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CHAPTER - V

SYNTHESIS, GROWTH AND CHARACTERIZATION OF METAL

ORGANIC SINGLE CRYSTALS

5.1 INTRODUCTION

he search for new NLO crystals which should have desired property

for device application in the aspect of high performance and easy to

grow optical quality with high NLO susceptibility continues. To bridge the

outstanding properties between organic and inorganic NLO crystals, attempt was

made to distribute π conjugate ligands like thiocyanate, thiourea, urea, allyl thiourea

and thiosemicarbazide in the lattice of inorganic salts (Zn, Cd, Hg, and Mn) to

produce metal organic NLO crystals [1]. Materials thus prepared are classified as semi

organics also called metal organics. Electron withdrawing ligands coordinated to the

metal increase the electro negativity of the metal, whereas electron donating ligand

decreases the electro negativity of the metal atom.

Choosing the suitable ligands and metal atoms, large differences in the electro

negativities can be introduced to assist in tuning the large NLO property resulting in

various metal-organic crystals. Metal organics have two kinds of charge transfer

mechanism; metal to ligand and ligand to metal apart from delocalization of

π-conjugate electrons of metal organic crystals. Metal organic crystals like bisthiourea

cadmium chloride (BTCC) [2], allylthiourea cadmium chloride (ATCC) [3], tris

(thiourea) zinc sulphate (ZTS) exhibited deff coefficients greater than of KDP.

T

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CHAPTER-V | Synthesis, Growth and Characterization of Metal Organic Single Crystals

Bimetallic thiocyanate complex NLO crystals have attracted much interest

because of their potential for combining high nonlinearity and chemical flexibility of

organics with physical ruggedness of inorganic. Zinc cadmium thiocyanate (ZCTC),

cadmium mercury thiocyanate (CMTC), manganese mercury thiocyanate (MMTC)

and its adduct complex crystals, tetrathiourea cadmium tetrathiocyanate zincate

(TCTZ), tetrathiourea mercury tetrathiocyanate manganate (TMTM) and manganese

mercury thiocyanate glycol monomethyl ether (MMTG) were identified to posses

large NLO properties.

In this investigation, single crystals of glycine zinc cadmium thiocyanate

(GZCTC) and cadmium manganese thiocyanate (CMTC) have been synthesized and

single crystals were grown with optimized growth conditions. The structural,

functional, optical, mechanical and SHG characteristics of the grown crystals have

been studied systematically.

5.2 REVIEW OF LITERATURE

X.Q.Wang et al (1999) reported the synthesis of non linear optical crystals of

coordination complex, zinc cadmium tetrathiocyanate (ZCTC) from aqueous solution.

The structural, optical and thermal properties were determined [4]. S.Guo et al (2000)

have been reported the growth of cadmium mercury thiocyanate dimethylsulphoxide

(CMTD) by temperature lowering method. The crystal structure has been determined

and relation between crystal structure and nonlinear optical properties were discussed

[5]. G.W.Lu et al (2001) have been investigated the Raman scattering of zinc

cadmium tetra thiocyanate (ZCTC) [6].

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G.Zhang et al (2001) have generated the violet 404 nm light by frequency

doubling of GaAlAs diode laser using a metal organic complex crystal ZCTC and its

refractive index and second harmonic generation phase matching angles are reported

[7]. X.Q.Wang et al (2001) have been reported the crystal growth and physical

properties of zinc cadmium thiocyanate (ZCTC). The dielectric constants,

piezoelectric strain constants, electro-optic co-efficient and direct current resistivities

have also measured [8]. X.Wang et al (2001) have grown the zinc cadmium

thiocyanate crystal by slow solvent evaporation [9].

Xue Ning Jiang et al (2001) have been grown single crystals of zinc cadmium

thiocyanate (ZCTC) as a source of blue-violet light by laser diode frequency doubling

from the NH4Cl.NH4SCN.H2O mixed solvent by using the solvent evaporation

method [10]. X.Q.Wang et al (2002) reported the growth of organometallic material,

bis(dimethyl sulfoxide) cadmium thiocyanate (DSTC) from aqueous solution. The

elemental analysis, X-ray powdered diffraction, Raman, infrared and optical

transmission spectroscopy and thermal analysis of the grown crystals were done [11].

