organic cocrystals: the development of ferroelectric ... · organic cocrystals: the development of...

materscichinacom linkspringercom Published online 14 July 2016 | doi 101007s40843-016-5049-ySci China Mater 2016 59(7) 523ndash530

Organic cocrystals the development of ferroelectricpropertiesGeetha Bolla1 Huanli Dong1 Yonggang Zhen1 Zhaohui Wang1 and Wenping Hu12

ABSTRACT Organic cocrystals are crystalline single-phasematerials composed of two or more molecular andor ioniccompounds generally in a stoichiometric ratio A feature oforganic cocrystals is that special optoelectronic propertiessuch as ferroelectricity are easy to realize in these materialsIn this perspective we systematically introduce the recentresearch advances in organic cocrystal ferroelectrics and westudy in depth the molecular structure and self-assemblingbehaviors of cocrystals for ferroelectric applications Finallycombined with an understanding of recent progress andachievements in this field we discuss the challenges andopportunities for ferroelectric materials based on organiccocrystals as well as the promising applications of thesematerials

Keywords organic cocrystal hydrogen bonding charge trans-fer ferroelectric

An organic cocrystal is a stoichiometric multi-componentmolecular crystal wherein the different components areassembled by heteromolecular interactions (Fig 1a) suchas hydrogen bonds halogen bonds charge transfer (CT)and πndashπ interactions [1] In cocrystalline systems thesupramolecular synthon [2] a fundamental concept ofcrystal engineering plays an important role Synthons arereliable patterns of intermolecular interactions that canbe used to generate supramolecular functional materialsThe first cocrystal was reported by Woumlhler [3] in 1844during his studies on quinone He mixed solutions ofquinone (colorless) and hydroquinone (yellow) and foundthat a crystalline substance formed Woumlhler called thiscrystalline material green hydroquinone Following thisfirst publication Ling and Baker reported several relatedcocrystals composed of halogenated quinones and greenhydroquinone also referred to as quinhydrone [4] In

1958 the first crystal structure of a monoclinic quin-hydrone crystal was reported [5] The crystal structureshowed that the quinone and hydroquinone moleculesformed alternating zigzag chains connected by OndashHOhydrogen bonds Another crystal engineering strategyrelies on the use of electronic donor (D) and acceptor(A) building blocks This process has gained consider-able interest because large molecules can be utilized andthe resultant DA cocrystals are stable Moreover thesetypes of cocrystal have the potential to be used for thefabrication of promising next generation materials withapplications in innovative electronic and photonic devices[6] CT interactions mainly refer to weak non-covalentattractive forces that arise between electron-rich donorsand electron-deficient acceptors forming so-called D-Acomplexes However these CT cocrystals have been inves-tigated to a lesser degree than cocrystals held together byhydrogen bonds

The properties of cocrystals are not thoroughly inves-tigated in terms of their technological applications espe-cially concerning optical nonlinearity (semi)conductivityferroelectricity andmagnetism [7] In the last decade mul-tifunctional organic materials have emerged as high-de-mand materials because they are versatile and have appli-cations in the area of molecular optoelectronics How-ever there has been limited success in developing optoelec-tronically active materials using traditional single-compo-nent materials However the formation of supramolecu-lar assemblies of materials into cocrystals through non-co-valent interactions allows material scientists to tailor theproperties of the final material by careful choice of mul-tifunctional component molecules yielding materials withunique physicochemical properties [8] Now having dis-

1 Key laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences Beijing 100190 China2 Tianjin Key Laboratory of Molecular Optoelectronic Sciences Department of Chemistry School of Science Tianjin University amp CollaborativeInnovation Center of Chemical Science and Engineering (Tianjin) Tianjin 300072 China

Corresponding authors (emails dhl522iccasaccn (Dong H) zhenygiccasaccn (Zhen Y))

523 copy Science China Press and Springer-Verlag Berlin Heidelberg 2016

SCIENCE CHINA Materials PERSPECTIVE

Figure 1 (a) General schematic representation of cocrystals (b) Typesof ferroelectric systems and the interactions in these crystal structures

cussed the background to cocrystallinematerials wewouldlike to explore their applications in particular wewill focuson organic ferroelectrics and studies into the correlationbetween their two dimensional (2D) surface arrangementsand properties [9]

