Dibutylammonium Hydrogen Oxalate: An Above-Room-TemperatureOrder−Disorder Phase Transition Molecular MaterialTariq Khan,†,‡ Yuanyuan Tang,† Zhihua Sun,*,† Shuquan Zhang,† Muhammad Adnan Asghar,†,‡

Tianliang Chen,† Sangen Zhao,† and Junhua Luo*,†

†Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, ChineseAcademy of Sciences, Fuzhou 350002, China‡University of the Chinese Academy of Sciences, Beijing 100039, China

*S Supporting Information

ABSTRACT: An above-room-temperature new phase transition molec-ular material, dibutylammonium hydrogen oxalate (1), has beensynthesized, which undergoes a reversible phase transition at 321.6 K.Differential scanning calorimetry (DSC) measurements confirm itsreversible phase transition with a large heat hysteresis of ∼10 K. Thedielectric constants of 1 exhibit an obvious step-like anomaly, showing thehigh and low dielectric constant states at high and room-temperaturephases, respectively, which reveal that 1 might be considered a potentialswitchable dielectric material. Variable-temperature single crystal X-raydiffraction analyses disclose that the origin of this phase transition is greatlyattributed to the coupling of the order−disorder change and reorientationof the dibutylammonium cation; that is, the frozen ordering of ethyl groupin butyl moiety triggered by the decreasing temperature forces thereorientation of cation, thus resulting in the structural phase transition.

■ INTRODUCTION

Reversible solid-to-solid structural phase transition materials(PTMs) have long drawn wide interest due to their applicationsin data communication, thermal energy storage, and mechanicalenergy transfer.1−7 Particularly, some properties of PTMsmaterials could be switched between at least two states, whichhave been used in switches, memory devices, and sensors.8,9

For example, their dielectric responses will be switchedbetween different states under the external stimulus. For themolecular entities, reorientational order−disorder motion ofthe moieties is considered one of the most efficient strategiesfor constructing smart PTMs, and intensive efforts have beenmade by researchers for exploring a large number of suchmaterials.10−30 For instance, reorientation of dimethylammo-nium (DMA) cation was recently noticed to induce the strikingdielectric responses in metal−organic frameworks,26 coupledwith the ferroelectric or antiferroelectric ordering in thetemperature range of 160−185 K.27 Below the phase transitiontemperatures, they are the multiferroic materials with thecoexistence of ferroelectricity, antiferroelectricity, and weakferromagnetic order.Similarly, many other PTMs have also been successfully

assembled; however, the low phase transition temperature (Tc)restricts their potential device applications.27−29 In contrast tolow temperature, the most popular range of Tc is 290−365K,30,31 which covers around the ambient temperature,possessing tremendously extensive applications includingpyroelectric sensor, solar, medical, textile, electronic, and

energy-saving applications.32−39 For example, various salts ofglycine molecule with inorganic counterions were discovered toexhibit promising ferroelectricity and pyroelectricity at ambienttemperature.36,39 A distinct representative pyroelectric materialof triglycine sulfate (TGS), having ferroelectric Tc around 323K, is considered a significant parameter for applications inpyroelectric detectors because of its large pyroelectriccoefficient.39 These above-room-temperature compoundsmake the researcher curious to discover substances whichoffer great response in potential applications near or aboveroom temperature. Recently, the work pertinent to thediscussion was done by Xiong et al., where the combinationof disordered cation and magnetic transition metal ion resultsin the formation of a magneto-dielectric molecule-basedfunctional material (i.e., triethylmethylammonium tetrabromo-ferrate) with the above-room-temperature phase transitionpoint.40 Significant spontaneous polarization and high Tc makethis above-mentioned compound a promising candidate for thedesign of molecule-based ferroelectrics, and promote newelectronic utilization of organic ferroelectrics. Therefore, thesefindings inspire us to investigate new above-room-temperaturePTMs, utilizing the various organic moieties. As a flexiblestructural primitive, dibutylamine molecule is sensitive to thetemperature change, which might be advantageous for