The vibrational spectra of the zinc cadmium thiocyanate (ZCTC) single crystal by

Raman spectroscopy and ab initio calculation with the molecular orbital (MO) theory

using a GAUSSIAN 98 program has been studied by X.Q.Wang et al (2002) [12].

X.Q.Wang et al (2002) reported that six kinds of macro defects found in the single

crystals of zinc cadmium thiocyanate. The morphology of the crystals was indexed.

The morphologies and distribution regularities of these defects were observed and

analyzed using optical microscopy. Their formation mechanisms and the methods of

eliminating these defects are discussed [13].

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X.N.Jiang et al (2002) discussed the morphology of the (1 0 0) face of zinc

cadmium thiocyanate crystal before and after its growth and observed highlands

formed by chains of small growth hillocks generated by spiral dislocation sources and

small cavities adjacent to the hillocks [14]. G.H.Zhang et al (2004) have studied the

effect of post growth thermal treatment on optical figures of merit of ZCTC crystal. It

was established that annealing with nitrogen gas improved bulk transparency and

homogeneity of the samples and decreased the volume and surface optical loss, and

increased the materials homogeneity [15]. K.Rajarajan et al (2006) reported the

growth of single crystals of bis(dimethylsulfoxide) tetrathiocyanato cadmium(II)

mercury(II) (CMTD) from a DMSO-water mixed solvent by slow solvent evaporation

technique [16].

Ginson.P.Joseph et al (2006) studied the growth of single crystal of

manganese mercury thiocyanate (MMTC) at room temperature by slow evaporation

technique. The fundamental characterizations were also investigated [17].

S.Gunasekaran et al (2006) have been grown the cadmium magnesium tetra

thiocyanate crystal by solution growth technique at room temperature. The structural,

functional, optical, dielectric and thermal properties were studied [18]. P.Nisha

Santha Kumari et al (2007) investigated the growth of zinc cadmium thiocyanate

(ZCTC) single crystal in silica gel using gel technique by the process of diffusion

[19]. K.Rajarajan et al (2007) have been grown tetrathiourea mercury (II)

tetrathiocyanato manganate (II) (TMTM) by slow evaporation technique. Structural,

functional, optical and thermal characterizations were also investigated [20].

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P.Nisha Santha Kumari et al (2008) investigated the magnesium doped

cadmium mercury thiocyanate crystal from silica gel by the process of diffusion and

comparative study with pure are discussed [21]. A.Bhaskaran et al (2008) reported

the synthesis of non linear optical tetrathiourea cadmium tetrathiocyanato zincate

(TCTZ) materials and single crystals were grown by the slow evaporation method in

an aqueous solution [22]. S.M.Ravi Kumar et al (2009) have been synthesized the

mercury cadmium chloride thiocyanate (MCCTC) in water methanol mixed solvent.

Optically good quality crystal was grown by slow evaporation technique under

optimized conditions [23]. Growth aspects of Ba and Ca doped cadmium mercury

thiocyanate (CMTC) single crystals from silica gel by the process of diffusion are

discussed by S.Kalainathan et al (2009) [24].

C.M.Raghavan et al (2009) have been studied the growth of cadmium

mercury tetrathiocyanate single crystals from acetone water (4:1) mixed solvent by

slow evaporation solution technique and studied their characterizations [25]. Ginson

P.Joseph et al (2009) grown manganese mercury thiocyanate bis-dimethyl sulfoxide

single crystal by slow evaporation method and studied the electrical, linear and

nonlinear optical properties [26]. T.Rajesh Kumar et al (2010) reported the growth

of bimetallic SCN ligand based single crystals of manganese mercury thiocyanate

(MMTC), cadmium mercury thiocyanate (CMTC) and zinc cadmium thiocyanate

(ZCTC) grown by slow solvent evaporation technique. The growth mechanism and

surface features are investigated by optical microscopic techniques such as scanning

electron microscopy (SEM) and atomic force microscopy (AFM) [27].