Ferroelectrics are polar (pyroelectric) crystals that showspontaneous electric polarization They have electroac-tive properties allowing the storage and switching ofpolarity (ferroelectricity) sensing of temperature changes(pyroelectricity) interchange of electric and mechanicalfunctions (piezoelectricity) and manipulation of light(through optical nonlinearities and the electro-opticaleffect) [10] Ferroelectric materials are mostly inorganicor organic-inorganic hybrid compounds for examplebarium titanate (BaTiO3) (BTO) sodium nitrite (NaNO2)potassium dihydrogen phosphate (KH2PO4 KDP) andtriglycine sulfate (TGS) [11] Historically ferroelectricscan be traced back to 1655 when Rochelle salt was firstisolated by Elie Seignette but pyroelectricity has been rec-ognized since ancient times However the piezoelectricityof Rochelle salt was established in 1880 by Curiersquos brotherand it took a further 40 years in 1920 until the term

lsquoferroelectricrsquo was coined by Joseph Valasek by analogywith ferromagnetism For some time Rochelle salt was theonly known ferroelectric material however in 1935 Buschand Scherer found KDP to be ferroelectric A furtherbreakthrough came in the 1940s during the Second WorldWar when BTO which has a simple perovskite structurewas prepared followed by von Hippelrsquos demonstrationof its ferroelectricity in 1945 [12] Following this periodof discovery piezoelectric transducers were developedand many new ferroelectric perovskite oxides were syn-thesized Among these new materials the most notableis lead zirconate titanate (PZT) Today PZT remains themost widely used ferroelectric material [13] In summaryinorganic ferroelectrics have been well explored and fororganic ferroelectrics the specifics are well establishedCurrently for organic ferroelectrics attention is given tothe use of multicomponent materials that exhibit ferro-electricity and polarization at temperatures at and aboveroom temperature

Conventional organic ferroelectrics [14] are of the or-der-disorder type (Figs 1b and 2) as will be discussed laterThe most interesting feature of ferroelectric materials istheir spontaneous polarization which can be reversed byinverting the external electric field During polarization re-versal in a coercive field the electric displacement (D) as afunction of the field (E) exhibits a hysteresis (a D-E loop)and ferroelectric materials undergo a paraelectric-to-fer-roelectric phase transition Furthermore dielectric sus-ceptibility usually obeys the Curie-Weiss law at high tem-peratures ie temperatures above the Curie temperature(Tc) Normally ferroelectric phase transitions are catego-rized using the following scheme 1 Order-disorder inthis scheme the ordering of the asymmetric molecules orions carrying permanent dipoles generates a spontaneouselectric polarization Sodium nitrite is a well-known exam-ple in which the bent NO2

2minus ion bears the electric dipole(Fig 2a) 2 Displacive type in the displacive mechanismthe relative displacements between the different chargesof the ions yield macroscopic polarization in the crystalan example being perovskite ferroelectrics such as BTO(Fig 2) The ferroelectric phenomenon can be utilizedfor a diverse range of technological applications such asnon-volatile memory elements (ferroelectric random ac-cess memory (FeRAM)) or ferroelectric field-effect tran-sistors (FeFET) and high-capacity condensers and capac-itors Furthermore ferroelectric crystals exhibit unusu-ally large electromechanical couplings ie the couplingbetween mechanical strain and electric polarization Thelarge electrostriction and piezoelectric effect can be utilized

524copy Science China Press and Springer-Verlag Berlin Heidelberg 2016

PERSPECTIVE SCIENCE CHINA Materials

Figure 2 (a) Conventional classifications of ferroelectric materials in inorganic chemistry The origin of the dipole moment p is shown by openarrows (b) Classical examples of organic ferroelectrics CT complexes with their classification and 2D packing Reprinted with permission from Ref[9] Copyright 2008 Nature Publishing Group

in actuators transducers ultrasonic motors piezoelectricelements and second-order optical nonlinearity for secondharmonic generation (SHG) activity along with the linearelectro-optic effect Moreover there are many challengesand these are broadly interdisciplinary for example the de-velopment of frequency multipliers and light modulatorsand photonic devices which are currently of great inter-est and also could be fabricated using crystal engineeringmethods

