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doi:10.1016/j.jc

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(R. Jayavel).

Journal of Crystal Growth 305 (2007) 156–161

www.elsevier.com/locate/jcrysgro

Growth and characterization of a new semiorganic non-linear opticalthiosemicarbazide cadmium chloride monohydrate

(Cd(NH2NHCSNH2)Cl2 �H2O) single crystals

R. Sankar, C.M. Raghavan, R. Mohan Kumar, R. Jayavel�

Crystal Growth Centre, Anna University, Chennai 600 025, India

Received 16 September 2006; received in revised form 18 February 2007; accepted 5 March 2007

Communicated by M. Roth

Available online 31 March 2007

Abstract

A new semiorganic non-linear optical thiosemicarbazide cadmium chloride monohydrate (TSCCCM) material has been synthesized.

TSCCCM single-crystals were grown from aqueous solution by slow cooling and slow evaporation methods. Solubility of TSCCCM has

been determined for various temperatures. The grown crystals were characterized by single-crystal X-ray diffraction (XRD) study, FTIR,

SHG test, UV–vis, and EDAX analyses. Single-crystal XRD study has been carried out to identify the lattice parameters. FTIR studies

confirm the functional groups present in the grown crystal. Optical transmission studies have confirmed that the grown crystals are highly

transparent. The SHG conversion efficiency of TSCCCM crystals has found 14 times higher than the KDP crystal. In TSCCCM crystal

structure, the planer p-organic molecules combine harmonically with inorganic distorted polyhedrons. The chlorine atoms in TSCCCM

must be involved in the coordinate polyhedra and have promoted the NLO property.

r 2007 Published by Elsevier B.V.

PACS: 77.84.�s; 78.20.Nv; 81.10.Dn

Keywords: A1. Growth from solution; A3. Semiorganic; B2. Non-linear optical crystal

1. Introduction

During the last few years, organic non-linear optical(NLO) crystals have attracted much interest due to theirsuperior properties over inorganic NLO materials, such ashigher susceptibility, faster response and the capability ofdesigning components on the molecular level. However,unlike inorganic NLO crystals, they have not come intowide use, owing to drawbacks such as the difficulty ofgrowing large perfect single crystals, poor physico-chemicalstability, and difficulties in cutting and polishing thedevices. Under these circumstances, crystals of complexesof organometallic materials with NLO effects which areexpected not only to retain high NLO effects, but also to

e front matter r 2007 Published by Elsevier B.V.

rysgro.2007.03.019

ing author. Tel.: +9144 22203571; fax: +91 44 22352870.

esses: rjvel@annauniv.edu, rjvel@hotmail.com

minimize some of the shortcomings of pure organic crystalshave been developed in other words, they have theadvantages of both organic and inorganic crystals in termsof their physico-chemical properties. This approach hasresulted in their practical use in frequency-doubling of laserradiation [1,2]. Since the theory of double-radical model(organic conjugated molecular groups are included inthe distorted polyhedron of coordination complex) wasbrought up in 1987 [3], metal–organic coordinationcompounds as NLO materials have attracted much moreattention for their considerable high NLO coefficients(contrast to inorganic materials), stable physico-chemicalproperties and better mechanical intension (contrast toorganic materials). With the guidance of this theory,many metal–organic coordination materials withgood NLO effect have been designed and synthesized[4–12]. The metal–organic coordination complexes can alsoprovide the following advantages: (i) an enhancement of

ARTICLE IN PRESSR. Sankar et al. / Journal of Crystal Growth 305 (2007) 156–161 157

the physico-chemical stability, (ii) the breaking up of thecentro-symmetry of the ligand in the crystal, and (iii) anincrease in NLO intensity, via metal–ligand bridginginteractions. The central metal ion (together with its hybridelectronic orbital) not only offers a certain anisotropic fieldto keep the NLO active chromopher ligands in afavourable acentric arrangement but also involved in theNLO processes.

Transition metal thiourea (TU), thiosemicarbazide(TSC), and thiocyanate (SCN) coordination complexesare potentially useful candidates for such organometallicsystems. As ligands with potential S and N donors, the TUmolecule and the SCN ion are interesting not only becauseof the structural chemistry of their multifunctionalcoordination modes, but also because of the possibility offormation of organometallic coordination complexes withNLO activities.

