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Comparative study on BIS thiourea cadmium acetate crystals using HRXRD,etching, microhardness, UV–visible and dielectric characterizations

V. Ganesh a,b, Ch. Snehalatha Reddy b, Mohd. Shakir c,d, M.A. Wahab c,G. Bhagavannarayana d, K. Kishan Rao b,n

a Department of Physics, Ganapathy Engineering College, Warangal-506005 Indiab Department of Physics, Kakatiya University, Warangal-506009 Indiac Crystal Growth Lab, Department of Physics, Jamia Millia Islamia, New Delhi-110025 Indiad Materials Characterization Division, National Physical Laboratory, New Delhi-110 025 India

a r t i c l e i n f o

Article history:

Received 26 June 2010

Received in revised form

26 October 2010

Accepted 27 October 2010

Keywords:

Optical materials

Crystal growth

Defects

Mechanical properties

a b s t r a c t

/1 1 1S oriented bis thiourea cadmium acetate (BTCA) crystal of diameter 15 mm and length 45 mm was

grown for the first time by the unidirectional Sankaranarayanan–Ramasamy (SR) method. The

conventional and SR method grown BTCA crystals were characterized by using high-resolution X-ray

diffraction (HRXRD), chemical etching, Vickers microhardness, UV–vis, dielectric studies and differential

scanning calorimetry. The HRXRD analysis indicates that the crystalline perfection of SR method grown

crystal is good without having any low angle internal structural grain boundaries. The transmittance of SR

method grown BTCA is 14% higher than that of conventional grown crystal. The dielectric constant was

higher and the dielectric loss was less in SR method grown crystal. The crystals grown by SR method

possess less dislocation density and higher microhardness.

& 2010 Elsevier B.V. All rights reserved.

1. Introduction

In the recent period, search for new nonlinear optical (NLO)materials has escalated because of their applications like secondharmonic generation (SHG), frequency mixing, electro optic mod-ulation, optical parametric oscillation, etc. [1]. For these applica-tions some times we need good quality, large size and defect freecrystals, which can be possible by a new technique calledSankaranarayanan–Ramasamy (SR) method [2,3]. The search fornew and efficient NLO materials has resulted in the development ofnew class of materials called semiorganics, which are superior toorganic and inorganic NLO materials [4,5]. Another importantadvantage of semiorganic materials is the high resistance to laserinduced damage. Thiourea molecules play an important role in thegrowth of nonlinear optical crystals like bis thiourea cadmiumchloride (BTCC) [6], bis thiourea zinc acetate (BTZA) [7], tristhiourea zinc sulphate (ZTS) [8]. Bis thiourea cadmium acetate(BTCA) is an efficient semiorganic NLO compound under thioureacategory [7], whose SHG efficiency is superior to KDP [9]. BTCAbelongs to the orthorhombic crystal system with space groupP212121 [9]. Few reports are available on growth, X-ray diffraction(XRD) and spectral studies on pure [10] and doped [11] BTCA

crystals. All these studies were confined to crystals grown byconventional slow evaporation technique.

The aim of the present communication is to report the growth oflarge size BTCA crystal by SR method for the first time and itscharacterization using HRXRD, chemical etching, Vickers micro-hardness, UV–vis, dielectric and DSC studies. Further, the resultsobtained are compared with that of crystals grown by conventionalslow evaporation method.

2. Experimental

2.1. Synthesis

BTCA was successfully synthesized using commercially avail-able AR grade thiourea and cadmium acetate substances in thestoichiometric ratio 2:1 [10] according to the following reaction:

(CH3COO)2Cd+2[CS(NH2)2]-Cd[CS(NH2)2]2(CH3COO)2

The synthesized salt was dissolved in deionized double distilledwater. The obtained product was purified by repeated recrystalli-zation before it was used for crystal growth. As meager informationis available in literature on solubility of this crystal, the solubility ofBTCA is determined in the temperature range 25–50 1C in theinterval of 5 1C. The solubility data of this crystal was prepared bythe method described earlier [12] and is shown in Fig. 1.

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0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved.

doi:10.1016/j.physb.2010.10.061

n Corresponding author. Tel.: +91 9866275026.

E-mail address: kishankotte@yahoo.co.in (K. Kishan Rao).