Ferroelectric organic cocrystalsRecently the CT strategy for the formation of cocrystalshas been applied to form a complex of tetrathiafulvalene(TTF) and p-chloranil (QCl4) resulting in π-stackedcolumns of alternating D and A (lsquolsquomixed stackrsquorsquo) moleculesin one dimension This one-dimensional stacking is vitalin the formation of ferroelectric properties (Figs 2b and 3)

[15] The most important organic molecules and binarysystems along with their properties are summarized inTable 1 Occasionally strong electron-lattice interactions(Peierls instability) trigger molecular displacements Thesedisplacements lead to D-A dimerization forming electricdipoles along the molecular stack In addition the flatD and A molecules are flexible enough to bend and thespontaneous bending deformation breaks the symmetrygenerating highly polar D-A chains The interactions andthe arrangement of the molecules in the crystal lattice leadto either a ferroelectric or antiferroelectric state dependingon the three dimensional (3D) arrangement of the polarchains The ferroelectric and antiferroelectric orderingstyles are both shown by TTF-QCl4 on phase transitionand the dielectric properties have been closely controlledin various ways depending on the chemical and physicalmodifications of the crystals However these CT com-



Figure 3 (a) Structural comparisons between an antiferroelectric squaric acid (top) and ferroelectric croconic acid (bottom) Reprinted with permis-sion from Ref [20] Copyright 2010 Nature Publishing Group (b) Chemical structures of generally used anilic acids and base molecules of phenazinebipyridines reported in literatures for ferroelectric organic adducts

Table 1 Properties of purely organic ferroelectrics compared with those of selected typical ferroelectrics of small single compounds and organic com-plexes compounds (modified from Ref [9])

Transition temperature (K) Dielectric constant

Tc TcD κRT κmax

Ps (μC cmminus2) temperature

Single-component organic molecules

Thiourea 169 185 30 104 32 120 K

TEMPO 287 288 10 16 05

CDA 397 25

TCAA 355 45 65 02 RT

Benzil 78 84 88 27 36 times 10minus3 70 K

DNP 46 40 22

TCHM 24 104 96 100 024 10 K

CT complexes

TTF-CA 81 84 40 500

TTF-BA 50 20

H-bonded supramolecules

Phz-H2ca 253 304 110 3 times 10 18 160 K

Phz-H2ba 138 204 30 17 times 103 08 105 K

CDA = cyclohexan-11prime-diacetic acid DNP = 16-bis (24-dinitrophenoxy)-24-hexadiyne TCAA = trichloroacetamide TCHM = tricyclohexyl-methanol TEMPO = 2266-tetramethyl-1-piperidinyloxy(tanane) Tc

D = transition point for deuterated compound κRT dielectric constant at roomtemperature κmax maximum dielectric constant at Tc Ps = spontaneous polarization

plexes exhibit a relatively narrow charge gap and hence arebad insulators In addition large dielectric losses degradethe spontaneous polarization preventing various ferroelec-tric applications near or above room temperature

Hydrogen-bonded organic compounds are another typeof cocrystalline material Concerning ferroelectricitythese materials can behave in a similar way to KDP Ferro-

electricity and the related properties of hydrogen-bondedsystems have been studied in many inorganic molecularsalt systems [16ndash17] andKDP is a well-known ferroelectricmaterial PO4

3minus ions are connected in a 3D interpenetrat-ing network of homonuclear OndashHO hydrogen bondswith short OO separations (250 Aring) A phase transitionis triggered by the collective ordering of protons and the



magnitude of spontaneous polarization of the system arisesmainly from the displacive motion of highly polarizablePO4