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P.Paramasivam et al (2011) have been synthesized cadmium manganese

thiocyanate (CMTC) and single crystals were grown by slow evaporation solution

growth technique using water as solvent [28]. T.Rajesh Kumar et al (2011) grown

the single crystals of bimetallic MMTC by slow cooling method and investigated the

second and third order optical nonlinearities. The influences of SCN ligand in

modifying the NLO properties are discussed and compared with other organometallic

crystals [29]. X.Liu et al (2011) have been grown the single crystals of manganese

mercury thiocyanate MMTC by the solvent evaporation method. The growth habit of

MMTC crystal crystallized under different conditions was investigated by means of

micro crystallization method. Various defects were found and formation mechanisms

and the methods to eliminate these defects are discussed [30].

5.3 GLYCINE ZINC CADMIUM THIOCYANATE (GZCTC) CRYSTA L

5.3.1 Synthesis

Glycine zinc cadmium thiocyanate (GZCTC) compound was synthesized by

two steps using GR grade potassium thiocyanate, zinc chloride, cadmium chloride and

glycine as raw materials. In the first step, an aqueous solution of glycine

(NH2CH2COOH) and zinc chloride (ZnCl2) were mixed and the resulting solution was

stirred for 30 minutes. Then the solution was maintained at 50 °C for solvent

evaporation and then glycine zinc chloride material was precipitated at the solution

vessel and filtered off. The precipitated glycine zinc chloride was purified by

recrystallization process.

( ) 1.5222222 →→+ ClCOOHCHNHZnZnClCOOHCHNH

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In the second step, aqueous solutions of potassium thiocyanate (KSCN) and

cadmium chloride (CdCl2) were mixed and the resulting solution was slightly warmed

with continuous stirring. Then aqueous solution of glycine zinc chloride was added to

the mixture of KSCN and CdCl2 under continuous stirring. The solution was

maintained at 50 °C temperature for material precipitation. The synthesized material

was then dissolved in hot solution for the removal of co precipitated KCl, which was

filtered off by filtration. The purity of the synthesized GZCTC was enhanced by

successive recrystallization processes.

( ) 2.524 422 ↓→+→+ KClSCNCdKCdClKSCN

( ) ( ) ( ) ( ) KClSCNCdCOOHCHNHZnClCOOHCHNHZnSCNCdK 242222242 +→+

3.5→

5.3.2 Crystal growth

The purified form of the synthesized salt of GZCTC was used as the raw

material for the growth process. 100 ml of saturated solution of GZCTC was prepared

using triple distilled water at room temperature. The slow evaporation method was

adopted for the growth of GZCTC single crystals, because small fluctuations in slow

cooling method lead to multi nucleation and hence the growth got affected. After the

homogenous mixing, saturated solution was filtered using a Whatman filter paper. The

filtered solution was tightly closed with thin plastic sheet, so that the rate of

evaporation could be minimized and then housed in the constant temperature bath

(CTB) maintained at 35 °C with an accuracy ± 0.02 °C. Good transparent GZCTC

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single crystal with well defined morphology was harvested from the solution after 25

days. Figure 5.1 shows the as grown single crystal of GZCTC. The grown crystal was

subjected to different characterization analyses.

Figure 5.1 As grown GZCTC single crystal

5.3.3 Single crystal X-ray diffraction (SXRD) analysis

To determine the crystal structure and crystallinity of the grown crystal, single

crystal X-ray diffraction measurements were made on the GZCTC crystal. The single

crystal X-ray diffraction studies were carried out using the instrument ENRAF

NONIUS CAD4/MAC4 X-ray diffractometer with MoKα = 0.71073Å radiation. It was

found that the GZCTC crystallizes into orthorhombic system. The cell parameter

values of GZCTC obtained by single crystal X-ray diffraction experiment are given

as; a = 8.552 Å, b = 8.569 Å, c = 8.587 Å, α = β = γ = 90 ° and cell volume

V = 629.1 Å3.

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5.3.4 Powder X-ray diffraction (PXRD) analysis

The GZCTC crystal was subjected to powder X-ray diffraction (PXRD) to

reconfirm the cell parameter values. The Kα radiation from a copper target

(λ = 1.5406 Ǻ) was used over the range 10 - 60°. PXRD pattern recorded for the

grown GZCTC crystal is shown in figure 5.2. The prominent peaks confirm the

perfect crystalline nature of the grown crystal and the diffraction planes were indexed

to orthorhombic structure.

