† Electronic supplementary informationinformation, CIF les, and additional d2082229–2082232. For ESI and crystallogformat see DOI: 10.1039/d1sc02729h
Cite this: Chem. Sci., 2021, 12, 14198
All publication charges for this articlehave been paid for by the Royal Societyof Chemistry
Received 19th May 2021Accepted 5th October 2021
DOI: 10.1039/d1sc02729h
rsc.li/chemical-science
14198 | Chem. Sci., 2021, 12, 14198–14
n and record-high performance ofenergy storage induced by a phase change inorganic molecular crystals†
Sachio Horiuchi *a and Shoji Ishibashi b
Dielectrics that undergo electric-field-induced phase changes are promising for use as high-power
electrical energy storage materials and transducers. We demonstrate the stepwise on/off switching of
large polarization in a series of dielectrics by flipping their antipolar or canted electric dipoles via proton
transfer and inducing simultaneous geometric changes in their p-conjugation system. Among
antiferroelectric organic molecular crystals, the largest-magnitude polarization jump was obtained as 18
mC cm�2 through revisited measurements of squaric acid (SQA) crystals with improved dielectric
strength. The second-best polarization jump of 15.1 mC cm�2 was achieved with a newly discovered
antiferroelectric, furan-3,4-dicarboxylic acid. The field-induced dielectric phase changes show rich
variations in their mechanisms. The quadruple polarization hysteresis loop observed for a 3-(4-
chlorophenyl)propiolic acid crystal was caused by a two-step phase transition with moderate
polarization jumps. The ferroelectric 2-phenylmalondialdehyde single crystal having canted dipoles
behaved as an amphoteric dielectric, exhibiting a single or double polarization hysteresis loop depending
on the direction of the external field. The magnitude of a series of observed polarizations was
consistently reproduced within the simplest sublattice model by the density functional theory
calculations of dipole moments flipping over a hydrogen-bonded chain or sheet (sublattice) irrespective
of compounds. This finding guarantees a tool that will deepen our understanding of the microscopic
phase-change mechanisms and accelerate the materials design and exploration for improving energy-
storage performance. The excellent energy-storage performance of SQA was demonstrated by both
a high recoverable energy-storage density Wr of 3.3 J cm�3 and a nearly ideal efficiency (90%). Because
of the low crystal density, the corresponding energy density per mass W 0r (1.75 J g�1) exceeded those
derived from the highest Wr values (�8–11 J cm�3) reported for several bulk antiferroelectric ceramics
ðW 0r\1:3 J g�1Þ, without modification to relaxor forms.
Introduction
Highly polarizable dielectrics have been used in diverse elec-tronic, mechatronic, and optoelectronic applications.1,2 Inparticular, phase-change dielectrics accompanied by a largepolarization jump are desired for high-power electrical energystorage, which is increasingly demanded with the expansion ofmodern commercialization.3,4 One of the most suitable dielec-trics is the antiferroelectric,5,6 in which permanent dipoles can
and Photonics (RIAEP), National Institute
logy (AIST), Tsukuba, Ibaraki 305-8565,
of Advanced Functional Materials (CD-
ustrial Science and Technology (AIST),
(ESI) available: Additional structuralata of theoretical calculations. CCDCraphic data in CIF or other electronic
206
be reversibly switched between antiparallel and parallelarrangements by changing the amplitude of an externallyapplied electric eld.7–11 The reversibility between the anti-ferroelectric (AFE) and ferroelectric (FE) phases yields electricpolarization (P) vs. electric eld (E) hysteresis, where the P–Ecurves exhibit double loops instead of the single loop exhibitedby ferroelectrics. The stored energy density Ws during theforward (antiferroelectric-to-ferroelectric phase) switching, therecoverable energy density Wr during the backward(ferroelectric-to-antiferroelectric AFE phase) switching, and theefficiency h can be evaluated through numerical integration ofthe P–E curves according to the following equations:
where Pm and Pr are maximum and remanent polarizations,respectively. While the voltage across conventional capacitorsdecreases linearly as they are discharged, the stronglynonlinear-type capacitors that exhibit the polarization jump canretain their voltage. This characteristic can simplify the elec-tronics required to deliver a constant voltage from a capacitor.In addition, antiferroelectrics can store energy at a high densitymore effectively than linear dielectrics and ferroelectrics.Especially high performance has been achieved with lead-containing antiferroelectrics,12–14 as exemplied by(Pb,La)(Zr,Ti)O3 (PLZT) compounds, which have beencommercially used in dc link condensers. Also, extensiveresearch has led to remarkable improvements in the electricstorage performance of lead-free alternatives.15–18 Ultrahighenergy storage has also been achieved by modifying theseantiferroelectrics into relaxors.19–22
Organic molecular compounds have additional advantageswhen used in energy storage devices, such as mechanical ex-ibility, low density, and environmental benignity, as well asgood dielectric strength. For instance, free-standing poly-crystalline ferroelectric lms have been prepared from smallorganic molecules.23 Over the past decade, we have discoveredantiferroelectric switching or analogous metaelectric transi-tions in several hydrogen-bonded compounds.24–27 Highly effi-cient energy storage in a squaric acid (SQA) crystal, whichcomprises an antiparallel array of polar sheets, has been high-lighted. The molecular dipole moments are reoriented throughproton tautomerism (also known as prototropy), which simul-taneously relocates the double bonds of a p-conjugated systemand a proton of the adjacent hydrogen bond.26 The next chal-lenge is to increase the stored energy density, which requiresincreasing the polarization jump DP and the switching eld Esw.