3minus ions Furthermore these protons can be replaced bydeuterons leading to a dramatic increase in crystallizingtemperature (Tc) Therefore both hydrogen bonding andproton dynamics are very important in ferroelectric mate-rials There are several prototypical materials for this typeof behaviors for example squaric acid The square planarC4O4H2 molecules of squaric acid form strong hydrogenbonds with neighboring squaric acid molecules againthese have a short OO distances (255 Aring) forming a 2Dnetwork via intermolecular OndashHO hydrogen bonds Inthis crystal a phase transition at T = 373 K is a collectiveorder-disorder of the protons and this crystal like KDPhas a significant HD isotope effect This constraint theso-called lsquolsquoice rulersquorsquo characterizes the correlated motionof protons True organic ferroelectric analogs of KDP aredesirable in particular multi-component crystal-engi-neered systems Displacive-type ferroelectricity in neutralcocrystals has been well discussed as has proton trans-fer in acid-base multi-component molecules mediatedby intermolecular hydrogen bonds Indeed initially asupramolecular system is a system of hydrogen-bondednonpolar molecules whereas subsequent proton transferyields an ionic supramolecular system These types offerroelectric compounds mainly include anilic acids as theproton donors D and pyridine bases as the acceptors AFor example anilic acid and bipyridine form ionic DAadducts on proton transfer of an anilic acid proton tobipyridine Furthermore these cocrystals have a variety ofsupramolecular arrangements as shown in Fig 3b

Subsequently the drawbacks of this pioneering workwere resolved by replacing CT interactions with hydrogenbonding cocrystals ie the lock arm supramolecular or-dering (LASO) method which has succeeded in producingroom temperature ferroelectric materials (Fig 4) a molec-ular design that allows donor and acceptor molecules toself-assemble into charge-transfer ferroelectric networksat ambient temperature [18ndash19] New structures arisingfrom this technique challenge the long-standing notionthat donorndashacceptor mixed-stack materials cannot exhibita ferroelectric Tc greater than room temperature Thedemonstration of ferroelectric properties in an organicnetwork affords us opportunities to produce these systemsin new forms such as electrically addressable hydrogelsferroelectric catalysts and charge-transfer-based sensitiz-ers for photovoltaics The combination of donorndashacceptorinteractions with hydrogen-bonded networks above roomtemperature [20] offers a promising supramolecular plat-form to design novel organic electronic structures

2D organic cocrystals with potential ferroelectricsRecently 2D H-bonded organic cocrystals of croconic acid(CA) and 3-hydroxyphenalenone (3-HPLN) have beenprepared by self-assembly on an Au(111) substrate asshown by scanning tunneling microscopy images (STM)in Fig 5 [21] and these 2D materials show ferroelectricat room temperature The CA3-HPLN system formscocrystals with a variety of stoichiometries One of theobserved structural building blocks consists of two CA andtwo 3-HPLN molecules Studies have shown that CA and3-HPLN can be cocrystallized as 2D crystals in a solvent-

Figure 4 Crystal structures of LASO complexes and structural formula of the electron donor and acceptor molecules used Reprinted with permissionfrom Ref [14] Copyright 2012 Nature Publishing Group



Figure 5 STM images of the self-assembled 2D molecular structures on an Au (111) substrate (a) CA (b) CA3-HPLN = 21 where (CA)2(3-HPLN)2tetramers are shaded red and (CA)2 dimers are shaded green (c d) two network architectures of CA3-HPLN = 11 that were found to coexist (packingpolymorphism) (CA)2(3-HPLN)2 (e) CA3-HPLN at approximately 15 and (f) 3-HPLN Reprinted with permission from Ref [20] Copyright 2010Nature Publishing Group

free approach when confined by a substrate The impor-tance of this study lies not only in the formation of a cocrys-talline material but the solvent-free approach and surface-supported cocrystallization Thus this method could be-come a quick and versatile test to identify miscible cocrys-tal constituents and inform solution-based cocrystalliza-tion furthering the discovery of new materials and en-abling the rational design of organic ferroelectric cocrys-tals We hope that this work will encourage the search forcocrystalline organic H-bonded systems particularly thosethat are prone to network formation which can preventcrystallization in centrosymmetric space groups thus al-lowing ferroelectric phenomena in 2D materials to be gen-erated

