Unusual Long-Range Ordering Incommensurate StructuralModulations in an Organic Molecular FerroelectricZhihua Sun,†,§ Jian Li,‡,§ Chengmin Ji,† Junliang Sun,*,‡ Maochun Hong,† and Junhua Luo*,†

†State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,Fuzhou, Fujian 350002, China‡College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China

*S Supporting Information

ABSTRACT: The incommensurate (IC) behaviors of ferroelectrics have beenwidely investigated in inorganic oxides as an exciting branch for aperiodicmaterials, whereas it still remains a great challenge to achieve such intriguingeffects in organic systems. Here, we present that successive ordering of dynamicdipoles in an organic molecular ferroelectric, N-isopropylbenzylaminiumtrichloroacetate (1), enables unusual incommensurately modulated structuresbetween its paraelectric phase and ferroelectric phase. In particular, 1 exhibits threedistinct IC states coupling with a long-range ordering modulation. That is, theincommensurately modulated lattice is ∼7 times as large as its periodic prototype,and the IC structure is well solved using a (3 + 1)D superspace group with themodulated wavevector q = (0, 0, 0.1589). To the best of our knowledge, 1 is thefirst organic ferroelectric showing such a long-range ordering IC structuralmodulation. In addition, structural analyses reveal that slowing down dynamicmotions of anionic moieties accounts for its modulation behaviors, which also results in dramatic reorientation of dipolarmoments and concrete ferroelectric polarization of 1 (∼0.65 μC/cm2). The combination of unique IC structural modulationsand ferroelectricity makes 1 a potential candidate for the assembly of an artificially modulated lattice, which will allow for a deepunderstanding of the underlying chemistry and physics of aperiodic materials.

■ INTRODUCTION

Ferroelectric materials, which show switchable spontaneouspolarization (Ps), have been widely used as the basic elementsfor electric−optical devices, nonlinear optical switches, andsensors.1−4 Generally, the emergence of ferroelectricity isinseparable from phase transitions, changing from a high-symmetry paraelectric phase (PEP) to a low-symmetryferroelectric phase (FEP).5 An exceptional case, however, isthe incommensurate (IC) lattice of the periodic distortions,which has been an important topic in condensed matterscience.6 For instance, the long-range IC charge fluctuation wasfound to exert a significant influence on the superconductivityof (Y,Nd)Ba2Cu3O6+x.

7 Particularly, because of the irrationalvectors, the incommensurately modulated ferroelectrics exhibita three-dimensional long-range order but lack universaltranslation of the lattice periodic symmetry.8 In terms of thisunique concept, IC ferroelectric materials have been proven tobe promising candidates for the design of artificially modulatedlattices.9 Scientists have achieved the modulated structures in afew ferroelectric oxides, such as Ba1−xCaxNb2O6,

10

Bi2Mn4/3Ni2/3O6,11 and YMnO3.

12 More recently, a relaxorferroelectric oxide of PbBiNb5O15 was reported to show quitestrong IC structural modulation.13 Despite some IC ferro-electric oxides, most inorganic materials require high-temper-ature syntheses and even contain environmentally poisonousmetals (e.g., lead). This becomes a big hindrance for further

development of optoelectronic devices based on IC ferro-electrics. Alternatively, molecular ferroelectrics have emerged ascompeting candidates, showing behaviors comparable to thoseof BaTiO3, including large Ps and high Curie temperature(Tc).

14 Because of the structural diversity and flexibility, organicferroelectrics have been developed as the key materials in mass-light, printable, and bendable electronic devices. However,reports on the organic IC ferroelectrics are quite sparce,15

owing to the lack of knowledge regarding control of themotions of dipole moments between periodic lattices. In thiscontext, it is a great opportunity to design the incommensur-ately modulated structures in the pure organic ferroelectricsystems.Structurally, the appearance of IC modulation shows up in

the system that contains the periodic lattices with differentranges but similar magnitudes.16 For ferroelectrics, the drivingsource to IC-modulated structures is the competition betweenferroelectric lattice and paraelectric prototype, which is closelyrelated to the positional freedom of molecular dipoles. Fromthe viewpoint of phase transition kinetics, this requires thatatomic disordering or reorientation undergoes a successivedeviation from the prototypic lattices (i.e., FEP or PEP); that is,the IC structure originates from the distortion of one periodic

Received: August 22, 2017Published: October 16, 2017

Article

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© 2017 American Chemical Society 15900 DOI: 10.1021/jacs.7b08950J. Am. Chem. Soc. 2017, 139, 15900−15906