Figure 5.2 Powder X-ray diffraction pattern of GZCTC

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5.3.5 Fourier transform infrared (FTIR) spectral analysis

Figure 5.3 shows the recorded FTIR spectrum of the grown crystal. The band

obtained from the spectrum of GZCTC crystal arises from internal vibrations of the

glycine and thiocyanate. The IR spectrum exhibits characteristic peaks at 3482, 2411,

2096, 1628, 1402, 1116, 895, 679, 610 and 457 cm-1. The C-N stretching vibration of

SCN functional group is observed at 2096 cm-1 for GZCTC complex. This peak

shifted from 2063 cm-1 of pure thiocyanate confirms the metal nitrogen co ordination

bond (Kinell and Strandbere 1959). The C-S bending vibration of SCN group is

obtained at 457 cm-1. The NH3+ vibration of glycine group is observed at 3482 cm-1.

The peak found at 1628 cm-1 is attributed to COO- stretching group of glycine. COO-

vibration of glycine appears at 1402 cm-1. The NH3+ vibration is observed at

1116 cm-1 and peaks at 679 and 610 cm-1 are assigned to COO- vibration of glycine.

The observed frequencies and their assignment of the crystal are given in table 5.2.

Figure 5.3 FTIR spectrum of GZCTC

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Table 5.1 Frequency assignments of GZCTC

GZCTC (cm-1) KSCN (cm-1) Glycine (cm-1) Assignments

3482 - 3175 NH3+

2411 - - CN stretching

2096 2063 - δ(CN)

1628 - 1612 COO- stretching

1402 - 1404 COO-

1116 - 1119 NH3+

895 746 892 υ (CS)

679 - 690 COO-

610 - 607 COO-

456 488 - δ(NCS)

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5.3.6 UV-Vis-NIR analysis

The optical absorption and transmittance spectra of GZCTC single crystal were

recorded in the range 190 to 1100 nm. Figure 5.4 represents the UV-Vis-NIR

absorption spectrum of GZCTC crystal and shows that the lower cutoff of this crystal

is found at 212 nm. Figure 5.5 shows the UV-Vis-NIR transmittance spectrum of

GZCTC crystal and the crystal is found to be transparent in the region of 280 to

1100 nm. Transmittance of light in the UV region is large for GZCTC crystal due to

d10 configuration of Zn and Cd atoms in the material.

Figure 5.4 Absorption spectrum of GZCTC

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Figure 5.5 Transmittance spectrum of GZCTC

5.3.7 Microhardness study

The microhardness was measured for the grown GZCTC crystal. Plot of load p

versus hardness number (Hv) is presented in figure 5.6 and it reveals that linear

pattern. Resistance offered by the GZCTC crystal increases sharply at lower loads due

to work hardening. Dislocations move on easy glide region and this movement occurs

over large distance without encountering barriers. As the load increases further it

results rapid increase in work hardening, there by slip occurs on crystal planes leading

to more interaction of dislocations. At higher loads the dislocation does not interact

with each other instead bypass the obstacles in their planes through cross slip. This

mechanism exhibits a low rate of work hardening [31 - 32].

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Figure 5.6 Variation of microhardness and stiffness constant with load for GZCTC

Figure 5.7 Graph between log p versus log d of GZCTC crystal

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Beyond the load 100 g, the material does not offer resistance to deformation

resulting in breaking of the crystal. To confirm the degree of hardness of the material,

log p versus log d curve was plotted as shown in the figure 5.7. The work hardening

coefficient ‘n’ was calculated from Kick’s law p=adn, and was found to be n > 2 for

the GZCTC crystal and this value suggest that microhardness value increases with

load [33]. The elastic stiffness constant (C11) gives an idea about tightness of bonding

between neighbouring atoms [34]. The stiffness constant for different loads has been

calculated using Wooster’s empirical formula C11=Hv7/4 and was depicted in the

figure 5.6. From the graph it is clear that the stiffness constant increases with increase

in load.