Physica B 406 (2011) 259–264

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In the present work, all the crystal growth experiments werecarried out using a constant temperature bath of accuracy 70.5 1C.The saturated solutions of BTCA of about 500 ml prepared at 35 1Care uniformly transferred into four 100 ml beakers and are allowedfor slow evaporation. Self nucleated seed crystals are allowed togrow on the base of the beakers. The rate of evaporation wascontrolled by fixing perforated lids on the beakers. Good qualitysingle crystals of dimensions 15�6�5 mm3 were obtained in aperiod of 30–40 days (shown in Fig. 2a).

The SR growth experimental setup in the present work is similarto that reported earlier [10]. It essentially consists of a glassampoule of diameter 15 mm and length 150 mm. A (1 1 1) orientedseed crystal collected from the conventional method has been cutand polished suitably to fix at the bottom of the ampoule. Sufficientcare has been taken to see that (1 1 1) face points upwards. Afterascertaining this, already prepared saturated solution is slowlytransferred into the ampoule. Now the ampoule is kept in a waterbath of size 30�30�40 cm3 made up of glass, which holds andprotects it. The ring heaters are positioned at the top and bottom ofthe ampoule. They are connected to dual channel temperaturecontroller, which maintains temperature constancy of 70.05 1C. Inthe present work, the temperature around the growth region andtop of the ampoule is maintained at 35 and 40 1C, respectively. Ithas been observed that good quality /1 1 1S oriented BTCA crystalof diameter 15 mm and length 45 mm is obtained in a period of 40days. To achieve a constant growth rate, the reduced solvent due toevaporation was compensated by adding freshly prepared solution.The grown crystal was carefully removed after cutting the ampoulewith a diamond cutter.

Fig. 2a and b shows the photograph of BTCA crystals grown byconventional and SR methods. The average growth rate for con-ventional and SR method grown crystals along /1 1 1S directionare 0.13 and 1.12 mm/day, respectively. The reason for highergrowth rate of the crystals grown by SR method appears to be theestablishment of temperature gradient, which helps in creating asuitable concentration gradient between top and bottom of theampoule [2]. Fig. 2c shows the slices of the SR grown crystal aftercutting and polishing with fine grade polishing paper.

3. Results and discussions

3.1. Powder X-ray diffraction studies

Powder X-ray diffraction pattern of the BTCA crystal has beenrecorded using PW1830 Philips analytical X-ray diffractometer

with CuKa radiation (35 kV, 30 mA). The lattice parameters werecalculated using Treor software with two theta values as the inputdata. The X-ray diffraction studies confirmed that the material

20 30 40 50 6030

35

40

45

50

55

60

65

Temperature (°C)

Con

cent

ratio

n (g

/100

mL

)

Fig. 1. Solubility curve of BTCA.

Fig. 2. BTCA crystals grown by (a) conventional method, (b) SR method and (c) cut

and polished ingots.

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crystallizes in orthorhombic class with space group of P212121 withcell parameters as a¼7.57 A, b¼11.88 A, and c¼15.71 A, which arein good agreement with the reported data [10,11].

3.2. High-resolution X-ray analysis

The crystalline perfection of the grown single crystals wascharacterized by HRXRD by employing a multicrystal X-ray dif-fractometer developed at NPL [13]. The arrangement and themethod of recording is similar to that reported earlier [2]. Fig. 3ashows the high-resolution diffraction curve (DC) recorded for atypical conventional grown BTCA single crystal specimen using(1 1 1) diffracting planes in symmetrical Bragg geometry. The solidline (convoluted curve) is well fitted with the experimental pointsrepresented by the filled circles. On deconvolution of the diffractioncurve, it is clear that the curve contains an additional peak, which is5600 away from the main peak. This additional peak depicts aninternal structural very low angle (tilt angle, ar10 of arc) boundary[14] whose tilt angle [misorientation angle, a (please see the insetin the Fig. 3a) between the two crystalline regions on both sides ofthe structural grain boundary] is 5600 from its adjoining region. TheFWHM (full width at half maximum) of the main peak and the verylow angle boundary are 23 and 9200 respectively. Though thespecimen contains a very low angle boundary, the relative lowangular spread of around 20000 of the diffraction curve and the low

FWHM values show that the crystalline perfection is reasonablygood. The effect of such very low angle boundaries may not be verysignificant in many device applications, but for applications likephase matching, it is better to know these minute details regardingcrystalline perfection. Thermal fluctuations or mechanical distur-bances during the growth process could be responsible for theobserved very low angle boundary. It is mentioned here that suchvery low angle boundaries could be detected with resolved peaks inthe diffraction curve only because of the high-resolution of themulticrystal X-ray diffractometer used in the present studies.