Here, we develop a series of prototropic organic dielectricshaving antipolar or canted electric dipoles. Excellent polariza-tion performance, with a DP exceeding 15 mC cm�2, is achieved
Fig. 1 Squaric acid (SQA): (a) crystal structure viewed along the b (ctetrazations P1 and P2. (b) Electric polarization (P) vs. electric field (E) hysteresisOrange and blue areas correspond to the recoverable energy densityWr (hysteresis loops and corresponding J–E curves with Ekh100 itetra and Ekh¼ 50 Hz.
by improving the dielectric strength of SQA and by preparingnew antiferroelectrics: deuterated SQA-d2 and furan-3,4-dicarboxylic acid (FDC). The materials development is accom-panied by the discovery of multiple phase changes in anothernew antiferroelectric: 3-(4-chlorophenyl)propiolic acid (CPPLA).The alternative approach to antiferroelectric phase switching isto exploit the crystal anisotropy of a ferroelectric having canteddipoles. For each prototropic antiferroelectric investigated, thepolarization of the polar subunit (i.e., a hydrogen-bonded sheetor chain) is theoretically simulated and its simple ippingmodel is examined to explain the polarization jump. Recordhigh energy-storage performances are also claried incomparison with the corresponding performances of inorganicantiferroelectrics.
Results and discussionSQA and deuterated SQA-d2
The SQA crystal28–30 is a layered antiferroelectric at temperaturesless than 373 K. Its structure belongs to the monoclinic P21/mspace group with pseudotetragonal symmetry. The two-dimensional hydrogen-bonding network constructs dipolarmolecular sheets, the polarities of which alternate along thectetra-direction. Although previous polarization hysteresisexperiments26 showed that the best eld-induced polarizationwas achieved at that time, their maximum eld amplitude wasset at �150 kV cm�1 to avoid electric breakdown of the testsingle crystal. Higher-quality single crystals enabled us toincrease the maximum eld strength (to 220–230 kV cm�1) inthis re-examination of the SQA and to conduct new tests on thedeuterated SQA-d2. In Fig. 1b and c, the entire switching processis shown together with a steep polarization jump in the corre-sponding P–E curve and a sharp peak in the correspondingcurrent density (J)–E curve. With an Ek[100]tetra conguration atroom temperature, the polarization jump DP was optimized to17.2 and 18.4 mC cm�2 for the SQA and SQA-d2 crystals,respectively. These polarizations are greatly improved
) direction. Arrows point to the directions of sheet (sublattice) polari-loops for SQA and its deuterated SQA-d2 crystals at room temperature.orange) and the unrecoverable energy densityWloss (blue area). (c) P–E110itetra configurations for an SQA crystal at room temperature and at f
compared with that previously reported for SQA (10.5 mCcm�2)26 and are the largest polarizations reported for organicmolecular antiferroelectric compounds.