Cite This: J. Am. Chem. Soc. 2017, 139, 15900-15906

lattice. Thus, a delicate balance must be achieved between thedynamic branch and static remaining (not strictly). The formeraffords a driving force to break through the energy barrier ofphase transitions, while the later provides cooperation topreserve its preliminary crystallographic feature. As shown inthe IC ferroelectric of NaNO2, the ordered orientation of NO2

−

groups inside the lattice leads to its sinusoidally modulatedstructure between 163 and 166 °C.17 Nevertheless, it ischallenging to achieve such intriguing effects in organicferroelectrics due to their more complicated structures.Here, we designed a new molecular ferroelectric that shows

diverse incommensurately modulated structures, N-isopropyl-benzylaminium trichloroacetate (1), by introducing the con-strained disordered moiety into a H-bonding system. Inaddition to notable ferroelectric properties, 1 displays threedistinct IC states between its PEP and FEP. Particularly, anunusual long-range ordered IC periodicity of ∼7 times as largeas its prototype lattice is established from the diffuse satellitereflections. At 150 K, the satellite reflections have to be indexedwith four integers as H = ha* + kb* + lc* + mq with amodulated vector of q = (0, 0, 0.1589) and m ≠ 0. The dynamicanions account for such incommensurately modulatedstructures as well as the long-range ferroelectric order.18 Tothe best of our knowledge, 1 is the first molecular ferroelectricwith such diverse IC structural modulations. The combinedferroelectricity and IC properties make 1 a potential candidatefor the artificially modulated lattices, which also allows for adeep understanding of the underlying concepts of chemistryand physics in aperiodic materials.

■ EXPERIMENTAL SECTIONSynthesis. Raw materials of 1 were synthesized by the reaction of

equivalent mole ratio of trichloroacetic acid and N-isopropylbenzyl-amine. Block crystals were grown from the aqueous solutions by thetemperature lowering method (in Figure S1), and its bulk purity wasverified by powder X-ray diffraction (PXRD, Figure S2) and elementalanalysis (C, H, and N). Anal. Calcd for C12H16Cl3NO2 (%): C, 46.10;H, 5.16; N, 4.48. Found (%): C, 46.05; H, 5.12; N, 4.43.Measurement Methods. The PXRD patterns were collected

using the MiniFlex II X-ray diffractometer. The differential scanningcalorimetry (DSC) experiment was carried out on a NETZSCH DSC200 F3 in the temperature range of 100−300 K with a heating/coolingrate of ∼10 K/min, and the specific heat (Cp) studies were performedon the physical property measurement system (model 6000, QuantumDesign USA).In electric experiments, single crystals covered by the silver

conduction paste on the surfaces were used as electrodes. Thecomplex dielectric permittivities (ε = ε′ − iε″, where ε′ and ε″ are thereal part and imaginary part, respectively) were measured by the two-probe AC impedance method, using an impedance analyzer(TH2828A) under an applied electric field of 0.5 V. Pyroelectriccurrents were measured along the b-axis of single crystals by using ahigh resistance meter/electrometer (Keithley 6517B), in which thereleased currents were recorded as a function of temperature. Thus,temperature-dependent polarization can be obtained by integrating thepyroelectric currents with respect to time. Polarization hysteresis loops(P−E loops) were measured with applied electric field parallel to its b-axis direction, using the Sawyer−Tower circuit method (RadiantPrecision Premier II).Polycrystalline samples of 1 with particle sizes of 72−100 μm were

adopted to measure its temperature-dependent second harmonicgeneration (SHG) effects. The fundamental light was generated from aNd:YAG laser (λ = 1064 nm, the pulse duration is 5 ns, and peakpower is ∼1.6 MW), and the SHG signals were collected using afluorescence spectrometer (FLS 920, Edinburgh Instruments),equipped with a variable-temperature system (DE202, 120-190 K).