In the case of metalorganic coordination complexes, theorganic ligand is usually more dominant in the NLO effect.As for the metallic part, focus is on the group (IIB) metals(Zn, Cd, and Hg) as these compounds usually have a hightransparency in the UV region, because of their closed d10

shell. Regarding the organic ligands, small p electronsystems such as thiocyanate (SCN)�, urea [OC(NH2)2], andthiourea [SC(NH2)2] have been used with remarkablesuccess. These ligands and their metal (group IIB)complexes are always colourless. Potential NLO materialslike bis(thiourea) cadmium chloride (BTCC) [2], triallyl-thiourea cadmium chloride (ATCC) [2] are examples of thisapproach. The structural analysis and detailed studies ofthe physico-chemical behaviour of these materials have ledto the conclusion that the central metal cannot be ignoredin calculating the NLO coefficients [2]. In the presentinvestigation, attempt has been made to grow singlecrystals of TSCCCM by both slow evaporation and slowcooling methods. It mainly focuses on the synthesis andoptimized growth condition needed for growing relativelybig size crystals of TSCCCM.

30 35 40 45

7.5

9.0

10.5

12.0

13.5

Con

cent

rati

on (

gm/1

00m

l)

Temperature (°C)

Fig. 1. Solubility curve of TSCCCM.

2. Experimental procedure

2.1. Material synthesis

All the starting materials were of analytical reagent, andthe synthesis and growth process were carried out inaqueous solution. The TSCCCM has been synthesized bytaking cadmium chloride monohydrate and thiosemicar-bazide in a 1:1 stoichiometric ratio. Thiosemicarbazidecadmium chloride monohydrate salt was synthesizedaccording to the reaction:

CdCl2Cadmium chloride

þNH22NH2CS2NH2Thiosemicarbazide

! CdðNH2NHCSNH2ÞCl2 �H2OThiosemicarbazide cadmium chloride monohydrate

.

The calculated amount of cadmium chloride monohy-drate was first dissolved in the deionized water of resistivity

18.2MO/cm. Then thiosemicarbazide was added to thesolution slowly with stirring. The prepared solution was ledto dryness at room temperature. The purity of thesynthesized salt was improved by successive recrystalliza-tion process. Care was taken during heating of the solutionand a maximum temperature 80 1C was maintained inorder to avoid decomposition.

2.2. Solubility

The solubility test of TSCCCM in water was performedin the temperature range between 30 and 45 1C. Thetemperature of the solution was maintained above chosenconstant temperature and continuously stirred using amotorizer magnetic stirrer to ensure homogenous tempera-ture and the concentration throughout the volume of thesolution. The saturated solution was allowed to reach theequilibrium about 1 day at a chosen temperature and thenthe solubility was gravimetrically analysed. A sample ofTSCCCM solution was taken in a warm pipette andweighed. The solubility was estimated by evaporating the10ml of solution in an oven at a constant temperature. Thesame process was repeated for different temperatures andthe solubility curve was obtained as shown in Fig. 1.

2.3. Bulk growth of TSCCCM

Bulk growth of TSCCCM single crystal was carried outfrom aqueous solution by slow cooling technique, in aconstant temperature bath controlled with an accuracy of70.01 1C. Three hundred millilitres saturated solution wasprepared at 45 1C and then filtered to remove any insolubleimpurities. The seed obtained from slow evaporation wasemployed for the bulk growth. The solution was main-tained at 45 1C for 2 days before seeding. The temperaturewas reduced at a rate of 0.01–0.2 1C/day as growthprogressed. The period of growth ranged from 30 to 35

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Fig. 2. TSCCCM single crystal grown from (a) slow-cooling technique

and (b) slow-evaporation method.

R. Sankar et al. / Journal of Crystal Growth 305 (2007) 156–161158

days. The TSCCCM single crystal grown from slowcooling technique is shown in Fig. 2a. Fig. 2b showsTSCCCM crystals grown from slow evaporation method.All crystals had good compositional stability. Samples werestored at room temperature and at 60% relative humidityshowed no degradation after several months.

Fig. 3. (a) The ring-shaped conjugated plane arranged along the same

optimum direction in the organometallic crystal TSCCCM and (b) the

distorted octahedron structure of TSCCCM crystal.