Received: June 2, 2015Revised: September 18, 2015

Article

pubs.acs.org/crystal

© XXXX American Chemical Society A DOI: 10.1021/acs.cgd.5b00758Cryst. Growth Des. XXXX, XXX, XXX−XXX

assembling smart PTMs with the order−disorder structuraltransitions.Here, we report an above-room-temperature smart PTM,

dibutylammonium hydrogen oxalate (1), which undergoes areversible phase transition at Tc = 321.6 K and demonstratesdielectric anomaly. As expected, crystal structural studies showthat the phase transition of 1 mainly originates from the order−disorder transformation of the terminal ethyl moiety of thedibutylammonium cation. Furthermore, the reorientation ofcation relative to the anionic plane also occurs, in combinationwith the ordering of ethyl group, which induces the structuralphase transition and symmetry breaking in 1. To the best of ourknowledge, 1 is a new concrete example with the above-room-temperature phase transition, triggered by the reorientationalorder−disorder transition in dibutylammonium. This order−disorder and the apparent motion of the dibutylammoniumcation in 1 are switched by temperature, which leads to thedielectric changes between low and high state. It is believed thatthis finding might provide the potential pathway to design thenew above-room-temperature PTMs with properties throughsuch organic moieties.

■ EXPERIMENTAL SECTIONSynthesis. All reagents and solvents were obtained commercially

and used without further purification. Compound 1 was synthesized byevaporating the aqueous solution, containing dibutylamine base andoxalic acid monohydrate, in stoichiometric amount of 1:1 molar ratio.The reaction mixture was stirred and heated for 5 min to dissolve thecomponents thoroughly. As the solution became cool, tiny colorlesscrystalline product was obtained. Subsequently, the resulting solutionwas filtered, and the filtrate was left undisturbed. Several days later,prism-like single crystals of 1 were obtained. The deuterated sample of1 was synthesized and recrystallized from D2O solvent three times. Inthe IR spectra of 1 (Figure S1, Supporting Information), the peaks atapproximately 1714 and 1643 cm−1 are assigned to stretching vibrationabsorption of the carbonyl group (CO) in the carboxyl andcarboxylate groups, respectively. These absorptions reveal theexistence of oxalic acid in 1, and only one carboxyl group isdeprotonated, leading to the reaction of 1:1 stoichiometric ratio. Thephase purity of 1 was confirmed by powder XRD (PXRD) patterns,which match very well with the results simulated from the single-crystal structure, as depicted in Figure S2.Thermal Measurements. Differential scanning calorimetry

(DSC) measurement was recorded on NETZCSCH DSC 200 F3instrument at heating and cooling rate of 5 K/min in the temperaturerange from 285 to 350 K. These measurements were accomplishedunder nitrogen atmosphere in aluminum crucibles. Specific heat (Cp)experiments were performed using a comparison method, of whichsapphire standard was used as the reference. We first run the baselineon the desired temperature range and then sapphire disks forcalibration were used as the standard prior to the measurement. Thenthe sample was measured and compared with the sapphire recordedformerly.Dielectric Measurements. For dielectric experiments, well-

ground powder samples connected by silver-conducting paste onelectrodes were used for measuring the complex dielectricpermittivities (ε = ε′ − iε″). The dielectric constants were measuredusing a TH2828A impedance analyzer at different frequencies of 1MHz, 100 kHz, 10 kHz, and 5 kHz with the measuring AC voltagefixed at 1 V.Single-Crystal Structure Determination. Variable-temperature