SHG intensities were compared with that of KH2PO4 (i.e., KDP), forwhich the SHG coefficient χ(2) is ∼0.39 pm/V.19

Single-Crystal X-ray Crystallography. Data of single-crystaldiffraction were collected on an Agilent Technologies SuperNova dualwavelength CCD diffractometer with Mo Kα radiation (λ = 0.71073Å) at different temperatures (100, 140, 150, 190, 240, and 290 K).Data reduction and multiscan absorption correction were performedby the CrysAlisPro software.20 Crystal structures except forincommensurately modulated structures were solved with directmethod and refined by the SHELX software.21 The non-hydrogenatoms were refined in the anisotropic modes, with positions of Hatoms being generated geometrically. Moreover, the crystal structuresat IC phase (ICP) were determined with the charge-flipping algorithmusing the program SUPERFLIP22 and refined against F2 by theprogram JANA2006.23 VESTA was used as a visualization of Fourierelectron density.24 Crystal data and structural information for 1 arelisted in Tables S1−S3, and CCDC 999204−999208 contain thecrystallographic data for this paper.

■ RESULTS AND DISCUSSIONIt is well-known that most ferroelectric materials are inseparablefrom the symmetry breaking phase transitions, which resultfrom quite subtle structure changes. For 1, the preliminary DSCand Cp−T measurements clearly disclose that it undergoessuccessive phase transitions (Figure 1). As shown in the DSC

traces, two pairs of exothermal peaks are observed at ∼150.3/146.1 and ∼159.3/155.6 K in the heating/cooling runs. Inaddition, the Cp−T curve also displays two thermal anomaliesat 149.9 and 159.1 K upon heating, which coincide with DSCresults. For convenience, the phase transition temperatures at146.1 and 155.6 K (in the cooling run) were labeled as Tc andTi, respectively.To deeply understand the origin of phase transitions, we

determined crystal structures of 1 at different temperatures(Table S1, Supporting Information). Structure analyses revealthat 1 crystallizes in the orthorhombic space group of Pbca (thepoint group of mmm) above Ti but transforms to Pb21a (thepoint group of mm2) below Tc. From the viewpoint ofsymmetry breaking, an Aizu notation of mmmFmm2 confirmsthat the phase transition of 1 belongs to the ferroelectric type.25

Between Tc and Ti, some satellite reflections around the mainreflections with strong intensities are clearly observed from thesingle-crystal X-ray diffraction patterns, reminiscent of thepossible IC structure modulations (as discussed below). In thiscontext, we denote the phase above Ti as PEP, the phasebetween Ti and Tc as incommensurate phase (ICP), and thephase below Tc as FEP (as shown in Figure 1).

Figure 1. DSC and the Cp−T traces of 1.

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At PEP (T = 240 K), the basic unit of 1 is composed of theH-bonding dimers, in which anions and cations are linked bynotable N−H···O hydrogen bonds with the donor−acceptordistances of 2.794 and 3.185 Å (in Figure 2 and Table S3).

Interestingly, the trichloromethyl moiety adopts a distortedditetrahedron geometry. This positional disordering is alsoconfirmed by their relatively elongated thermal ellipsoids(Figure S4). In contrast, the atoms of organic cations aresolved at the exclusive positions without any disordering, whichbehave as the static part. As a result, the sterically chemicalconfiguration of 1 could be vividly described as a rotator−statorassembly via intermolecular N−H···O hydrogen bonds, asshown in Figure 2. It is known that molecular motions in therotator−stator systems have succeeded in the assembly offerroelectrics, such as 4-methoxyanilinium tetrafluoroborate 18-crown-6,26 in which motional behavior of rotators gives rise tothe ferroelectric properties. It should be expected that dynamicsof the rotator in 1 will also enable the potential ferroelectricresponses.As temperature decreases below Tc, the ordering of dynamic

anions results in the breakdown of its prototypic symmetry. AtFEP (T = 100 K), the asymmetric unit of 1 is doubledcompared to that of PEP, containing two H-bonding dimerswith slightly distinct steric configuration (named A and B,Figure 2). This is solidly identified from the differences ofdonor−acceptor distances for N−H···O H-bonds (i.e., 2.777/3.210 Å for the dimer A and 2.856/3.059 Å for B, in Tables S2and S3). Another distinguishable change is that the anioniccomponents become ordered, with all the Cl atoms located atdefinite positions. The trichloromethyl group shows an idealtetrahedron symmetry with almost equivalent C−Cl bondlengths (∼1.76 Å) and Cl−C−Cl bond angles (∼107°). Thecationic configuration displays quite small alterations; that is,the isopropylaminium group deviates farther away from thebenzene ring plane and thus compacts the space for N−H···Ohydrogen bonds. This will enhance energy barriers to activatethe possible dynamic motions of rotators, favoring an orderedarrangement of molecular dipoles.27 Hence, the frozen orderingof such dynamic components affords a driving force to itssymmetry breaking (Figure S5); that is, the paraelectric