Immediately aer the discovery of antiferroelectricity, Kittelintroduced the Landau-type macroscopic model comprisingtwo interpenetrating sublattices with opposite polarizations.7
For the antiferroelectrics and a dipole-canted ferroelectricexamined herein, the sublattices can also be dened by theperiodic array of the polar subunits (chains or sheets) of iden-tical polarities. The SQA crystal contains two sublattices withpolarizations P1 and P2 (¼ �P1) being parallel to the h110itetradirection. As theoretically simulated elsewhere,31 the externaleld induces a 90� rotation of P1 or P2, causing the polarization
jump DP ¼ ffiffiffi2
p jP1j (16.4 mC cm�2) along the h100itetra direction,rather than a 180� ip of P1 or P2, which would give DP ¼ 2jP1j(23.2 mC cm�2) along the h110itetra direction. Notably, thisinterpretation does not change under this revision because theeasy switching axes are h100itetra and the observed polarization
jump DP is very similar toffiffiffi2
p jP1j, as clearly shown in Fig. 1c.
Furan-3,4-dicarboxylic acid (FDC) with large polarization
While the polar crystal structures of ferroelectrics usually revealsome additional hidden crystal symmetries (pseudosymme-tries), similar key signatures are available for researcherssearching for new antiferroelectric candidates. Here, we havediscovered the new antiferroelectric furan-3,4-dicarboxylic acid(FDC) crystals, which exhibit the second-best polarization jumpamong organic antiferroelectrics. In the Cambridge StructureDatabase, all three available datasets concern the identicalmonoclinic polymorph (denoted as the a-form hereaer) grownfrom aqueous solution. In the rst structural analysis (ref code:
Fig. 2 Furan-3,4-dicarboxylic acid (FDC): (a) chemical structure. (b) Mole(top) and a-directions (bottom). (c) Molecular arrangements in the g-formArrows represent the polarity of the hydrogen-bonded chains. (d) P–E hytemperature and at various frequencies.
14200 | Chem. Sci., 2021, 12, 14198–14206
FURDCB), Williams et al. found that both acidic protons weredisordered by crystal symmetry (space group P21/m).32 Later,Semmingsen et al. redetermined the crystal structure at T¼ 125K and found that the appearance of weak Bragg spots wascaused by the antipolar arrangement of protons with twofoldperiodicity along the c-axis (ref code: FURDCB01).33 Our carefulreassessment conrmed the validity of the latter structure evenat room temperature. The c/2-translation symmetry is hidden inthis antipolar structure. In the presence of pseudosymmetry,the antipolar and polar structures can be interconverted byrearranging the protons with minimalized modulation of thehost lattice. The actual crystal structure belongs to the P21/cspace group (#14). The global crystal symmetry except the twoprotons has a mirror plane normal to the molecular plane, inaddition to the aforementioned c/2-translation. The hydrogen-bonded molecular chains parallel to the b-direction havelongitudinal dipoles whose polarities alternate along the c-direction.
Two additional polymorphs were newly generated by subli-mation under reduced pressure: rhombus plates of b-FDC(major form; see ESI Fig. S1† for detailed molecular packingwith orthorhombic Pbcm symmetry) and rectangular plates of g-FDC (minor form; triclinic). Although both crystal forms aresimilar to a-FDC with respect to their hydrogen-bondedmolecular sequence (Fig. S2†), polarization switching is hardlyexpected in the b-FDC crystal, which lacks the correspondingpseudosymmetry.
The crystal structure of g-FDC exhibits a C-centered mono-clinic lattice but exhibits only inversion symmetry. Instead ofthe unconventional space group C�1, we used the equivalenttriclinic space group P�1 for the structural determination. The a/2-translation symmetry is hidden in this antipolar structure.
cular arrangements in the a-form crystal viewed along the stacking c-crystal viewed along the stacking a- (top) and b-directions (bottom).
steresis loops with Ek[�102] configurations in the g-FDC crystal at room
The large unit cell of the g-FDC crystal accommodates foursublattices P1, P2 (¼ �P1), P3, and P4 (¼ �P3). Therefore, ip-ping P1 and P3 (or P2 and P4) gives rise to DP ¼ j2P1 + 2P3j.
We demonstrated the expected phase-change phenomenadescribed in the preceding paragraph for the g-FDC crystal(Fig. 2d). The results of P–E hysteresis measurements with theapplied ac electric eld conguration Ek[�102] show the doubleloop characteristic of the antiferroelectric–ferroelectric transi-tion. The observed polarization jump DP, which was as large as15.1 mC cm�2, is slightly smaller than those of SQA but thesecond-highest among those of organic antiferroelectric crys-tals. The switching at �70 kV cm�1 is accompanied by a largehysteresis width of �50 kV cm�1.