X-ray single-crystal diffraction data of 1 were collected using SuperNova CCD diffractometer with graphite monochromated Mo Kαradiation (λ = 0.71073 Å) at low (175 K) and high (330 K)temperatures, respectively. Crystal structures were solved by the directmethods and refined by the full-matrix least-squares method based onF2 using the SHELXLTL software package.41 All non-hydrogen atoms

were refined anisotropically. The positions of hydrogen atoms weregenerated geometrically, located at the cation and the hydrogen atomsin carboxyl groups as determined from the Fourier electron densitymap. Due to the disorder characteristic of the terminal ethyl moiety,we split the carbon atoms C1 and C2 into two sites C1A, C1B andC2A, C2B, with occupancy of 0.60, 0.40 and 0.60, 0.40, respectively.Some atoms also show slight disorder; we try to solve the disorder, butthe disorder was too small to split. Crystallographic data and details ofdata collection and refinement are listed in Table S1. CCDC1046677−1046678 for 1 contains the supplementary crystallographicdata for this paper, which can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

■ RESULTS AND DISCUSSIONThermal Properties of 1. DSC measurement is an effective

and useful method to confirm the existence of a reversiblephase transition triggered by external temperature stimulus.42,43

Here, compound 1 was subjected to the preliminary DSCmeasurement (Figure 1a), which shows a couple of exothermic

(311.4 K) and endothermic (321.6 K) peaks, revealing areversible phase transition with a large thermal hysteresis of∼10.2 K. Such a large thermal hysteresis is also supported bythe estimation from the different scanning rates (Figure S3),which is the characteristic of a first-order phase transition for1.44 In addition to DSC, the Cp−T curve further confirms thepresence of phase transition in 1 (Figure 1b). The line shown atthe bottom of Figure 1b denotes the measurement obtainedfrom sapphire standard, which is used as the reference. The

Figure 1. (a) DSC curves and (b) temperature dependence of Cp of 1.

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curve of Cp in the heating mode gives a sharp peak at 321.6 K,which is consistent with the DSC result. The enthalpy (ΔH)associated with the endothermic peak (in Cp) was calculated as8.887 J/g, and the corresponding total entropy change (ΔS)was estimated to be 6.020 J/mol·K−1 by using the equation ΔS= ΔH/Tc. Moreover, the pronounced thermal anomaly was alsoobserved in differential thermal analysis curve at Tc, as depictedin Figure S4. This curve matches quite well with the resultsobtained from the DSC and Cp measurements.Variable-Temperature Powder X-ray Diffraction of 1.

Variable-temperature powder X-ray diffractions (VT-PXRD)offer the most powerful information on phase transitionmaterials. In order to assist the exploration of phase transitionin 1, finely ground powder was checked in VT-PXRD atdifferent temperatures of 298, 308, 333, and 343 K, respectively(Figure 2). The experimental patterns obtained at low and high

temperature in the VT-PXRD match well with the simulateddata obtained from single crystal X-ray diffraction. However,drastic changes were found between the data obtained at low-temperature phase (LTP) and high-temperature phase (HTP),strongly confirming the occurrence of phase transition in 1. Forinstance, during the warming process, the Bragg diffractions of(101), (101 ), (022), (031), and (104) planes of LTP exhibitobvious shifts at HTP, respectively. Moreover, it is noteworthythat the profile patterns recorded at low temperature, beforethe phase transition point and on cooling back to 298 K fromhigh temperature, appear to be the same. Such an observationreveals the reversible phase transition of 1, which is consistentwith the thermal measurements of DSC and Cp, providing asolid proof of the existence of reversible phase transition in 1.Crystal Structural Analysis of 1. X-ray diffraction analyses

of 1 were performed at 175 K (the low-temperature phase,LTP) and 330 K (high-temperature phase, HTP), respectively.Distinct differences of structural parameters are observedbetween the HTP and LTP, which further confirm theexistence of phase transition. In the HTP, 1 crystallized inthe monoclinic crystal system with space group P21/c, and cellparameters are a = 17.868(2), b = 5.5508 (3), c = 14.4018(10)Å, β = 112.102(11)°, Z = 4, V = 1323.5(2) Å3, as shown inTable S1. In the LTP, 1 transforms into the triclinic crystalsystem with a centrosymmetric space group P1 and the cellparameters are a = 5.5144 (3), b = 9.4520 (5), c = 13.4040 (5)Å, α = 74.239(4)°, β = 84.747(4)°, γ = 81.205(4)°, Z = 2, V =