prototype of Pbca changes to a polar space group Pb21a,reminiscent of possible ferroelectricity in 1.What makes 1 more intersting is the emergence of ICP

between Tc and Ti. As shown in Figure 3, at 100 and 240 K, the

main reflections are identified by the (hkl) indices and conformto the universal translation of long-range lattice periodicsymmetry. In contrast, quite strong satellite reflections areclearly observed around the main diffraction peaks at 150 K(Figure 3e). Viewed from the reciprocal lattice of diffractionpatterns (Figure 3b), the linear distibution of satellitereflections reveals the first-order modulation along its c*-axis,whereas no modulated vectors can be observed along a*- andb*-axes. Therefore, these diffraction patterns can be indexedwith four parameters as H = ha* + kb* + lc* + mq with q = (0,0, 0.1589) and m ≠ 0. As the q vector is not an irrational value,this unique feature confirms that the crystal structure of 1 is tobe incommensurately modulated.16 Especially, all the reflectionconditions follow the rule of (h0lm: h + m = 2n; 0klm: l = 2n;hk00: k = 2n), which suggests its modulated lattice should bePbca(00g)0s0, similar to the paraelectric phase of Pbca. Thiscentrosymmetric feature agrees well with the absence ofpyroelectric and quadratic nonlinear optical properties at 150K. At 148 K, the main diffractions transform to an acentric basiccell, showing an average superstructure of Pb21a(00g)s00.However, it fails to obtain the accurate modulated structuremodel by refining the crystal data, and we could only obtain anaverage structure from the main diffractions (Figure S6,Supporting Information). As the temperature further decreasesto 146 K, the intensities of satellite reflections become muchweaker, and the main diffractions still adopt an acentric basiccell, close to its ferroelectric phase of Pb21a at 100 K (FigureS6d). Hence, it is proposed that 1 shows an unusual multilevelmodulation of IC structures, including centrosymmetric ICP(150 K), intermediate ICP (148 K), and acentric ICP (146 K).This result discloses that the modulated structures in 1 aregradually inhibited. In detail, the stable ICP at 150 K adopts acentrosymmetric supergroup of Pbca(00g)0s0, whereas theunstable IC states at 146 and 148 K exhibit polar characteristics.In this regard, we can conclude that the polarization shouldtotally disappear at Ti. As far as we are aware, 1 should be thefirst example of organic molecular ferroelectrics showing such

Figure 2. Perspective views of 1 at (a) PEP and (b) FEP. At PEP, itsasymmetric unit consists of one H-bonding dimer, and the anion ishighly disordered. In comparison, two H-bonding dimers with slightlydistinct steric configurations are included, and all the anions becometotally ordered at FEP.

Figure 3. (0hk) Plane diffraction patterns for 1 at 100 K (a,d), 150 K(b,e), and 240 K (c,f). All the main reflections at 150 K possess thesame forbidden reflection law and diffraction intensities with those atPEP.

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rich and distinct IC structural modulations, which suggests that1 has a strong tendency to be modulated.Moreover, we adopt the (3 + 1)-dimensional superspace

group directly with the charge-flipping algorithm to solve itsexact IC structure. The details of its superspace groupdetermination are presented in Supporting Information. Therefined structure model of ICP has a small residual electrondensity (the largest peak is ∼0.37 e/Å3), along with very low Rvalues for the main diffractions and first-order satellitereflections (I > 3σ). Figure 4 depicts the approximate

superstructures of 1 at 150 K, of which the asymmetric unitis greatly expanded in comparison to that of PEP and FEP. Inits superstructure, the modulated cell length of c-axis is almost∼7 times as large as that of its periodic prototype, whereas thea- and b-axes remain unchanged. This feature suggests that theIC structural modulation of 1 is well developed with long-rangeordered modulations, coincident with the observation ofsatellite reflections. Structurally, this behavior is closely relatedto the positional and occupational modulations induced by thedisordering of trichloroacetate anions. As shown in Figure 4a,highly disordered anions were initially modeled according to allthe reflections. A more reasonable mode was finally determinedby cutting chlorine atoms with the occupancies below 0.5 (inFigure 4b). Obviously, the large molecular freedom oftrichloroacetate groups affords abundant possibilities to berelaxed in an energetically favorable state, which might promoteits long-range ordering modulations. Similar molecular motionshave been observed in other molecular IC ferroelectrics, such astrichloroacetamide.28 Based on this study, it is deduced that theunique structural modulations of 1 are driven and dominatedby the occupational and positional disordering of anions. Indetail, for this rotator−stator assembly, anionic moieties arehighly disordered at PEP, acting as dynamic rotor-like units. AtICP, the rotors still preserve disordered feature but have anordered occupancy, leading to the incommensurately modu-lated structures of 1 (see eqs 9−11, Supporting Information).Upon further cooling below Tc, thermal motions of dynamicparts are totally frozen into an ordered state, corresponding toits FEP.Microscopically, the frozen ordering of dynamic molecular