Although the a-form crystal is predicted to show a similarlylarge DP, its small size precluded satisfactory measurements.For the b-FDC crystal, no signatures of phase-changephenomena were detected at least up to 180 kV cm�1 in themeasurements at room temperature and at 80 �C, as suggestedby the aforementioned structural assessments.
Antiferroelectric with multiple switching processes
We discovered the new antiferroelectric CPPLA crystal whileseeking for additional hidden crystal symmetries (pseudo-symmetries) in the reported crystal structures. Das et al. re-ported a polar monoclinic crystal structure for the iodine-substituted derivative 3-(4-iodophenyl)propiolic acid (ref code:BEFSUB),34 in which we noticed pseudo-inversion symmetry,suggesting a possible ferroelectric. They also reportedparaelectric-like crystal structures of CPPLA, in which thehydrogen-bonded molecular sequence pinpoints the disordered(or centered) hydrogen atom on each hydrogen bond because ofinversion symmetry (ref code: SUHSET).34,35 In contrast, our
Fig. 3 3-(4-Chlorophenyl)propiolic acid (CPPLA): (a) chemical structurroom temperature (top) and the corresponding J–E curve obtained frswitching (bottom). (c) A hydrogen-bonded polar molecular ribbon (top)and arrowheads represent the polarity of ribbons. The dotted cell corres
reexamination of the single-crystal structure by X-ray diffractionrevealed additional weak Bragg spots indicative of the doubledperiodicity as (a, b, c) ¼ (�ap � cp, �ap + cp, bp). The unit-celldoubling is caused by the antipolar ordering of asymmetri-cally located hydrogen atoms in the revised crystal structure,which contains two sublattices with polarizations P1 and P2 (¼�P1) and thus suggests possible antiferroelectricity along the b-direction.
Consistent with this expectation, the P–E curve (top panel ofFig. 3b) shows a double hysteresis loop when the maximumeld amplitude of 70 kV cm�1 is applied in the Ek[110] cong-uration (instead of the ideal Ekb conguration because of thecrystal shape). However, the application of a higher eldamplitude induced an additional polarization jump, causinga quadruple polarization hysteresis loop. This behavior is themanifestation of eld-induced successive phase transitionswith the antiparallel dipoles ipped half-by-half. Therefore, theintermediate phase is regarded as the ferrielectric state.
Amphoteric behavior of a dipole-canted ferroelectric
While most ferroelectric crystals have fully aligned dipoles, theferroelectric 2-phenylmalondialdehyde (PhMDA) single crystalexhibits a canted arrangement of dipolar chains.36 Here, wereport amphoteric behavior, where a single or double polari-zation hysteresis loop is obtained depending on the direction ofthe external eld (Fig. 4a). In the orthorhombic crystal withspace group Pna21, the PhMDA molecules form polar hydrogen-bonded chains (Fig. 4b). As indicated by small open arrows inFig. 4c, each chain has a sublattice polarization P1 along the[102] direction or P2 along the [�102] direction. Here, the crystalsymmetry demands P1x ¼ –P2x, P1y ¼ P2y ¼ 0, P1z ¼ P2z; the netpolarization 2jP1zj then emerges in the c-direction. Specically,
e. (b) P–E hysteresis loops obtained with different maximum fields atom the quadruple hysteresis loops, indicating multiple field-inducedand the crystal structure viewed along the b-direction (bottom). Arrowsponds to the pseudosymmetry of a/2.
Fig. 4 2-Phenylmalondialdehyde (PhMDA): (a) P–E hysteresis loops at room temperature, as measured with a triangular ac electric field. Theapplied electric field configurations of Eka and Ekc are normal and parallel to the spontaneous polarization, respectively. (Inset) The chemicalstructure of PhMDA. (b) Hydrogen-bonded polar molecular sequences viewed along the c-direction. (c) Schematics of changes in sublatticepolarizations (small arrows) of the hydrogen-bonded chains and the total bulk polarization (large arrows) during the (i) ferroelectric polarizationreversal and (ii) field-induced phase change.
