Journal of Physics and Chemistry of Solids 71 (2010) 1774–1779

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Journal of Physics and Chemistry of Solids

0022-36

doi:10.1

n Corr

E-m

(B. Kum

journal homepage: www.elsevier.com/locate/jpcs

Structural, dielectric, optical and ferroelectric property of urea succinic acidcrystals grown in aqueous solution containing maleic acid

B.K. Singh a, N. Sinha b, N. Singh a, K. Kumar a, M.K. Gupta a, Binay Kumar a,n

a Crystal Lab, Department of Physics and Astrophysics, University of Delhi, North Campus, Delhi 110007, Indiab Department of Electronics, SGTB Khalsa College, University of Delhi, Delhi 110007, India

a r t i c l e i n f o

Article history:

Received 24 June 2010

Received in revised form

6 August 2010

Accepted 10 September 2010

Keywords:

A. Organic compounds

B. Crystal growth

C. X-ray diffraction

D. Dielectric properties

D. Ferroelectricity

97/$ - see front matter & 2010 Elsevier Ltd. A

016/j.jpcs.2010.09.010

esponding author. Tel.: +91 9818168001; fax

ail addresses: b3kumar69@yahoo.co.in, bkum

ar).

a b s t r a c t

Urea-succinic acid crystals have been grown at room temperature from aqueous solution in the

presence of maleic acid by a slow evaporation technique. The structural parameters were determined

using powder X-ray diffraction (XRD) and found to have monoclinic symmetry (space group P21/m)

with a¼9.902, b¼17.510, c¼5.555 A and a¼g¼901, b¼96.461. The transparency and optical analysis

were carried out using UV–vis analysis. The optical band gap is found to be 4.71 eV. The presence of

various functional groups was confirmed by FTIR analysis. The samples have shown piezoelectric

behavior with a fairly good piezoelectric charge coefficient (d33) of 5 pC/N, when it is poled at 7 kV/cm.

The hysteresis loop was plotted and the remnant polarization and coercive field were found to be

2.8 mC/cm2 and 4 kV/cm, respectively. The dielectric analysis was carried out as a function of

temperature at various frequencies and the results were also discussed.

& 2010 Elsevier Ltd. All rights reserved.

1. Introduction

Succinic acid is a common metabolite formed by plants,animals and microorganisms and belongs to C4 dicarboxylic acidproduced as an intermediate of the tricarboxylic acid cycle (TCA),and also as one of the fermentation products of energymetabolism [1]. It belongs to the monoclinic system in both theferroelectric and paraelectric phases. In the paraelectric phase, thecrystal belongs to centrosymmetric space group P21/C, while inthe ferroelectric phase it becomes non-centrosymmetric spacegroup P21. Being a ferroelectric crystal, it finds many importantorgano–opto-electronic applications such as capacitors, nonvola-tile memory devices, actuators, high-performance gate insulators,etc. [2–6]. Owing to its biological as well as industrial application,it has attracted the attention of many researchers to explore itspotentiality for industrial application. Further, in recent times, thetrend is towards the development of organic ferroelectricmaterials [4,7,8]. Attempts have been made to grow crystals ofsuccinic acid with some other organic compounds (co-crystals)/metals in order to enhance its performance in various applications[9–11]. The growth of succinic acid (SA) and urea-succinic acid(USA) crystals and their properties were investigated earlier[12,13]. However, it does not deal with the piezoelectriccharacterization of the material. In the present paper, we report

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ar@physics.du.ac.in

the growth and the characterization of urea succinic acid crystalsgrown in aqueous solution containing maleic acid (M-USAhereafter). The effect of the presence of maleic acid on variousproperties of USA crystals has been investigated and discussed.