motions during phase transitions would induce the notabledielectric response and ferroelectric polarization. As shown inFigure 5, temperature-dependent dielectric constants of 1

display sharp anomalies around Ti and Tc, coinciding well withits PEP−ICP−FEP phase transitions. In the vicinity of themaximum peak position at Tc, the ε′−T traces display sharpchanges and clear frequency-dependent responses. The ε′values are much larger than those in both higher and lowertemperature regions, which obey the Curie−Weiss law of 1/ε′= (T − T0)/C. The fitting curve affords the Curie constant C of∼148 K ( f = 1 MHz, inset in Figure 5a), falling around those ofo t h e r m o l e c u l a r f e r r o e l e c t r i c s , s u c h a s[NH2CH2COOH]2HNO3, RbHSO4,

29 and bis(imidazolium)L-tartrate.30 In particular, it is noteworthy that the shoulder-likedielectric anomalies are also observed around Ti, suggestive ofits ICP-to-PEP phase transition. As shown in NaNO2 (inset inFigure 5b), this shoulder-like dielectric anomaly has evidencedits IC properties.17 For 1, the emergence of successive dielectricchanges agrees fairly well with its thermal and structuralanalyses, which also confirms 1 is a ferroelectric with ICbehaviors.In addition to remarkable dielectric anomalies, ferroelectric

materials display switchable spontaneous polarizations inducedby long-range ordering of molecular dipoles.32 Here, weperformed the measurement of Ps versus electric field (E) onthe crystals of 1. At 160 K (above Ti), the dependence ofpolarization to the applied field is linear, which reveals theparaelectric properties of 1 (Figure S8). As temperaturedecreases in the range of Ti and Tc, it fails to record clearhysteresis loops in this narrow temperature range. However,upon cooling below Tc, the traces become broader with acurved tail and grow into the typical ferroelectric loops (at∼145 K), as illustrated in Figure 5. Such nonlinear P−E loopsare the characteristic behaviors for ferroelectrics. The betterhysteresis loops can be obtained by further decreasing the

Figure 4. Superlattice approximants of 1 at 150 K. (a,b) 1a × 1b × 7cmodulated approximant along its a- and c-axes. (c,d) Reasonable ICsuperstructure with 1a × 1b × 7c modulated approximant along its a-and c-axes, which cutoff the atoms (Cl1b, Cl2b, Cl3b and Cl4b, Cl5b, Cl6b)to where occupancies are below 0.5. The diameter of Cl atoms wasenlarged in order to clearly show the modulation function ofoccupancy.

Figure 5. Dielectric properties of 1 measured on the single crystalsalong the b-axis direction. (a) Temperature dependence of thedielectric constant (ε′). Inset: Reciprocal of dielectric constants versustemperature, which obeys Curie−Weiss law in the vicinity of Tc. (b)Comparison of dielectric constants for 1 and NaNO2, of which likeanomalies confirm their IC behaviors.

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temperature from its Tc. At 135 K, the rectangle-like hysteresisloop is recorded, like that of triglycine sulfate (Figure S9). Thesaturated polarization Ps is estimated to be ∼0.65 μC/cm2, andthe remanent polarization (Pr) is ∼0.63 μC/cm2. This Ps figureis fairly consistent with the result simulated using Landautheory (Figure 6b) and slightly larger than that of some other

ferroelectrics, such as Rochelle salt (∼0.2 μC/cm2), guanidi-nium aluminum sulfate hexahydrate (∼0.35 μC/cm2), trichlor-oacetamide (∼0.2 μC/cm2), and ammonium sulfate (∼0.25μC/cm2).33 The related coercive electric field (Ec ∼ 4.8 kV/cm) is also comparable with that of other molecularferroelectrics (Table S4).Ferroelectric materials are a subgroup of pyroelectrics, of