Applications for future studies of organic ferroelectricsFerroelectric materials have important technological appli-cations because of their superior piezoelectric propertiesIn the last a few years a number of newmolecular ferroelec-tric materials have been developed and these are soft flex-ible lightweight environmentally friendly and biocom-patible in addition they can be easily processed at rela-tively low temperatures and printed onto soft substratesThese materials are promising and have a broad range of

applications including sensing actuation energy harvest-ing data storage and memory devices (such as FeRAMand FeFETs) Another distinct advantage of molecular fer-roelectrics is that they can be directly printed as demon-strated in an all-printed ferroelectric active-matrix sensornetwork and in energy harvesting devices that convert me-chanical vibrations into electricity With this backgroundwe expect that newly developed molecular ferroelectricsbased on crystal engineering design strategies using cocrys-tals design will enter the field soon thus enabling manymore applications As mentioned above organic cocrys-tals are a popular research topic in material science in theform of CT complexes In addition photonic and opto-electronic compounds are becoming increasingly popularTherefore the synthesis of a wide variety of organic cocrys-tals is highly valuable and applications of the aspects ofcrystal engineering to organic cocrystals will allow the de-sign of highly desirable material properties The applica-tion of molecular cocrystals and understanding their crys-tal structures and interactions will significantly advance thepreparation of improved functional materials The designof the hydrogen-bonded supramolecular materials that areferroelectric at and above room temperature have excellentcharge transport properties have diverse applications and



show photonic behavior will lead to further developmentsin molecular electronics in the next couple of decades

Received 11 April 2016 accepted 6 June 2016published online 14 July 2016

1 Aitipamula S Banerjee R Bansal AK et al Polymorphs salts andcocrystals whatrsquos in a name Crystal Growth Design 2012 122147ndash2152

2 Desiraju GR Supramolecular synthons in crystal engineering-anew organic synthesis Angew Chem Int Ed 1995 34 2311ndash2327

3 Woumlhler F ldquoUntersuchungen Uumlber des Chinonsrdquo Annalen ChemPharm 1844 51 145ndash163

4 Ling AR Baker JL XCVI halogen derivatives of quinone PartIII Derivatives of quinhydrone J Chem Soc Trans 1893 631314ndash1327

5 Matsuda H Osaki K Nitta I Crystal structure of quinhydroneC12H10O4 Bull Chem Soc Jpn 1958 31 611ndash620

6 Zhu W Zheng R Zhen Y et al Rational design of charge-transferinteractions in halogen-bonded co-crystals toward versatile solid-state optoelectronics J Am Chem Soc 2015 137 11038ndash11046

7 Li J Liu Y Zhang Y et al Molecular ferroelectrics where electron-ics meet biology Phys Chem Chem Phys 2013 15 20786

8 Zhu W Zheng R Fu X et al Revealing the charge-transfer interac-tions in self-assembled organic cocrystals two-dimensional pho-tonic applications Angew Chem Int Ed 2015 54 6785ndash6789

9 Horiuchi S Tokura Y Organic ferroelectrics Nat Mater 2008 7357ndash366

10 Lines ME Glass AM Principles and Applications of Ferroelectricsand Related Materials New York Oxford University Press 1977

11 K Uchino Ferroelectric Devices New York CRC Press 200012 Von hippel A Breckenridge RG Chesley FG et al High dielectric

constant ceramics Ind Eng Chem 1946 38 1097ndash110913 Shirane G Takeda A Phase transitions in solid solutions of

PbZrO3and PbTiO3(I) small concentrations of PbTiO3 J Phys SocJpn 1952 7 5ndash11

14 Tayi AS Shveyd AK Sue ACH et al Room-temperature ferroelec-tricity in supramolecular networks of charge-transfer complexesNature 2012 488 485ndash489

15 Torrance JB Girlando A Mayerle JJ et al Anomalous nature ofneutral-to-ionic phase transition in tetrathiafulvalene-chloranilPhys Rev Lett 1981 47 1747ndash1750