Fig. 3b shows the DC for a typical SR grown BTCA single crystalspecimen using (1 1 1) diffracting planes recorded under identicalconditions. As seen in the figure, the DC is quite sharp without anysatellite peaks. The full width at half maximum of the diffractioncurves is 2200, which is very close to that expected from the planewave theory of dynamical X-ray diffraction [15]. The single sharpdiffraction curve with low FWHM indicates that the crystallineperfection is reasonably good without having any internal struc-tural grain boundaries, which indicates that crystal grown by SRmethod possess better quality than grown from conventionalmethod.

3.3. Chemical etching studies

For fabrication of devices we need good quality crystals withminimum defects. The quality of the grown crystals is usuallyassessed by knowing the imperfections, particularly dislocations incrystals. Dislocations influence a number of physical properties likeplasticity, mechanical strength, etc. Hence it is necessary to knowthe density and distribution of dislocations in a crystal. For thispurpose chemical etching technique has been employed to studydislocations on as-grown (1 1 1) faces of these crystals by usingMagnus MLX microscope fitted with Motic (1 0 0 0) camera. Asmeager information is available on etching studies of these crystals[16], a number of enchants were tried and good etching action wasrevealed by methanol+CdCl2. Successive etch–reetch experimentsconfirm that the etch pits are formed at dislocations sites only.Fig. 4a and b shows the etch pattern on (1 1 1) face of BTCA crystalobtained from conventional and SR methods, respectively. Theshape of the etch pits is more or less squares slightly elongatedalong the diagonal. The average dislocation density is 4.5�103/cm2

and 3.1�102/cm2 for conventional and SR grown slices, respec-tively. These studies suggest that the crystals grown by SR methodare of better quality with less dislocation density.

3.4. Microhardness studies

The structure and molecular composition in crystals greatlyinfluence mechanical properties. Microhardness testing is one ofthe simplest and best methods to understand the strength of thematerials. Hardness of a material is a measure of resistance offeredby the lattice for permanent deformation. Microhardness measure-ments were made on as-grown (1 1 1) faces of BTCA crystals usingLeitz–Wetzlar hardness tester fitted with a Vickers diamondindenter. Hardness values Hv are calculated from the expression,

Hv ¼ 1:854P=d2 ð1Þ

where P is the load applied in Kg and d is the diagonal length in mm.The variation of hardness Hv with load P ranging from 10 to 120 g forboth the crystals is illustrated in Fig. 5. The Hv value increases initiallyup to a load of 30 g, beyond this load; Hv decreases upto 60 g andthereafter attains a load independent value. Similar kind of behavior isobserved for PbS and BaFCl [17]. The load independent hardnessvalues for conventional and SR grown crystals are 69 and 75 Kg/mm2,respectively. In the present study, it is important to note that at all

0

250

500

750

1000

Diff

ract

ed X

-ray

inte

nsity

[c/s

]

Glancing angle [arc s]

22"

BTCA (SR) (111) Planes MoKα1(+,−,−,+)

0

100

200

300

400

500

Diff

ract

ed X

-ray

inte

nsity

[c/s

]

Glancing angle [arc s]

56"

23"

92"

BTCA(conventional)(111) Planes MoKα1(+,−,−,+)

-200 -100 1000 200

-200 -100 1000 200

Fig. 3. High-resolution X-ray diffraction curves recorded for BTCA single crystal

grown by (a) conventional and (b) SR method.

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loads, the crystals grown by SR method show higher hardness valuesthan crystals grown conventionally. Such a trend has been observedby earlier workers [18,19] on other conventional and SR growncrystals. When the material is deformed by the indenter, dislocationsare generated near the indentation site. The major contribution to theincrease in hardness is attributed to the high stress required forhomogenous nucleation of dislocations in the small dislocation-freeregion indented [20]. Hence, the higher hardness value of SR growncrystal appears to be due to greater stress required to form disloca-tions and also absence of liquid inclusions. The load variation can beinterpreted by using Meyer’s law,

P¼ Adn ð2Þ

where A is a constant and n is the Meyer’s index (or work-hardeningco-efficient). The value n is determined from the slope of the plotsbetween ln P vs ln d. From these plots, the estimated values of n are2.28 and 2.35 for conventional and SR grown crystals respectively.According to Onistch [21] and Hanneman [22], the value of n is 1–1.6for hard materials and above 1.6 soft materials. Thus our crystalbelongs to moderately harder category.