663.55(6) Å3. These remarkable differences between HTP andLTP structures can also be perceived from the variation in thecell parameters. For 1, the anomalies of unit cell parametersduring phase transition are measured as a function oftemperature from 280 to 335 K. As shown in Figure 3, all

the cell parameters of 1 are stable in LTP until the phasetransition point at 321.6 K, where these values exhibit sharpchanges and then become stable again at HTP. Its unit volumeis doubled at Tc. The temperature-dependent lattice constantsof a-axis demonstrate a considerable 3-fold increment while b-and c-axes exhibit a computable change. The abrupt changes inthe cell parameters in the vicinity of Tc ascertain theconfirmation of phase transition which corresponds well tothe DSC and Cp measurements.

45,46

From the symmetry breaking point of view, the structure of 1changes from a high centrosymmetric space group of P21/c atHTP (point group of C2h) to a low centrosymmetric spacegroup of P1 at LTP (point group Ci). The symmetry breakingphenomenon occurs during the transition, showing an Aizunotation of 2/mF1. Symmetric elements decrease by half fromfour (E, i, C2, σh) at HTP to two (E and i) at LTP, in strictaccordance with Landau phase transition theory. As shown inFigure S5, the change of its spatial symmetric operationsindicates that the symmetric elements (2 and m) disappear in 1in LTP, which might be aroused by slight reorientations ofmolecules. According to the Curie symmetry principle, thespace group of P1 is a subgroup of the one of C2/c, of whichthe maximal nonisomorphic subgroups include Cc, C2, and P1 .At HTP, the asymmetric unit contains one protonated

dibutylammonium cation and one monodeprotonated oxalateanion, while the LTP asymmetric unit is composed of onedibutylammonium cation and two halves of oxalate anions (seeFigure 4). Packing structures of 1 in both states are stabilizedby the extensive O−H···O and N−H···O hydrogen bondingnetworks (Figure 5). As shown, a one-dimensional array ofhydrogen bonding (O−H···O) is constructed involving theoxygen atoms of carboxylate and carboxylic acid moiety of theadjacent oxalate anions. Arrays of these interactions betweenthe anionic moieties are aligned parallel to each other,extending in the direction of the b-axis. Moreover, dibuty-lammonium cations sandwiched between the neighboringoxalate anions chains form a butterfly-like skeleton via N−H···O hydrogen bonds with carbonyl oxygen atoms of themonodeprotonated anion. Thus, a strip-like motif is created byhydrogen-bond interactions along the c-axis, which imposes agreat impact on the packing structures. The hydrogen-bondingskeleton formation by the anion−anion and anion−cationintermolecular interactions constitutes the two-dimensional

Figure 2. Variable-temperature powder XRD patterns of 1.

Figure 3. Temperature-dependence of (a) cell parameter changes forthree axis lengths and (b) cell volume and three angles in the rangefrom 280 to 335 K for 1.

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supramolecular array, which extends in the bc plane as clearlyseen in the packing diagram.