Chemical Science Edge Article
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the conguration of chain dipoles is antiparallel in the a-direction components and parallel in the c-direction compo-nents. The ferroelectricity observed with the Ekc congurationcorresponds to the polarization reversal induced by ippingboth P1 and P2 according to process (i) in Fig. 4c.
In the Eka conguration, the antiferroelectric-like switchingwith a DP of 5.8 mC cm�2 appears at �110 kV cm�1, which issubstantially greater than the magnitude of the coercive eldalong the c-direction (�20 kV cm�1). The most plausiblemechanism is process (ii) in Fig. 4c. The increasing/decreasingelectric eld ips either P1 or P2, which causes the observationof DPx ¼ 2jP1xj through a 90� rotation of the spontaneouspolarization from the the c- to the a-direction and vice versa. Inthis example, the antiferroelectric-like functionalities can beachieved even by rotating polarizations through the eld-induced transition between two different ferroelectric phases.
Fig. 5 Comparison of field-induced polarizations of prototropicantiferroelectrics and an amphoteric dielectric, PhMDA. Experimen-tally observed polarization jump jDPexpj at room temperature versuscalculated total polarization jDPcalj assuming a fully aligned sublatticepolarization.
Theoretical evaluation of polarization
The experimentally optimized spontaneous polarizations oforganic ferroelectrics have recently been reproduced withexcellent accuracy using density functional theory (DFT) calcu-lations and the Berry phase formalism of electric polarization.37
Although the corresponding theoretical evaluations are rare fororganic antiferroelectrics, SQA represents an example in whichthe microscopic switching process has been successfully iden-tied in terms of a sublattice polarization model throughcomparisons with experimental data.31 In the present work, thesublattice polarizations are simulated for other prototropicantiferroelectrics (FDC, CPPLA, and benzimidazoles). Except forthe g-FDC crystal, the two sublattices, which interpenetrateeach other, construct the antipolar or dipole-canted structure.First, one of them is extracted as a periodic polar crystal latticeand its sublattice polarization P1 is computed. Together with itssymmetry-related sublattice polarization P2, the theoreticalpolarization DPcal is calculated by ipping either P1 or P2. Forthe g-FDC crystal, two of four sublattices were extracted forcalculating the crystallographically independent P1 and P4. Thesublattice polarizations were calculated for different degrees ofpolar distortion l between the reference symmetrized
14202 | Chem. Sci., 2021, 12, 14198–14206
(hypothetical paraelectric, l ¼ 0) and fully polar (ferroelectric, l¼ 1) congurations. As shown in Fig. S3,† the smooth l-dependence conrms the validity of each simulation.
In the g-FDC crystal, each hydrogen-bonded chain is highlypolarized and the resultant large jDPcalj of 13.2 mC cm�2
directed along the [�102] is similar to the experimentallyobserved polarization jump jDPexpj (15.1 mC cm�2). Note thatnearly the same jDPcalj values were computed for both the a-and b-forms, which have similar hydrogen-bonded molecularsequences (Fig. S2†). Likewise, excellent agreement betweenjDPcalj and jDPexpj is conrmed for a series of prototropic anti-ferroelectrics, as demonstrated in Fig. 5. Regarding the CPPLAcrystal, the entire polarization jump of the quadruple hysteresisloops (4.3 mC cm�2) is explained well by the full alignment ofchain polarizations (the Ek[110]-direction component of DPcal is4.8 mC cm�2).
DFT calculations were also carried out for the dipole-cantedferroelectric PhMDA to evaluate its amphoteric behavior quanti-tatively. The calculated sublattice P1 is (3.0, 0.0, 4.5) mC cm�2,yielding its counterpart P2 ¼ (�3.0, 0.0, 4.5) mC cm�2 bysymmetry. The resultant spontaneous polarization Ps ¼ P1 + P2 ¼(0, 0, 2jP1zj) is (0, 0, 9.0) mC cm�2 and coincides with the
d The data from ref. 26 for SQA. e The data in ref. 24 for three benzimidazoles was used in the analysis.
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experimental Ps (9 mC cm�2) emerging in the c-direction. With theEka conguration, the predicted polarization jumpDPx¼ 2jP1xj ¼6.1 mC cm�2 agrees well with the experimental DP of 5.8 mC cm�2.