3. Characterization studies

Single-crystal X-ray diffraction (XRD) analysis wascarried out using a Bruker AXS diffractometer with MoKa (l ¼ 0.770 A) radiation to identify the structure and toestimate the lattice parameter values. To confirm the NLOproperty, Kurtz powder SHG test was performed on thegrown crystals. The FTIR spectrum of TSCCCM crystalwas recorded in the range 400–4000 cm�1 employing aPerkin-Elmer spectrometer by KBr pellet method in orderto study the metal complex coordination. Linear opticalproperties of the crystals were studied using a ShimadzuUV–vis spectrophotometer. The energy dispersive X-raydiffraction (EDAX) analysis was employed to find thestoichiometric composition, which was done using INCA

200 system connected to a LEO-Stereo scan 440 scanningelectron microscope.

4. Results and discussion

Single-crystal XRD study of TSCCCM was performedwith a specimen of dimension 0.17� 0.21� 0.33mm3 cutfrom the as-grown crystals. A Bruker AXS diffractometerwas used to collect the intensity data using a Mo Karadiation. Least square refinement of 25 reflections wasdone in the range 20–301. From single crystal X-raydiffraction analysis, the lattice parameters of TSCCCMwere obtained as a ¼ 10.108 A, b ¼ 13.917 A, c ¼ 6.888 A,and b ¼ 124.071. Thus, TSCCCM crystallizes in mono-clinic system with non-centrosymmetric space group Cc.Fig. 3a shows the infinite chain structure of TSCCCM,

which can be expressed as [CdCl2CH5N3S]a with theapproximate planar CdCl2(CH5N3S) as the unit of thechain (the five non-hydrogen atoms of the organic ligandsare in a plane) and these ligand planes are parallely stackedalong chain direction. The distorted octahedron coordina-tion consists of Cd at centre and three Cl, two S and one Nat six vertices. In this organometallic TSCCCM crystal

ARTICLE IN PRESSR. Sankar et al. / Journal of Crystal Growth 305 (2007) 156–161 159

structure, the planar p-organic molecule combines harmo-nically with inorganic distorted polyhedron. Also thedistorted octahedron has been realized as mentioned inthe double-radical structure model. TSC molecules haveused two coordination atoms, N and S, to chelate with thecentral Cd ions and form the conjugated ring-shaped plane.The whole crystal is linked as a distorted octahedralstructure by S and Cl with central atom, Cd. Fig. 3 showsthe planar conjugated ring and the octahedral structure inthe TSCCCM crystal. The structure consists of infantchains of TSCCCM parallel to a-direction as shown inFig. 3a. Each chain elements is an octahedron with centralCd-atoms and vertices occupied by three Cl�, two S� andone N atoms. It is slightly distorted so as to accommodatethe geometrical constraints of the thiosemicarbazideentity (Fig. 3b). The link between consecutive octahedrais given by each Cl� and S atoms. The cohesion of thestructure (Fig. 3b) normal to the chain axis is due tohydrogen bonds between water molecule and the N atomsof thiosemicarbazide.

The FTIR spectral analysis of Cd[NH2C(S)NHNH2]Cl2 �H2O was carried out between 4000 and 450 cm�1. Theobserved spectrum is shown in Fig. 4. In the high-energyregion, there is a broad band between 2100 and 3500 cm�1.Intense sharp peak was observed at 3174 cm�1 due to O–H(–H2O) vibration. The involvement of NH3

+ in hydrogenbonding is evident by the fine structure of the band in thelower-energy region. The bands appear in the region 1649and 1610 cm�1 is assigned for n(CQS). The peak at1649 cm�1 is due to asymmetrical NH3

+ bending mode. Thewell-resolved sharp peak at 1460 cm�1 is due to symme-trical NH3

+ bending. Since the stretching vibrations of theOH bonds in water molecules and the deformationvibrations of the water molecules fall into the region ofthe deformation vibrations of NH2

+ and NH3+ groups, it is

difficult to determine the presence or absence of watermolecules on the basis of the FTIR spectrum alone. Thisband is shifted to 1613 cm�1 region in the case ofcomplexes indicating that sulphur is involved in metalcoordination. n(NH2) is absent in the region 2063 cm�1,which disappears in the case of complexes, revealing thatamino-nitrogen is one of the coordinate sites. The band at

50.0

100.0

150.0

%T

500.01000.01500.02000.03000.04000.0

1/cmTSCCG

482.2

684.7

752.2

1085.8

1205.4

1379.0

1460.0

1610.4

1649.0

2368.4

3174.6

3388.7

3475.5

Fig. 4. FTIR spectrum of TSCCCM single crystal.