2. Experimental details

In the present work the solution growth method was used togrow M-USA crystals using water as solvent. In this process, 1 molof maleic acid has been added in the aqueous solution containingthe stoichiometry ratio of urea and succinic acid. Differentialscanning calorimetry (DSC) study was performed using TAInstrument DSC Q200 in the range �50 to 275 1C in the nitrogenatmosphere. The obtained crystals were finely ground andsubjected to powder XRD in the range 10–701 of 2y with a stepangle of 0.021 with step time 0.2 s using a PW3710 Philipsdiffractometer (CuKa radiation). To determine the optical trans-mittance range and hence to know the suitability of M-USAcrystals for optical applications, the UV–vis transmittancespectrum was recorded with a Perkin Elmer Lambda 25 spectro-photometer in the range 190–900 nm. FTIR analysis of M-USAcrystals was carried out in the middle infrared region extendingfrom 400 to 4000 cm�1 using a Bruker 66 V FT-IR spectrometer.The hysteresis loop was traced using a computer controlled P-Eloop tracer. Crystals were poled by applying a field of 0.7 kV/mmat 40 1C for 10 min. The piezoelectric charge coefficient d33 (pC/N)was measured using Piezometer (PM300, Piezotest). The variation

B.K. Singh et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1774–1779 1775

of dielectric constant and ac conductivity at different frequencies(100 Hz to 100 kHz) of as grown samples was studied from roomtemperature to 150 1C using an HP 4284 A impedance analyzer.

Fig. 2. Plot of differential scanning calorimetry (DSC) for M-USA crystal.

3. Results and discussion

3.1. Crystal growth

Urea succinic acid crystals were grown from aqueous solutioncontaining 1 mol% of maleic acid by the slow evaporation techniqueusing water as a solvent. About 25 g of analytical grade urea andsuccinic acid were used as starting materials in the 2:1 molar ratio.In addition 1 mol% of maleic acid was added to the solutioncontaining a stoichiometric ratio of urea and succinic acid. Thesolution was then stirred for nearly 4 h at 35 1C to obtain ahomogeneous solution and then kept in a constant temperaturebath at 35 1C. After a period of nearly 4 days, optically transparentcrystals having dimensions ranging from 2�2�1 to 5�3�2 mm3

were obtained (Fig. 1).

3.2. DSC analysis

The DSC curve of M-USA crystal is shown in Fig. 2. The sampleis found to undergo a sharp endothermic transition near 151 1C,which corresponds to the melting point of the crystal. Whencompared to the USA crystals the presence of maleic acid wasfound to decrease its melting point by �3 1C [13]. A very sharpcurve near melting point shows the high crystallinity of thesample. After melting, another exothermic peak is observedaround 168 1C where gaseous fragments such as carbon dioxideand ammonia might be liberated by the complete breakage of theion from M-USA crystals. As evident from the curve, the materialis very stable up to its melting point, which makes it suitable forpossible application in lasers, where the crystal is required towithstand high temperatures [14].

3.3. XRD analysis

The obtained XRD pattern is subjected to cell refinement usingFullProf software and found to consist of monoclinic symmetry(space group P21/m) with cell parameters a¼9.902, b¼17.51,

Fig. 1. Photograph of as grown M-USA crystals.

c¼5.555 A and a¼g¼901, b¼96.461. A very good correlationbetween the experimental and observed values was observedafter refinement (Fig. 3a). However, a deviation from the earlierreported value of lattice parameter for USA crystals was observed[13]. This may be due to polymorphism in USA crystal due to thepresence of maleic acid in the aqueous solution. A well indexedXRD pattern is shown in Fig. 3b. In addition, Miller’s indices ofXRD peaks for M-USA crystal are shown in Table 1. The lowintensity peaks, which are not visible in the pattern are shown inthe inset Fig. 3b.

3.4. UV–vis analysis

The optical properties of materials are important as theyprovide information on the electronic band structures, localizedstates and types of optical transitions. The desired lower cutoff inthe transmittance analysis should be between 200 and 400 nm foreffective application of these NLO materials for SHG from diodelasers [15]. To determine the optical transmittance range andhence to know the suitability of these crystals for opticalapplications, the UV–vis transmittance spectrum was recordedin the range 190–900 nm and is shown in Fig. 4. A prominentcharacteristic absorption edge is observed at 286 nm, and noabsorption was observed between 286 and 900 nm. The goodtransmission property of the crystal in the entire visible regionsuggests its suitability for second harmonic generation. A highervalue of optical transparency was observed for the present samplewhen compared to SA or USA crystals [13]. Energy band gap hasbeen calculated from the UV–vis absorbance data. For this, the(ahn)1/2 vs. (hn) plot has been drawn (where a is the absorptioncoefficient) and the energy band gap is found to be 4.71 eV(Fig. 4). The higher and wider energy band gap shows that thedefect concentration in the grown crystals is very low and thecrystal has a large transmittance in the visible region.