5.3.8 Dielectric study

Dielectric studies of the grown GZCTC crystal have been carried out at various

frequencies and temperatures. The experimental detail and formula for calculated

dielectric constant has been discussed in the section 2.8.7 of second chapter. The

dielectric constant and dielectric loss versus frequency are plotted in figure 5.8 and

5.9 respectively. The dielectric constant as a function of both frequency and

temperature were measured and the dielectric constant decreases with increase of

frequency, which is usually referred to as anomalous dielectric dispersion. Large

dielectric constant at low frequency is attributed to various polarization mechanisms

of molecules. At lower frequency, the polarization follows the alternation of the

electric field resulting that the dielectric constant is independent of the frequency.

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Figure 5.8 Frequency dependence of dielectric constant of GZCTC crystal

Figure 5.9 Frequency dependence of dielectric loss of GZCTC crystal

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At higher frequency, ionic polarization cannot follow the field variations and

ceases but the existence of electronic polarization results in low dielectric constant.

The dielectric loss was also measured as a function of both frequency and

temperature. At low frequencies, the dielectric loss was found to be maximum. The

measure of low dielectric loss at higher frequencies is due to dipole rotation. At high

frequencies, the orientation polarization ceases and hence the energy need not be spent

to rotate dipoles. It is also observed that both dielectric constant and dielectric loss

depend on the temperature and increase slightly with the increase of temperature at a

constant frequency. The characteristic of low dielectric loss at high frequencies

clarifies that the grown samples possess enhanced optical quality with lesser defects.

It is obvious that the variation of dielectric constant with temperature is small, which

infers that the crystals are of good chemical homogeneity [35].

5.3.9 Second harmonic generation measurement

Second harmonic generation test was performed to find the non-linear optical

property of glycine zinc cadmium thiocyanate crystal using the Kurtz and Perry

method [36]. A high intensity Nd:YAG laser (λ =1064 nm) with a pulse duration of

10 ns was passed through the powdered sample and the SHG behaviour of the

GZCTC crystal was confirmed from the emission of green radiation by the crystal.

The input laser of energy 5.3 mJ/p was incident on the powdered sample and the

second harmonic signal of 4.5 mV for the grown GZCTC was obtained. For the same

input energy, the second harmonic single for KDP was 18.5 mV.

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5.4 CADMIUM MANGANESE THIOCYANATE (CMTC) CRYSTAL

5.4.1 Synthesis

Cadmium manganese thiocyanate (CMTC) compound was synthesized using

cadmium chloride, manganese chloride and potassium thiocyanate based on the

following equations. The required quantities of salts were mixed in the stoichiometric

ratio:

( ) 5.524 422 ↓→+→+ KClSCNCdKCdClKSCN

( ) ( ) 6.524242 ↓→+→+ KClSCNMnCdMnClSCNCdK

In the first step, an aqueous solution mixture of cadmium chloride (CdCl2) and

potassium thiocyanate (KSCN) was prepared and the resulting solution was slightly

warmed up with continuous stirring. Then, aqueous solution of MnCl2 was added to

the mixture of KSCN:CdCl2 and then maintained below 50 °C for solvent evaporation.

The synthesized material was then dissolved into hot solution for the removal of co-

precipitated KCl, which is removed from the solution by filtration. Then by

recrystallization processes high purity CMTC compound was obtained.

5.4.2 Crystal growth

The synthesized and purified CMTC was taken to the growth process. 100 ml

of saturated solution of CMTC was prepared using triple distilled water at room

temperature and the solution was thoroughly stirred for one hour. The stirred solution

was then filtered by Whatman filter sheet and transferred into 100 ml beaker with

perforated holes. The solution vessels were kept in constant temperature bath (CTB) at

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35 ºC with an accuracy of ± 0.02 ºC and solution was allowed for solvent evaporation

and the grown crystals were harvested after 30 days from the mother solution. Figure

5.10 shows the grown single crystals of CMTC. The structural, functional, optical,

mechanical and SHG characteristics of the grown crystal have been studied

systematically.