1001 cm�1 is due to n(N–N) vibration which resists a readshift 1085 cm�1 in the complexes. Reduction of lone pairand bonded pair repulsion between N–N bonds uponcomplex may be due to sulphur. An intensive band near1610 cm�1 and bands in the region 1379–1085 cm�1 areclose to absorption bands of thiosemicarbazide. Theformer band connected with deformation vibrations ofNH2 and NH groups and the latter may be assigned tovalence vibrations of CQS and C–N groups. The narrowbands at 684, 752, and 1085 cm�1 and wide splitt bands at1460, 1379, and 1205 cm�1 correspond to the vibrations ofCl� groups of n1, n2, and n4, respectively. The complex islikely to coordinate with a tetrahedra CdN2S core.The stoichiometric composition of the as-grown

TSCCCM single crystal was determined by energydispersive analysis (EDAX) using XRD INCA 200 systemconnected to a LEO-Stereo scan 440 scanning electronmicroscope. The presence of nitrogen, chlorine, sulphur,and cadmium in the crystal were identified by scanning atseveral points. The EDAX measurements confirmed theabsence of any impurities such as Fe, Na, and Si in theTSCCCM single crystal. Fig. 5 shows the EDAX spectrumof TSCCCM single crystal. The peaks attributed to thepresence of Cd, Cl, N, and S at different energies isdepicted. From the chemical and EDAX analyses, theempirical formula for the crystal was determined to beCd(SC(NH2)NHNH2)Cl2. The structure determinationcarried out on the crystal revealed the molecular formulato be Cd(SC(NH2)NHNH2)Cl2.The UV–vis–NIR spectrum gives information about the

structure of the molecule because the absorption of UVand visible light involves the promotion of the electron in sand p orbitals from the ground state to higher energystates. Transmission spectral analysis is important for anyNLO material because a NLO material can be of practicaluse only if it has wide transparency window. To find thetransmission range of TSCCCM, the optical transmissionspectrum was observed for the wavelength between 200 and1200 nm. A crystal thickness 3mm was used for thismeasurement. A graph of transmission vs. wavelengthignoring the loss due to reflection is shown in Fig. 6.From the graph, it is evident that TSCCCM crystal has UV

Fig. 5. EDAX study of TSCCCM crystal.

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300 400 500 600 700 800 900 1000

0

20

40

60

80

100

Tra

nsm

ittan

ce %

TSCCC

Wavelength (nm)

Fig. 6. The transmittance spectrum of TSCCCM single crystal.

Table 1

SHG efficiencies of TSCCCM, organic and inorganic crystals relative to

potassium dihydrogen phosphate (KDP) equaling 1.0

Compound NLO efficiency Transparency range (nm)

KDP 1.0 200–1500

TSCCCM 14 280–1200

Urea 10 210–1400

BBO 28 198–3300

DLAP 40 220–1950

KTP 215 350–4500

Table 2

Laser damage threshold value of TSCCCM and its comparison with

different organic and inorganic NLO crystals

S. no. Crystals Laser damage threshold (mW/cm2)

1 Sodium para-nitro phenalate 52.37

2 BTCC 50.03

3 LiNbO3 100

4 L-Arginine hydrofluoride 120.31

5 KDP 200

6 TSCCM 725

7 Urea 1500

8 BBO 5000

9 DLAP 13000

R. Sankar et al. / Journal of Crystal Growth 305 (2007) 156–161160

cut-off below 280 nm, which is sufficient for SHG laserradiation of 1064 nm or other applications in the blueregion.