Energy-storage performance
The P–E curves of FDC, CPPLA, and PhMDA crystals were alsomeasured at higher temperatures (Fig. S4†). The FDC crystal
Fig. 6 Energy-storage performance of organic molecular antiferroelecperformance for high-performing inorganic AFE or relaxor ceramics. (aenergy storage efficiency versus the recoverable energy density per urepresent organic antiferroelectrics (1, SQA; 2, g-FDC; 3: DFMBI; 4,La0.02)(Zr0.55Sn0.45)0.995O3;12 b, (Pb0.98La0.02)(Zr0.49Sn0.51)0.995O3;13 c, PbHNbO3–0.10Bi(Mg2/3Nb1/3)O3;17 g, (Ag0.91Bi0.03)NbO3),18 and inorganic0.55(Bi0.5Na0.5)TiO3–0.45(Sr0.5Bi0.5)TiO3;20 j, 0.78(Bi0.5Na0.5)TiO3–0.22Na
revealed very weak temperature dependence up to 420 K. ForCPPLA and PhMDA as well as for SQA,26 both the DP andswitching eld decrease with heating, and thus the storedenergy densities are diminished from the room-temperatureperformance. Regarding the energy-storage performance of allof the prototropic antiferroelectrics, Table 1 summarizes theroom-temperature performance obtained according to eqn (1)–(3). Fig. 6 shows a plot of these data in comparison with those of
tric (AFE) crystals at room temperature in comparison with reported) The field-induced maximum polarization vs. switching field. (b) Thenit volume (top) or weight (bottom). Bold alphanumeric charactersTFMBI; 5, TCMBI; 6, CPPLA), inorganic antiferroelectrics (a, (Pb0.98-fO3;14 d, Ag0.94La0.02NbO3;15 e, Ag(Nb0.85Ta0.15)O3;16 f, 0.90(K0.5Na0.5)antiferroelectric relaxors (h, 0.76NaNbO3–0.24(Bi0.5Na0.5)TiO3;19 i,NbO3;21 k, Ag(Nb0.45Ta0.55)O3).22
bulk antiferroelectric ceramics and their relaxor modications,which exhibit high or ultrahigh energy-storage densities.Among the prototropic antiferroelectrics, SQA (and SQA-d2)crystals exhibit both the highest recoverable energy-storagedensity (Wr ¼ 3.3 J cm�3) and nearly ideal efficiency (h ¼0.90). As shown in Fig. 6a, the excellentWr of SQA is justied byits best performance with respect to both the maximum polar-ization Pm and the switching eld Esw (the average of forwardand backward phase switching elds). However, theWr, Pm, andEsw values are smaller than those of the bulk antiferroelectricceramics and their relaxor modications,12–22,38–44 as evident inFig. 6a and b. The magnitude relation of the performance isdrastically different in Fig. 6c, which is a replot of the dataagainst the stored energy density per weight ðW 0
rÞ instead ofthat per unit volume (Wr). This difference is attributed to thecrystal density r of the organic antiferroelectrics (1.3–1.9 g cm�3) being substantially lower than those45 of lead-containing (8.5–10.3 g cm�3) and lead-free antiferroelectrics(4.6–8.1 g cm�3). The W 0
r (1.75 J g�1) of SQA (r ¼ 1.88 g cm�3)exceeds those derived from the highest Wr values (approxi-mately 8–11 J cm�3) reported for several bulk antiferroelectricceramics ðW 0
r\1:3 J g�1Þ. Notably, the SQA crystal retains thehighest h without resorting to its modication to relaxor forms.These ndings are encouraging for the future applications ofthese materials in portable and/or mobile devices.
Conclusions
Various dielectric phase-change phenomena have beendemonstrated via studies on revisited SQA, newly developedantiferroelectrics, and the “amphoteric dielectric” PhMDA. Interms of the magnitude of the eld-induced polarization jump,the best (�18 mC cm�2) and second-best performances (�15 mCcm�2) among organic antiferroelectrics were achieved. Inparticular, the improvement of the dielectric strength of SQAresulted in excellent energy-storage performance, includinga high recoverable energy-storage density (Wr ¼ 3.3 J cm�3),while maintaining nearly ideal efficiency (h ¼ 90%). Theadvantage of organic molecular systems is their low crystaldensity, which resulted in corresponding energy densities permass as high as W 0
r ¼ 1:75 J g�1, which exceeds those derivedfrom the highest Wr values (�8–11 J cm�3) reported for severalbulk antiferroelectric ceramics ðW 0
r\1:3 J g�1Þ. Notably, thepresent SQA crystal has not yet been modied into relaxorforms.