3.5. Photoluminescence studies

Photoluminescence studies are preferred over optical absorp-tion to detect the lower concentrations of defects since theimpurity on absorption of light gives rise to the bound excitedstate from which it returns to its ground state abiding inthe analysis of color centre creation mechanism. The roomtemperature photoluminescence (PL) spectrum with excitation

Fig. 3. (a) Reitveld refinement and (b) powder X-ray diffraction pattern of M-USA

crystal at room temperature.

Table 1Miller’s indices of XRD peaks of M-US crystal.

h k l 2h (obs) 2h (cal) 2h (obs–cal)

0 0 1 16.03 16.0378 �0.0078

2 0 0 18.02 18.0157 0.0043

1 1 1 19.975 19.9684 0.0066

1 2 1 21.92 21.9172 0.0028

�1 3 1 23.47 23.4741 �0.0041

2 1 1 26.065 26.0747 �0.0097

�1 5 0 27.65 27.5712 �0.0112

�3 3 0 31.425 31.4368 �0.0118

0 0 2 32.415 32.4011 0.0139

�3 5 0 37.99 37.9657 0.0243

�4 0 1 38.345 38.3236 0.0214

�3 5 1 40.195 40.1708 0.0242

�3 1 2 40.605 40.5989 0.0061

�3 6 0 41.935 41.9435 �0.0085

1 5 2 43.87 43.8648 0.0052

�4 2 2 47.815 47.8476 �0.0326

�1 9 0 48.725 48.7376 �0.0126

�4 6 1 50.335 50.3096 0.0254

�2 3 3 53.5901 53.5908 �0.0007

�3 4 3 58.78 58.7641 0.0159

3 3 3 62.67 62.6818 �0.0118

Fig. 4. UV–vis spectrum of M-USA crystal. The optical band gap is shown in the

inset.

Fig. 5. Photoluminescence spectrum of M-USA crystal at room temperature.

B.K. Singh et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1774–17791776

wavelength 242 nm is shown in Fig. 5. A small peak at 484.6suggests that the sample is excited at 242 nm. PL spectra have astructure consisting of the main band at 392 nm (3.16 eV). Thisreveals that the grown crystal has a blue emission property. Thisblue band emission at 2.34 eV may be attributed to radioactiverecombination between donors and acceptors, i.e., the protona-tion of amino group to the carboxyl group and also this strongemission indicates there may be the presence of intrinsic defectsin the forbidden band region.

3.6. FTIR analysis

The FTIR spectrum of the M-USA crystal is shown in Fig. 6. Thesample is first mixed to KBr and a pallet was made for taking thespectrum. The sharp peaks at 3443 and 3238 cm�1 in the higherfrequency region are associated with asymmetric stretching ofNH2 and symmetric stretching of NH2, respectively. The bands at3337 and 1029 cm�1 are due to N–H and C–N stretchingvibrations, respectively. The C–H stretching bands are super-imposed upon the O–H band near 2940.29 cm�1. The sharp bandat 1699 is due the CQO stretching of the –COOH group. The band

Fig. 6. FTIR spectrum of as grown M-USA crystal at room temperature.

Table 2Assignment of functional groups.

Frequency in wavenumber (cm�1)

Assignment offunctional groups

Force constantK (N/m)