The most widely used technique for confirming the SHGfrom prospective second-order NLO material is the Kurtzpowder technique [13]. In addition to identify the materialswith non-centrosymmetric crystal structure, it is also usedas a screening technique to identify the materials with thecapacity for phase matching. The SHG intensity from thematerial is measured as a function of particle size. TheSHG intensity increases with the increase of particle sizeand remains essentially constant at a particle size greaterthan the coherence length confirming the phase matchingbehaviour of the material [14–16]. Powder SHG measure-ment was carried out for TSCCCM following theKurtz and Perry technique with 1064 nm laser radiation.A Quanta Ray of Nd:YAG laser producing pulses with awidth of 10 ns and a repetition rate of 10Hz was used.Because the SHG efficiency has been shown to dependstrongly on the particle size, crystals of TSCCCM weregrounded separately and sieved into different particle sizeranges below 106, 106–125, 125–150micron and above150micron. All the powdered samples were tightly packedin the separate microcapillary tubes of uniform diameter(1.5mm). The input laser energy incident on the capillarytube was chosen as 6mJ. The SHG was confirmed by theemission of green radiation (532 nm) and the optical signalwas collected by a photomultiplier tube (PMT). The opticalsignal incident on the PMT was converted into voltageoutput at the CRO. The TSCCCM shows a powder SHGefficiency of 14 times that of standard NLO material KDPand the measurement of SHG output at various particlesizes show the increasing SHG intensities with increasingparticle sizes, thus proving the phase matching property ofTSCCCM. The results were compared with the other NLOmaterials as shown in Table 1.

The laser damage threshold measurement was made onTSCCCM single crystals using a Q-switched Nd:YAG laser

for 20 ns laser pulses operating in (TEM00) mode atwavelength of 1064 nm. The laser beam divergence was2.5mrad. The output intensity of the laser was controlledwith a variable attenuator and delivered to the test samplelocated at the near focus of the converging lens. The lenswith a focal length of 5 cm was used, and it is useful insetting the spot size to the desired value. The sample wasmounted on the goniometer, which was used to positionthe differences in the beam. During laser radiation, thepower meter records the energy density of the input laserbeam for which the crystal gets damaged. Single andmultiple laser damage measurements were made on thepolished face of the grown crystal. The energy density wascalculated by using the following formula: energydensity ¼ E/A (MW/cm2), where E is the input energy inmillijoules and A is the area of the circular spot size. In thepresent study, the laser damage threshold energy densitywas found to be 725MW/cm2. The laser damage thresholdvalue is lower when the crystal is subjected to multipleshots. It reveals the fact that the single-shot damagethreshold is higher than the multiple-shot damage. Apartfrom the thermal effect, multiphoton ionization is animportant cause of laser-induced damage. For the pulsewidths of several nanoseconds, the thermal effects areunavoidable, while for the picosecond pulse widths, thethermal effects are negligible. This is because the thermaleffects take several nanoseconds to build up and could takeseveral milliseconds to decay. The observed damagethreshold value is greater than that of standard KDP andother known organic single crystals [17–19]. The damagethreshold value of TSCCCM was compared with otherNLO materials as shown in Table 2.

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5. Conclusions

A new semiorganic NLO material, TSCCCM, has beensynthesized and crystals were grown by slow-cooling andslow-evaporation methods. The solubility of TSCCCMwas estimated for different temperatures and it indicatesthe low solubility in water. The lattice parameter valueshave been evaluated by single-crystal XRD analysis. Thestructure consists of infant chains of TSCCCM parallel tothe a-direction. Each chain element is an octahedron withcentral Cd-atom and the vertices are occupied by three Cl�,two S� and one N atoms. In the TSCCCM crystal, thechlorine atom must be involved in the coordinatepolyhedra and some how promotes the NLO effect. TheSHG conversion efficiency of TSCCCM powder is 14 timeshigher than that of the KDP crystal. The melting point ofTSCCCM (230 1C) is higher than that of LAP crystal(140.8 1C). The non-linearity of TSCCCM may be due toan organic and inorganic ring structure of the complexcompound including the cadmium ion. The presence offunction groups, stoichiometric composition, transmit-tance, and thermal behaviour of TSCCCM crystal wereanalysed by FTIR spectra, EDAX, UV–vis, and thermalstudies, respectively. These studies reveal that TSCCCM isa potential material for frequency conversion applications.

Acknowledgements

One of the authors (R.S.) acknowledges the All IndiaCouncil for Technical Education (AICTE), New Delhi, forthe award of National Doctoral Fellowship (NDF). Theauthor is thankful to Prof. P.K. Das, Department of

Inorganic and Physical Chemistry, Indian Institute ofScience, Bangalore, India, for providing laser facilities.

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