In addition to the antiferroelectrics exhibiting conventionaldouble hysteresis loops, the CPPLA crystal was found to exhibitquadruple polarization hysteresis loops caused by the two-stepphase transition with moderate polarization jumps. The ferro-electric PhMDA single crystal represents a novel “amphotericdielectric” that can exhibit a single or double polarizationhysteresis loop depending on the direction of the appliedexternal eld relative to the directions of its canted dipolemoments. Irrespective of the details of their variation, all of theobserved polarizations are consistently explained by the DFTcalculations combined with the simplest sublattice model. Sucha theoretically precise prediction ability provides a powerful tool
14204 | Chem. Sci., 2021, 12, 14198–14206
for improving the energy-storage performance; it will acceleratethe materials design and exploration as well as deepening ourunderstanding of the microscopic phase-change mechanisms.
Experimental sectionPreparation and electric measurements
Commercially available SQA, FDC, PhMDA, and CPPLA werepuried by repeated recrystallizations and/or temperature-gradient vacuum sublimation. The temperature-gradientsublimation under reduced pressure gave single crystals withshapes of elongated rectangular plates in the case of FDC,parallelogram-shaped plates in the case of CPPLA, and thickplates in the case of PhMDA. The bipyramidal crystals of SQAand SQA-d2, which were grown by recrystallization from hotdeionized water and 99.5% D2O, respectively, were cut witha blade for electric measurements according to previously re-ported procedures.26 All of the electric measurements wereconducted using single crystals with painted silver electrodes.The P–E hysteresis measurements were performed by applyinga high-voltage triangular wave eld and various alternatingfrequencies to single crystals, which were immersed in siliconeoil to prevent atmospheric discharge. The system used to eval-uate ferroelectrics (Toyo Corporation, FCE-1) compriseda current/charge–voltage converter (model 6252), an arbitrarywaveform generator (Biomation 2414B), an analog-to-digitalconverter (WaveBook 516), and a voltage amplier (NF Corpo-ration, HVA4321).
Crystallographic studies
The crystallographic data and experimental details aresummarized in Table S1.† X-ray diffraction data were collectedfrom single crystals at room temperature using graphite-monochromated Mo Ka radiation (l ¼ 0.7107 A) and a four-circle diffractometer equipped with a two-dimensionaldetector [hybrid pixel detector (Rigaku AFC10 with PILA-TUS200K)]. CrystalStructure crystallographic soware packages[Molecular Structure Corp. (MSC; Woodlands, TX) and RigakuCorp. (Tokyo)] were used for the direct method and for therenement of the structures. Final renements of the non-hydrogen atoms were performed with anisotropic thermalfactors. The hydrogen-bonded hydrogen atoms were found bydifferential Fourier synthesis and were rened isotropically; theremaining hydrogen atoms were calculated in their idealgeometrical positions.
Theoretical calculations
First, for the experimentally obtained AFE structures, hydrogenpositions were computationally optimized to minimize the totalenergy. Next, for each system, all of the atoms except for onepolar subunit were removed from the unit cell (l¼ 1). Referencenonpolar structures (l ¼ 0) were constructed by symmetriza-tion. The polarization as a function of l was calculated using theBerry phase approach.46,47 All the calculations were performedusing the QMAS code48 based on the projector augmented-wavemethod49 and the plane-wave basis set. To describe the
electronic exchange–correlation energy, the Perdew–Burke–Ernzerhof (PBE) version of the generalized gradient approxi-mation (GGA)50 was used.
Author contributions
S. H. prepared the puried single crystals, performed thedielectric measurements, conceived the study design, and wrotemost of the paper. S. I performed the theoretical calculations.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was partially supported by JSPS KAKENHI GrantNumber 21H04679 and JST CREST Grant Number JPMJCR18J2,Japan. Part of the computation in this work was done using thefacilities of the Supercomputer Center, the Institute for SolidState Physics, the University of Tokyo.
Notes and references
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2 K. Uchino and J. R. Giniewicz, Micromechatronics, MarcelDekker, New York, 2003.
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