16 Katrusiak A Szafrański M Ferroelectricity in NH N hydrogenbonded crystals Phys Rev Lett 1999 82 576ndash579

17 Mochida T Izuoka A Sugawara T et al Organic hydrogen-bondeddielectrics quantum paraelectricity based on tautomerizationof 9-hydroxyphenalenone derivatives J Chem Phys 1994 1017971ndash7973

18 Tayi AS Kaeser A Matsumoto M et al Supramolecular ferro-electrics Nat Chem 2015 7 281ndash294

19 Blackburn AK Sue ACH Shveyd AK et al Lock-arm supramolec-ular ordering a molecular construction set for cocrystallizingorganic charge transfer complexes J Am Chem Soc 2014 13617224ndash17235

20 Horiuchi S Tokunaga Y Giovannetti G et al Above-room-tem-perature ferroelectricity in a single-component molecular crystalNature 2010 463 789ndash792

21 Kunkel DA Hooper J Bradley B et al 2D cocrystallization fromH-bonded organic ferroelectrics J Phys Chem Lett 2016 7435ndash440

Acknowledgments This work was supported by the National Nat-ural Science Foundation of China (91222203 91233205 5122230691027043 and 91433115) the Ministry of Science and Technology ofChina (2013CB933403 2013CB933500 and 2014CB643600) the Strate-gic Priority Research Program of the Chinese Academy of Sciences(XDB12030300) and the Chinese Academy of Sciences

Author contributions Bolla G wrote the paper and Dong H Zhen YWang Z and Hu W gave discussion in depth on the content of this article

Conflict of interest The authors declare that they have no conflicts ofinterest



Geetha Bolla obtained her PhD degree at the University of Hyderabad India and she is now a postdoctoral researcher inthe Laboratory of Organic Solids Institute of Chemistry Chinese Academy of Sciences (ICCAS) Her research interests aredesign synthesis and applications of organic cocrystals

Huanli Dong is an associate professor of the ICCAS She grew up in Shandong province China She received her PhDdegree from the ICCAS in 2009 after getting MSc degree at Fujian Institute of Research on the Structure of Material CASin 2006 She is presently focusing on the self-assembling of molecular materials and the applications of molecular materialsin optoelectronic devices Presently she has more than 110 peer review publications with citation more than 2900 times(H-index=24)

Yonggang Zhen is an associate professor of the ICCAS He grew up in Shanxi province China He received his PhD degreefrom the ICCAS in 2011 He is presently focusing on the design and synthesis of novel organic semiconductors

Zhaohui Wang is a professor of the ICCAS He received his PhD from East China University of Science and Technology in1999 He then joined Max Planck Institute for Polymer Research as a postdoctoral research fellow In 2005 he joined theICCAS and was promoted to full professor His research focuses on the design and synthesis of novel organic semiconduc-tors

有机共晶铁电材料的研究进展Geetha Bolla1 董焕丽1 甄永刚1 王朝晖1 胡文平12

摘要 ldquo有机共晶rdquo作为一种由两种或者两种以上分子按照一定比例形成的多组份体系在实现一些物理化学特性方面显示了独特的优势譬如铁电特性本文系统介绍了有机共晶铁电材料方面的研究进展深入分析了有机共晶在实现铁电性能方面的分子结构特征共晶体系组装策略以及铁电性能最后结合对这些研究进展和结果的分析作者指出了利用有机共晶实现铁电特性研究领域中所存在的各种挑战与机遇以及有机共晶铁电材料潜在的应用前景



Figure 1 (a) General schematic representation of cocrystals (b) Typesof ferroelectric systems and the interactions in these crystal structures

cussed the background to cocrystallinematerials wewouldlike to explore their applications in particular wewill focuson organic ferroelectrics and studies into the correlationbetween their two dimensional (2D) surface arrangementsand properties [9]