which the polarization is sensitive to temperature andpyroelectric current can be generated under the externalthermal stimuli.34 Temperature-dependent polarization andpyroelectric current of 1 are depicted in Figure 7a. Uponheating, the significant current peak was produced in thevicinity of Tc, of which the direction can be easily reversed byaltering the signs of external electric field (inset in Figure 7a).Temperature-dependent polarizations obtained by integratingpyroelectric current shows that the Ps value for 1 is ∼0.65 μC/cm2 at 135 K, which coincides with the experimental resultmeasured by P−E hysteresis loops. The direction reversal ofpyroelectric current and polarization confirms ferroelectricproperties for 1. Moreover, the SHG effect of 1 is also stronglydependent on its structures; that is, SHG signals emerge at FEPwith the χ(2) value of 0.52 pm/V but fully disappear at PEP andICP (in Figure 7b). The tendency of SHG effects versustemperature is consistent with that of Ps, following the Landautheory equation of χ(2) = 6ε0βPs, where ε0 and β aretemperature-independent constants (the formula derivation isshown in Supporting Information). This result is considered to

be another solid indicator for the ferroelectric phase transitionin 1.35

It is clear that positional and occupational disorder ofdynamic moieties dominate long-range ordered IC modulationsof 1, as well as its ferroelectricity. To understand the origin ofintrinsic polarization, structure analyses for collective alignmentof molecular dipole moments are illustrated in Figure 8. AboveTc, the disordered anions adopt a mirror symmetry, and thusthe dipole moment is canceled out (i.e., μs = 0), correspondingto its PEP. However, such IC structural modulations arestrongly dependent on thermal motions of dynamic parts,which become fully ordered below Tc. All the anionic moieties

Figure 6. Ferroelectric properties of 1. (a) P−E hysteresis loopsmeasured along the b-axis direction at different temperatures ( f = 30Hz). (b) Experimental and theoretical P−E loops simulated by usingt h e e x p r e s s i o n o f L a n d a u t h e o r y , i . e . ,

α β= − + −G T T P P EP( )12 0 c s

2 14 s

4.31

Figure 7. (a) Temperature dependence of Ps obtained by integratingpyroelectric currents in the positive (green) and negative (brown)poling voltage. (b) Temperature-dependent SHG effects for 1. Inset:SHG intensities vs the wavelength at different temperatures.

Figure 8. Diagram for the generation of molecular dipole moments(μs) and Ps in 1. (a) Origin of μs. (b) Generation of Ps along its b-axis,as indicated by the red arrows.

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deviate away from the mirror symmetry at FEP, showing atwisted angle of 5.8° (Figure 8a). Thus, the reorientation ofdipoles gives rise to the components along the b-axis. Accordingto the point electric charge model (Figure S10), the permanentdipole moment and electric polarization for 1 are, respectively,estimated to be 3.65 × 10−29 C·m and ∼1.2 μC/cm2, inaccordance with experimental pyroelectric and P−E hysteresismeasurements. As a result, the interplays between unique ICstructural modulations and bulk ferroelectricity make 1 apromising candidate for the future assembly of artificiallymodulated periodic structure.In summary, we have designed and characterized a new

organic molecular ferroelectric, which shows unique ICstructural modulations between its FEP and PEP. In particular,three distinct IC states coupling with a long-range structuralmodulation are observed, evidenced by an exact (3 + 1)Dsuperspace group. Such a modulation suppresses the long-rangeordered arrangement of electric dipoles at ICP; however, thefreezing of active rotators leads to the reorientation of dipolesas well as its ferroelectric behaviors. The comprehensiveunderstanding of structural modulation and ferroelectricity willbe inspiring to study the underlying concepts of physics andchemistry in aperiodic materials.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/jacs.7b08950.

Crystals, PXRD, and basic physical properties; details forthe refinement of IC structure (PDF)X-ray data for crystal structures (CIF)CheckCIF/PLATON report (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*junliang.sun@pku.edu.cn*jhluo@fjirsm.ac.cnORCIDJunliang Sun: 0000-0003-4074-0962Junhua Luo: 0000-0002-7673-7979Author Contributions§Z. Sun and J. Li contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work is supported by NSFC (21622108, 21525104,21601188, 91422301, 21373220, 51402296, and 51502290),the NSF of Fujian Province (2015J05040), the StrategicPriority Research Program of the Chinese Academy of Sciences(XDB20000000), the Youth Innovation Promotion of CAS(2014262 and 2015240), and State Key Laboratory ofLuminescence and Applications (SKLA-2016-09).

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