3.5. Optical transmittance studies

The UV–vis transmittance spectra of conventional and SRmethod grown BTCA crystal along /1 1 1S are recorded in thewavelength region 200–650 nm by Perkin-Elmer Lambda 25

spectrophotometer. The crystals are transparent in the entireUV–vis region with a lower UV cut off wavelength at 230 nm(Fig. 6). From the spectrum, it is observed that conventional and SRgrown BTCA crystals have transmittance up to 45% and 59%respectively in the higher wavelength region. This indicates theimproved transparency of the BTCA crystal grown by SR method.The reason for higher transparency of SR grown crystals appears tobe due to absence of inclusions.

3.6. Dielectric studies

The dielectric study was carried out using an impedanceanalyzer (Model 4284A). Silver paste was coated on both sides ofthe transparent good quality BTCA crystals and then placedbetween two copper electrodes to form the parallel plate capacitor.The sample dimensions 6�4�1.5 mm3 was used for the dielectricmeasurements. The dielectric constant (er), dielectric loss (tan d)and ac conductivity (sac) of the conventional and SR grown crystalsalong /1 1 1S direction were measured in the frequency range of100 Hz to 2 MHz at room temperature. From Fig. 7a it is observedthat the dielectric constant has high values in the lower frequencyregion and then decreases with increase in frequency. Further, thedielectric constant in the SR method grown crystal is higher thanthe conventional grown crystal. Fig. 7b represents the plot of thedielectric loss versus applied frequency, from this figure it is clear

50 μm 50 μm

Fig. 4. Etch pit pattern on BTCA crystals grown by (a) conventional method and (b) SR methods. (etching time �2 s).

0 25 50 75 100 125 15040

50

60

70

80

90

Conventional grown BTCASR grown BTCA

P (g)

Hv

(Kg/

mm

2 )

Fig. 5. Plot of Vickers hardness against load for (1 1 1) BTCA crystals.

200 300 400 500 600 7000

10

20

30

40

50

60

70

Conventional grown BTCA

SR grown BTCA

Wavelength (nm)

%T

Fig. 6. UV–vis spectral analysis of BTCA crystals.

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that tan d value for SR grown crystal is much less than conventionalgrown crystals. At low frequencies the dipoles can easily switchalignment with the changing fields. As frequency increases thedipoles are able to rotate less and maintain phase with the field;thus they reduce their contribution to the polarization field; andhence the observed reduction in dielectric constant and dielectricloss. The characteristic of low dielectric loss with high frequency forthe sample suggest that the crystal is almost free from structuraldefects which is in tuned with HRXRD results. Fig. 7c shows the acconductivity vs applied frequency, the sac has been evaluated fromthe following formula

sac ¼ 2pue0ertand ð3Þ

where e0 is the vacuum dielectric constant, er the relative dielectricconstant for the BTCA crystal and u is the frequency of applied acfield. From Fig. 7c it is clear that sac is almost zero up to 500 kHz for

SR and 100 kHz for conventional grown crystals and then increasessharply for higher frequencies suggesting that this parameter playsa vital role for nonlinear optical material.

3.7. DSC studies

In the present study, DSC of BTCA (15.050 mg of sample) hasbeen employed using Mettler Toledo in the temperature range 25–500 1C at a heating rate of 10 1C/min in the Nitrogen atmosphere(Fig. 8). The DSC thermogram shows a sharp endothermic peak at154.8 1C, signifying the melting point of the crystal. Further, thesharpness of the peak indicates good degree of crystalline perfec-tion of the sample.

4. Conclusions

/1 1 1S directional BTCA crystal of 15 mm diameter and45 mm length was successfully grown by SR method with anaverage growth rate 1.12 mm/day. The HRXRD study indicates thatthe grown crystal do not have any internal structural grainboundaries. The SR method grown /1 1 1S BTCA has 14% highertransmittance as against conventional method grown crystal. Thedielectric constant was higher and dielectric loss was less in SRmethod grown crystal when compared to conventional growncrystal. The crystal grown by SR method has much higher hardnessvalue than conventional method grown crystals. Etch pit density isless in the SR method grown crystal compared to conventionalmethod grown crystals. DSC studies suggest that these crystals arethermally stable upto 154.8 1C.

Acknowledgement

The authors thank UGC—New Delhi, for the financial assistanceunder DRS-SAP.

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conductivity (ac) of conventional and SR grown BTCA single crystals.

Fig. 8. DSC curve of BTCA crystal.

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