] ORCID
Jun Harada: 0000-0001-7346-8446 Yukihiro Takahashi: 0000-0002-3252-9502 Tatsuo Hasegawa: 0000-0001-5187-7433 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI grants JP16H04126, JP16H06353, JP18K19049, and JP19H00884 as well as by a Grant-in-Aid for Scientific Research on Innovative Areas “π-System Figuration: Control of Electron and Structural Dynamism for Innovative Functions” (JP17H05135) and by the Yamada Science Foundation, the Suharada Memorial Foundation, and The Murata Science Foundation. A part of this work was conducted at the Advanced Characterization Nanotechnology Platform of the University of Tokyo, which is supported by the “Nanotechnology Platform” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. The authors would like to thank Dr. A. Kobayashi (Hokkaido University) for access to a Bruker D8 ADVANCE powder X-ray diffractometer and Dr. H. Yamada (National Institute of Advanced Industrial Science and Technology) for providing access to an atomic force microscope. The authors would also like to thank Ms. M. Maeda (TA Instruments) for DSC measurements and Ms. M. Takehisa (Hokkaido University) for P−E hysteresis loop measurements.
CONCLUSIONS In summary, we have synthesized new plastic/ferroelectric ionic molecular crystals [AH][ReO4] (1) and [AH][IO4] (2), whose ferroelectricity was attributed to the reorientation of the polar organic cations (AH = 1-azabicyclo[2.2.1]heptanium). Owing to their multiaxial ferroelectricity and plastic malleability, their crystalline powders can be easily processed into bulk polycrystalline forms that exhibit ferroelectricity. The ferroelectric properties and phase structures of 1 and 2 were compared to those of previously reported plastic/ferroelectric crystals, especially to 3 and 4, which exhibit closely related molecular and crystal structures. In sharp contrast to 3 and 4 and other multiaxial molecular ferroelectric crystals, crystals of 1 show remarkably low Ec values at room temperature, which enables low-voltage operation and high-frequency performance of ferroelectric switching of thin crystalline films. Crystals of 1 also demonstrate a hitherto unprecedentedly high pyroelectric performance in the polycrystalline forms, together with 9356
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DOI: 10.1021/jacs.9b03369 J. Am. Chem. Soc. 2019, 141, 9349−9357