3480 OH 677

3443 Asymmetric stretching of NH2 657

3337 N–H stretching 617

3238 Symmetric stretching of NH2 581

2940 C–H stretching superimposed upon 474

O–H stretching 482

1699 CQO stretching 1167

1644 NH2 scissor superimposed with 150

CQC stretching 174

1506 N–H bending 126

1455 CH3 bending 116

C–N anti-symmetric stretching 807

1414 C–O–H bending 808

1329 C–N stretching 673

1203 C–O stretching 585

1176 C–O stretching 559

1029 C–N stretching 403

904 O–H out of plane 46

778 NH2 rocking 34

642 NH2 out of plane wagging 23

620 CH bending 21

B.K. Singh et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1774–1779 1777

at 1644 cm�1 is due to NH2 scissor. The NH2 bending is observedto yield a less intense peak 1506.02 cm�1. A sharp intense peak at1414 cm�1 is assigned to C–O–H bending vibration. The stretch-ing vibrations due to C–O and C–N bands are observed at 1203,1176 and 1329 cm�1, respectively. The bending vibrations of theC–H band are also located at 620 cm�1. The various bands are asshown in Table 2. Infrared absorptions occur when moleculesabsorb a photon of light which promotes the molecule into anexcited vibrational state. This vibrational energy is a characteristicof a particular bond or pair of bonds. Most important IRabsorptions lead to increased bond stretching motions in amolecule, which acts like a spring, stretching and contractingat some frequency, v. The spring motion follows Hooke’s

law: v¼ 1=2pcffiffiffiffiffiffiffiffiffik=m

p, where k is the force constant and m the

reduced mass given by m¼m1m2/(m1+m2), where m1 and m2 arethe masses of the bonded atoms (kg/atom). An increase in k

indicates an increase in bond strength, and can therefore be used asa probe of things that affect bonding in molecules. The forceconstant (k) for all these frequency assignments can be determinedby rearranging Hooke’s law as k¼4p2v2m. The force constant forvarious fundamental vibrations of the grown crystal is mentionedin Table 2. The evaluation of k is carried out to find out thestretching, bending and interaction force constants correspondingto each substituent and its position in the molecule. The values offorce constant (k) for C–O stretching were found to be 559 N/m andfor CQO, k value is 1167 N/m. These values are in good agreementwith the double bond characteristics values between carbon andoxygen atoms. The calculated set of force constants tabulated hereis well in the vicinity of the observed frequencies.

3.7. Dielectric and ac conductivity analysis

Fig. 7 shows the variation of dielectric constant with tempera-ture at various frequencies for unpoled samples. A sharp increase inthe dielectric constant was observed beyond 120 1C. A sharp peakaround 145 1C was observed, which possibly may be due toferroelectric to paraelectric phase transition. However, as the peakis very close to its melting point, it cannot be termed as Curietemperature with surety. The sharp decrease in the dielectricconstant after this may be due to its melting process and possiblythe system melts before reaching its Curie temperature. A slightincrease in the dielectric constant with frequency is observed.The dielectric constant of the sample was found to be in the range55–80. Fig. 7 shows the variation of the dissipation factor (tan d)with temperature. A sharp rise in the tan d is observed beyond120 1C, which possibly may be due to increase in the ionization ofthe sample at high temperature, which increases the conductivityprocesses and hence dissipation factor simultaneously.

s is directly related to the imaginary part of dielectric constantas s¼eooe00, where eo,o are permittivity of free space and angular

B.K. Singh et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1774–17791778

frequency, respectively. Fig. 8a shows the variation of acconductivity with inverse of temperature. A smooth increase inthe ac conductivity is observed till 120 1C, beyond which a sharpincrease at 1450 1C is observed. The activation energy (Ea) forconduction above the onset of the sharp increase of acconductivity for 1 kHz was calculated by fitting the equationsac ¼ soe�Ea=kBT and is found to be 0.69 eV. The variation of acconductivity with frequency at different temperatures below itsmelting point (151 1C) is shown in Fig. 8b. The frequencydependence part of the ac conductivity is described by powerlaw given by Jonscher [16,17] as s(o)¼sdc+Aos, where sdc is thedc conductivity, and A is temperature dependant constant and s isthe dimensionless frequency exponent parameter in the range0rsr1. For an ideal Debye dielectric and ionic-type crystals n

corresponds to 1 and 0, respectively. Further, this dc conductivityis basically due to the long-range movement of free chargespresent in the sample, while the frequency dependence of theconductivity is generally explained by a hopping conductionmechanism. It is evident from the curve that the ac conductivity

Fig. 7. Variation of the real part of dielectric constant with temperature at

different frequencies. The inset shows the variation of dielectric loss with

temperature at different frequencies.