Ferroelectrics are polar (pyroelectric) crystals that showspontaneous electric polarization They have electroac-tive properties allowing the storage and switching ofpolarity (ferroelectricity) sensing of temperature changes(pyroelectricity) interchange of electric and mechanicalfunctions (piezoelectricity) and manipulation of light(through optical nonlinearities and the electro-opticaleffect) [10] Ferroelectric materials are mostly inorganicor organic-inorganic hybrid compounds for examplebarium titanate (BaTiO3) (BTO) sodium nitrite (NaNO2)potassium dihydrogen phosphate (KH2PO4 KDP) andtriglycine sulfate (TGS) [11] Historically ferroelectricscan be traced back to 1655 when Rochelle salt was firstisolated by Elie Seignette but pyroelectricity has been rec-ognized since ancient times However the piezoelectricityof Rochelle salt was established in 1880 by Curiersquos brotherand it took a further 40 years in 1920 until the term

lsquoferroelectricrsquo was coined by Joseph Valasek by analogywith ferromagnetism For some time Rochelle salt was theonly known ferroelectric material however in 1935 Buschand Scherer found KDP to be ferroelectric A furtherbreakthrough came in the 1940s during the Second WorldWar when BTO which has a simple perovskite structurewas prepared followed by von Hippelrsquos demonstrationof its ferroelectricity in 1945 [12] Following this periodof discovery piezoelectric transducers were developedand many new ferroelectric perovskite oxides were syn-thesized Among these new materials the most notableis lead zirconate titanate (PZT) Today PZT remains themost widely used ferroelectric material [13] In summaryinorganic ferroelectrics have been well explored and fororganic ferroelectrics the specifics are well establishedCurrently for organic ferroelectrics attention is given tothe use of multicomponent materials that exhibit ferro-electricity and polarization at temperatures at and aboveroom temperature

Conventional organic ferroelectrics [14] are of the or-der-disorder type (Figs 1b and 2) as will be discussed laterThe most interesting feature of ferroelectric materials istheir spontaneous polarization which can be reversed byinverting the external electric field During polarization re-versal in a coercive field the electric displacement (D) as afunction of the field (E) exhibits a hysteresis (a D-E loop)and ferroelectric materials undergo a paraelectric-to-fer-roelectric phase transition Furthermore dielectric sus-ceptibility usually obeys the Curie-Weiss law at high tem-peratures ie temperatures above the Curie temperature(Tc) Normally ferroelectric phase transitions are catego-rized using the following scheme 1 Order-disorder inthis scheme the ordering of the asymmetric molecules orions carrying permanent dipoles generates a spontaneouselectric polarization Sodium nitrite is a well-known exam-ple in which the bent NO2

2minus ion bears the electric dipole(Fig 2a) 2 Displacive type in the displacive mechanismthe relative displacements between the different chargesof the ions yield macroscopic polarization in the crystalan example being perovskite ferroelectrics such as BTO(Fig 2) The ferroelectric phenomenon can be utilizedfor a diverse range of technological applications such asnon-volatile memory elements (ferroelectric random ac-cess memory (FeRAM)) or ferroelectric field-effect tran-sistors (FeFET) and high-capacity condensers and capac-itors Furthermore ferroelectric crystals exhibit unusu-ally large electromechanical couplings ie the couplingbetween mechanical strain and electric polarization Thelarge electrostriction and piezoelectric effect can be utilized












Tc TcD κRT κmax



Thiourea 169 185 30 104 32 120 K

TEMPO 287 288 10 16 05

CDA 397 25

TCAA 355 45 65 02 RT


DNP 46 40 22

TCHM 24 104 96 100 024 10 K

CT complexes

TTF-CA 81 84 40 500

TTF-BA 50 20


Phz-H2ca 253 304 110 3 times 10 18 160 K

Phz-H2ba 138 204 30 17 times 103 08 105 K



































































Tc TcD κRT κmax



Thiourea 169 185 30 104 32 120 K

TEMPO 287 288 10 16 05

CDA 397 25

TCAA 355 45 65 02 RT


DNP 46 40 22

TCHM 24 104 96 100 024 10 K

CT complexes

TTF-CA 81 84 40 500

TTF-BA 50 20


Phz-H2ca 253 304 110 3 times 10 18 160 K

Phz-H2ba 138 204 30 17 times 103 08 105 K


























































organic cocrystals: the development of ferroelectric ... · organic cocrystals: the development of...

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