Figure 5.10 As grown CMTC single crystals by slow evaporation method

5.4.3 Single crystal X-ray diffraction (SXRD) analysis

Crystal structure and degree of crystalline perfection of the grown crystal was

investigated by single crystal X-ray diffraction study. The unit cell parameters of the

grown CMTC crystal was estimated. From the single crystal X-ray diffraction

analysis, CMTC has been identified to be tetragonal crystallographic system. Cell

parameter values of the CMTC crystal are compared with the literature and are given

in table 5.2.

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Table 5.2 Lattice parameter values of CMTC crystal

Sample a (Å) b (Å) c (Å) Cell volume

(Å)3

CMTC (Present Work) 12.539 12.856 8.786 1256

CMTC (P.Paramasivam

et al 2011) 12.083 12.134 8.557 1254

5.4.4 Powder X-ray diffraction (PXRD) analysis

The CMTC crystal was ground into fine powder and powdered sample of

CMTC was spread over a square centimeter area and placed in a beam of

monochromatic X-rays. The mass of powder was rotated about all possible axes. From

the θ value for each peak, the lattice spacing d was obtained. Powder X-ray diffraction

pattern of CMTC crystals is shown in figure 5.11. From the X-ray diffraction pattern,

the prominent peaks confirm the crystalline nature of the grown crystal.

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Figure 5.11 Powder X-ray diffraction pattern of CMTC

5.4.5 High resolution X-ray diffraction (HRXRD) analysis

The experimental detail of HRXRD has been discussed in the section 3.6.4 of

third chapter. High resolution diffraction curve (DC) recorded for a CMTC crystal is

shown in figure 5.12. As seen in the figure, the DC is quite sharp without any

satellite peaks which may otherwise be observed either due to internal structural grain

boundaries [37] or due to epitaxial layer which may sometimes form in crystals grown

from solution [38]. The full width at half maximum (FWHM) of the diffraction curve

is 8 arc sec, which is very close to that expected from the plane wave theory of

dynamical X-ray diffraction [39]. The single sharp diffraction curve with very low

FWHM indicates that the crystalline perfection is quite good.

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Figure 5.12 HRXRD pattern of CMTC crystal

5.4.6 Fourier transform infrared (FTIR) spectral analysis

Fourier transform infrared (FTIR) spectrum of CMTC crystal was recorded in

the wavelength range 400-4000 cm−1. The recorded FTIR spectrum of CMTC crystals

is shown in the figure 5.13. The absorption peak at 748 cm−1 refers to the CS

stretching vibration. The peaks at 2082 cm−1, 1368 cm−1 and 538 cm−1 correspond to

the CN stretching, CN asymmetric stretching and SCN symmetric stretching

vibrations respectively [40-41]. The frequency assignment for CMTC crystals are

given in table 5.3.

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Figure 5.13 FTIR spectrum of CMTC

Table 5.3 Frequency assignments of CMTC

Wave number (cm-1) Assignments

538 SCN symmetric stretching

748 CS stretching

1368 CN asymmetric stretching

2082 CN stretching

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5.4.7 UV-Vis-NIR analysis

UV-Vis spectrum was recorded in the range 190 – 1100 nm for the grown

cadmium manganese thiocyanate (CMTC) crystal is shown in figure 5.14. The

spectrum shows that UV cut off wavelength of CMTC crystal occurs at 212 nm and

transparency region of CMTC lies in the region of 190 - 1100 nm. There are two

peaks at 230 nm and 327 nm in the transmission spectrum of CMTC, which are due to

the low energy d-d transition of Mn2+ ions [42].

Figure 5.14 Transmittance spectrum of CMTC

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5.4.8 Microhardness study

Vickers microhardness profile as a function of the applied load is shown in

figure 5.15. It is evident from the plot that the microhardness value of CMTC

increases with increase of load. The same trend observed for CMTC as like in

GZCTC, the same explanation could be applicable for CMTC crystal. The stiffness

constant for different loads has been calculated using Wooster’s empirical formula

C11=Hv7/4 and was depicted in the figure 5.15. From the graph it is clear that the

stiffness constant increases with increase in load. The work hardening coefficient ‘n’

was calculated from Kick’s law p=adn, and was found to be n>2 for the CMTC crystal

and log p versus log d curve was plotted as shown in the figure 5.16.