In the HTP crystal structure of 1, it is noteworthy that butylgroups of dibutylammonium cation are highly disordered,where the terminal carbon atoms present two atomicoccupations of C1A/C2A and C1B/C2B, respectively. Thesplitting makes the high-temperature structure of 1 moreacceptable. Such apparent disordered characteristics at HTPcan also be deduced from the thermal ellipsoids of C1 and C2atoms, which are relatively larger than those of otherneighboring atoms. In reality, this is not the actual state butan average result of the atomic disordering, as shown in FigureS6. With the temperature decreasing from 330 to 175 K, it isinterestingly found that the disordered terminal carbon atomsare frozen and its low-temperature structure becomes totallyordered, which does not possess sufficient thermal energy toswitch between sites, as demonstrated in Figure 4b. Thisobvious order−disorder change would possibly results from thelarge freedom and flexibility of the cationic moieties,particularly the terminal ethyl parts of the butyl chain.However, the hydrogen-bonding interaction between thenitrogen atoms and oxalate anion makes the internal carbonatoms incapable of undergoing such severe order−disorderbehavior. The deuterated sample of 1 shows no difference inDSC measurements with 1. Therefore, the role of hydrogenbonding in the phase transition would be excluded (Figure S7).In other word, such an observation reveals that order−disordertransformation of butyl part of the cation moiety is the drivingforce for the phase transition in 1.Besides the order−disorder transformation, another striking

feature of the structural change is the remarkable reorientationof the cation, which could be deduced from the angle betweenthe normal line of the anionic plane and the C8−N1 line of thecation. For comparison, the angles between the cationic lineand the line dropping vertically through the anionic plane arecalculated, as depicted in Figure 6. Since the adjacent oxygen

atoms of two alternate oxalate anions are nearly parallel,showing no obvious changes at both LTP and HTP, the planeconsisting of carboxyl oxygen atoms is thus assigned as areference plane. Furthermore, since another ordered butylmoiety is almost the same in the two phases, it is reasonable toappoint C8−N1 as a reference line. From Figure 6, it can beseen that the C8−N1 line inclines slightly with the angle of5.70° between the vertical line passing through the plane of theanion and the line of the cation in LTP, while it becomes more

Figure 4. (a) Asymmetric unit of 1 at 330 K and (b) asymmetric unitof 1 at 175 K.

Figure 5. Packing diagram of 1 at (a) LTP and (b) HTP. The dashedlines show the hydrogen bonds. Carbon and hydrogen atoms in thecationic moieties are omitted for clarity.

Figure 6. Order−disorder transformation and reorientation of thedibutylammonium cation during the phase transition of 1. The circleexhibits order−disorder transition, and schematic diagram in therectangle shows the changes of included angle between the normal lineof anionic plane and C8−N1 line of cation.

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tilted with the angle of 17.03° in HTP, induced by the draggingbehavior of disordered terminal carbon atoms. Such distinctreorientation of 1 is not only demonstrated by the inclinedposition change of cation, but also confirmed by the alterationof bond length in the hydrogen bonds between anion andcation. In fact, the inclination of cation has a great impact onthe hydrogen-bonding interactions of 1 (Figure 5), where theN atom plays an important role. Generally speaking, theacceptor/donor length of hydrogen bonds would causevariation with changing temperature, due to thermal expansionor contraction. In contrast, however, the length of N1−H2···O3is shortened from 2.883 (LTP) Å to 2.858 (HTP) Å. Thecoexistence of elongation and shortening of acceptor/donordistances might be reminiscent of reorientation of cationicmoieties. The above-mentioned analyses support the fact thatthe reorientational order−disorder motions of the terminalethyl moiety between two phases are responsible for phasetransition of 1.From the above studies, it can be deduced that the phase

transition of 1 is mainly induced by order−disorder change ofthe dibutylammonium cations together with the reorientationmotion. In detail, single-crystal structural analyses reveal thatone of the butyl groups in the cation is disordered at HTP.With decreasing temperature, it becomes more ordered belowthe Tc. Meanwhile, reorientational transformation of thedibutylammonium cation is also contributing to the structuralphase transition of 1; that is, the cationic moiety reorients fromits tilting position, affording a change of 11.3° for the includedangle between the normal line of anionic plane and the cationiccounterpart. Hence, the phase transition of 1 would be ascribedto the coupling of order−disorder change and reorientation ofthe dibutylammonium cation. To the best of our knowledge,this observation is unprecedented and differs from other PTMscontaining the dibutylammonium cation, in which hydrogenbonds play an important role for the structural phasetransition.21