Fig. 8. Variation of ac conductivity with (a) temperature at diffe

increases exponentially with increase in frequency. After fittingthe curve it is seen that the value of s is not linear withtemperature. It is found that the s increases in temperature range30–80 1C from 0.73 to 0.81, while a decrease in s is observedin the temperature range 80–150 1C from 0.81 to 0.73. This maybe due to (1) the distribution of defects in the crystals [18] or (2)the onset of transition form ferroelectric to paraelectric phase.Further it is evident from the value of s in all temperature rangesthat the sample shows an ideal Debye dielectric behavior.A similar trend is observed for dc conductivity (obtained afterfitting Fig. 8b). The dc conductivity at room temperature is foundto be 3.305�10�8 O m�1.

3.8. Poling and d33 measurements

The crystals were subjected to piezoelectric characterization.For this, the poling conditions were optimized by observing theresponse of the crystals under different applied electric fields,temperatures and durations of poling. It is observed that thecrystal can withstand high dc electric field up to 10 kV/cm,beyond which cracks were observed to develop in the crystals.Hence a moderate dc field (7 kV/cm) at 40 1C for 10 min was usedto pole these crystals. A steady increase in the value ofpiezoelectric charge coefficients (d33) was observed as polingfield is increased. The d33 value as high as 5 pC/N was achieved.

3.9. Hysteresis loop

Fig. 9a shows the electrical polarization vs. electric field for M-USA crystal at room temperature. From the hysteresis loop, thevalues of coercive field (Ec) and remnant polarizations (Pr) werefound to be 4.0 kV/cm and 2.8 mC/cm2, respectively. It is foundthat the M-USA crystals cannot sustain a switching ac electricfield higher than 6.5 kV/cm (cracks developed in the crystal).Hence we cannot apply a field more than this value to achievesaturation polarization. Further, the values of remnant polariza-tion corresponding to the increasing coercive field (up to 4 kV/cm)were plotted and the asymptote of the curve was drawn by fittingthe curve using the asymptotic equation (Fig. 9b). The value of theasymptote gives the actual value of Pr, which is found to be4.71 mC/cm2. The actual saturation polarization was calculated ina similar way and found to be 5.6 mC/cm2.

rent frequencies and (b) frequency at different temperature.

Fig. 9. Plot for (a) hysteresis loop of unpoled as grown M-USA crystal at room temperature and (b) the variation of remnant polarization with coercive field at room

temperature for M-USA crystal.

Table 3Comparison of various parameters of crystals of SA, USA and M-USA.

Parameters SA [12] USA [13] M-USA[present case]

System Monoclinic Monoclinic Monoclinic

Space group P21/c P21/c P21/m

a (A) 5.5261 5.640 5.555

b (A) 8.8807 8.241 9.902

c (A) 5.1051 12.257 17.510

a (deg) 90 90 90

b (deg) 91.490 96.7 96.46

g (deg) 90 90 90

e0 (at RT for 1 kHz) �0.01 �160 �57

tan d (at RT for 1 kHz) (%) �10 �3.5 0.8

Optical transparency (%) �85 �91

lcutoff (nm) 240 240 286

MP (1C) 240 (decompose) 154 151

d33 (pC/N) – – 5

Pr (mC/cm2) – – 2.8 mC/cm2

B.K. Singh et al. / Journal of Physics and Chemistry of Solids 71 (2010) 1774–1779 1779

4. Conclusion

Urea succinic acid crystals were grown in the aqueous solutionin the presence of Maleic acid by the slow evaporation solutiongrowth method. The maleic acid was found to strongly influencethe physical and chemical properties of USA crystals. An improvedtransparency with lower cutoff wavelength was achieved, whichmakes these crystals more suitable for laser applications.The piezoelectric and ferroelectric property of M-USA crystals isbeing reported for the first time. A fairly high value of piezo-electric charge coefficient (5 pC/N) and remanent polarization(2.8 mC/cm2 with Ec�4 kV/cm) for the present samples wasachieved. In the current scenario, where there is a vast demandin the research community to search for organic ferroelectriccrystals, these crystals can stand good for various applications.The growth of urea succinic acid crystals in the presence of maleic

acid has been found to enhance various properties of USA crystalsignificantly. A comparative table listing various physical/chemi-cal parameters is shown in Table 3.

Acknowledgements

We are thankful for the financial support received from theDepartment of Science and Technology (SERC Sanction no. 100/(IFD)/1637) and DU DST PURSE Grant. Mr. B.K. Singh andMr. M.K. Gupta are thankful to DST for SRF in SERC project.

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