5.4.9 Dielectric study

The dielectric properties are correlated with the electric field distribution within

the crystal. In order to carry out dielectric measurements, CMTC crystals were cut and

then polished to obtain a good surface finish. The temperature and frequency ranges

chosen for this study are 35 - 50°C and 100 Hz - 100 kHz, respectively. The dielectric

constant was calculated using the relation given in the section 5.3.8. Frequency

dependence of dielectric constant (εr) and dielectric loss of CMTC crystal at different

temperatures is shown in figure 5.17 and 5.18 respectively.

The dielectric constant (εr) decreases with increasing frequency, which is

usually referred to as anomalous dielectric dispersion. Large dielectric constant at low

frequency is attributed to various polarization mechanisms of molecules. Polarization

process occurs as a function of time.

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Figure 5.15 Plot of Vickers hardness and stiffness constant versus load for CMTC

Figure 5.16 Plot of log p versus log d for CMTC crystal

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Figure 5.17 Frequency dependence of dielectric constant of CMTC crystal

Figure 5.18 Frequency dependence of dielectric loss of CMTC crystal

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It is obvious that the time taken for polarization is high at lower frequency,

hence irrespective of the polarization mechanism, measure of dipole moment per unit

volume would be high, resulting that the dielectric constant is independent of the

frequency. At higher frequency, time taken for change of polarization direction is

small. The only polarization that involves with movement of electronic charges

contributes the polarization rather than ions resulting low dielectric constant. The low

value of dielectric loss with high frequency for CMTC crystal suggests that the CMTC

possess enhanced optical quality with lesser defects and this parameter is of vital

importance for nonlinear optical materials in their applications [43, 44]. It is obvious

that the variation of dielectric constant with temperature is small, which indicates that

the crystals are of good chemical homogeneity [35].

5.4.10 Second harmonic generation measurement

The powdered sample of CMTC crystal was irradiated with fundamental beam

of 1064 nm from Q-switched Nd:YAG laser to study the non-linear optical property

(second harmonic generation) of cadmium manganese thiocyanate (CMTC) crystal

using the Kurtz and Perry method [36]. A high-intensity Nd: YAG laser (λ =1064 nm)

with a pulse duration of 10 ns was passed through the powdered sample of CMTC to

test the second harmonic generation (SHG). Emission of green light from the sample

confirms the SHG nature and the output power from CMTC sample was compared to

KDP. The input laser of energy 4.2 mJ/p was incident on the powdered sample and

the second harmonic signal of 5 mV for the grown CMTC was obtained. For the same

input energy, the second harmonic single for KDP was 40 mV.

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5.5 CONCLUSION

Metal organic non linear optical materials glycine zinc cadmium thiocyanate

(GZCTC) and cadmium manganese thiocyanate (CMTC) have been synthesized

successfully and purified by recrystallization process. The single crystals of GZCTC

and CMTC were grown from aqueous solution by slow evaporation method.

Orthorhombic and tetragonal structure of the grown crystals of GZCTC and CMTC

were identified by single crystal X-ray diffraction analysis. The crystallinity and

lattice parameter values were reconfirmed by powder X-ray diffraction study. The

crystal perfection was analysed by HRXRD, which reveals that the grown crystals are

free from defects and dislocations. FTIR studies emphasis that the co-ordination of

ligands with metal ions and the functional groups belonging to thiocyanate were

present in both GZCTC and CMTC crystals. The presence of glycine molecule was

confirmed by FTIR analysis for GZCTC crystal.

The optical suitability of the GZCTC and CMTC crystals was studied by taking

UV-Visible spectrum and the results show that there is no absorption in the entire UV-

visible region. The second harmonic generation efficiency of the GZCTC and CMTC

powder samples were confirmed by Kurtz powder technique, which reveals that the

emission of green light from GZCTC and CMTC. Dielectric studies describe that

GZCTC and CMTC possess low dielectric constant and low dielectric loss and hence

they are promising materials for electro optic applications. The mechanical strength of

the grown crystals was characterized by Vicker’s micro hardness and hardness of the

GZCTC and CMTC crystals were increases with the increase of load due to

dislocation motion.

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