Dielectric Studies of 1. Temperature-dependent dielectricconstant measurement is of great importance and is asignificant indicator of phase transition in a compound, whichreveals the degree of electric polarizability of the material. Formolecular crystals, dielectric transition between the low- andhigh-dielectric states could be established by the disorder orreorientation transformation.47 The structural analyses of 1have disclosed the order−disorder mechanism of its phasetransition. To further confirm this feature, we measured itstemperature-dependent dielectric permittivities (ε = ε′ − iε″,where ε′ is the real part and ε″ is the imaginary part) onpowder pressed pellets; Figure 7 displays its dielectricpermittivities at different frequencies from 300 to 344 K. Theobvious step-like dielectric anomaly was clearly recorded at311.4 K in cooling mode, which corresponds well to thermalanalyses. Upon cooling to 312 K, the dielectric constantdecreases progressively, showing an evident reduction with asteep slope around Tc. Subsequently, the permittivity exhibits aplateau with a slight decrease. The values of ε′, for instance,display the changes from 4.9 to 5.0 to 4.3−4.4 ( f = 100 kHz)during its phase transition, at which the variation was quitesharp in the vicinity of Tc. The imaginary part (ε″) also exhibitsdistinct change around the transition point (at f = 100 kHz),which further affirms the occurrence of the phase transition at311.4 K. By decreasing the temperature below 311 K, the cationbecomes more ordered with a slight reorientation, and thenrealigns perpendicularly to the anionic plane. It is known that

the dielectric permittivity reflects the degree of electricpolarizability and is microscopically associated with dipolarmotions, such as molecular rotations or disorder.48 Therefore, itis assumed that the switching dielectric response results fromthe order−disorder phenomenon in the terminal ethyl moietiesof the alkyl groups, along with reorientation of the cation. Inother words, the disordering and the synergetic reorientationexert predominant influences on the dynamics of the cations.Because of the thermally induced switching behavior, asdemonstrated by dielectric properties as a function oftemperature, 1 might be a potential switchable dielectricmaterial.49

■ CONCLUSIONS

In the present work, we present an above-room-temperatureswitchable dielectric material, which displays a reversible phasetransition at 321.6 K, being confirmed by thermal analyses,temperature-dependent dielectric measurement, and single-crystal structural analyses. The origin of phase transition of 1 ismainly attributed to the coupling of order−disorder change andreorientation of the dibutylammonium cation. In detail, thefrozen ordering of ethyl groups caused by cooling down forcesthe reorientation of cations relative to the anionic plane, andthus results in the phase transition. We believe that this newfinding might offer a new route to design potential phasetransition materials.

■ ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.cgd.5b00758.

IR spectrum, PXRD patterns, DSC curves at differentscanning rates, DTA, packing diagram showing hydro-gen-bond geometries, thermal ellipsoids maps of cation,deuterated sample DSC and tables of compound 1(PDF)Crystallographic data (CIF)Crystallographic data (CIF)

Figure 7. Temperature-dependent dielectric constants performed onthe powder pressed pellets of 1 under different frequencies in coolingmode.

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■ AUTHOR INFORMATIONCorresponding Authors*E-mail: sunzhihua@fjirsm.ac.cn.*E-mail: jhluo@fjirsm.ac.cn.

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was supported by NSFC (21222102, 21373220,51402296, 21171166 and 21301172), the 973 Key Program ofMOST (2011CB935904), and the NSF for DistinguishedYoung Scholars of Fujian Province (2014J06015). Z. S. and S.Z. thanks the support from “Chunmiao Projects” of HaixiInstitute of Chinese Academy of Sciences (CMZX-2013-002and CMZX-2015-003). T. K. is thankful for the CAS-TWASPresident program of the University of the Chinese Academy ofSciences for financial support.

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Crystal Growth & Design Article

DOI: 10.1021/acs.cgd.5b00758Cryst. Growth Des. XXXX, XXX, XXX−XXX

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