37

3. NONLINEAR OPTICALLY IMPORTANT MATERIALS

In this Chapter, the literature survey on inorganic NLO materials, organic

NLO materials, semiorganic NLO materials, BTCC and BTCI single crystals and

their characterization studies are presented.

3.1 Inorganic Materials In the initial stages, the materials explored for nonlinear optical applications

had always been inorganic. Many inorganic crystals are well studied in-terms of their

physical properties. Since these materials are mostly ionic bonded, it is always easier

to synthesize inorganic materials. Often these have high melting points and high

degree of chemical inertness. High-temperature oxide materials are well studied for

diverse applications like piezo electricity, ferroelectricity, pyroelectricity and electro-

optics. Hence when the search for new materials began in NLO, scientists often

trusted their intuition, screened the known materials, and were fairly successful.

Some of the most useful crystals discovered are LiNbO3, KNbO3, potassium

dihydrogen phosphate (KDP) and its analogues, potassium titanyl phosphate and its

analogues, beta barium borate, etc [33].

As a useful ultraviolet (UV) NLO material, K[B5O6 (OH)4]. 2H2O (KB5) is the

first NLO crystal discovered in the series of borates [34]. After that various borate

crystals including β-BaB2O4(BBO), LiB3O5(LBO), Sr2B2Be2O7(SBBO),

BiB3O6(BiBO) and the latest Ca4LnO (BO3)3 (CLnOB, where Ln = Gd, La, Y) have

38

been studied as promising NLO crystals. The family of the various borate crystals

thus plays a very important role in the field of nonlinear optics [35]

KDP crystals, having 40×40 cm2 cross section area have been grown by

Sasaki and Yokotani[36]. They have adopted conventional temperature reduction

method (TRM) and three-vessel method using constant-temperature and constant

supersaturation technique .Owezarek and Sangwal [37] have reported the results of

the dependence of tapering angle θ and micromorphology of tapered faces of KDP on

the concentration of Fe3+ and Cr3+ impurities at various supersaturations. Surface

topographics of the (100) and (101) faces of as grown KDP crystals were observed ex

situ by atomic force microscopy [38] .

Xun Sun et al [39] have proved that light scattering in KDP crystal aggravates

with the increasing concentration of EDTA in the growth solution. The effect of swift

heavy ions on the dielectric properties of doped and undoped ADP crystals was

studied by Bhat et al[40]. Electrical conductivity measurements and activation

energies on gel grown KDP crystals added with same ammonium compounds were

reported by Freeda and Mahadevan[41]. Xue et al [42] have studied the second-order

nonlinear optical (NLO) properties of doped lithium niobate (LN) crystals

(abbreviated as M:LN, where M = Mg2+, Zn2+ and ln2+ respectively). It was observed

that the second-order NLO response of doped lithium niobate(LN) crystals decrease

with increasing dopant concentration in the crystal. Priya et al [43] have grown pure

and impurity (urea and thiourea) added KDP single crystals and reported the electrical

conductivity measurements along a- and c- directions at various temperatures. This

report gives evidence to prove that the conduction in KDP is protonic and mainly due

39

to the anions and not the cations. Ion transport in Au+ doped/undoped KDP crystals

with Kl/Nal as additives was reported by Ananda Kumari and Chandramani [44].

The conductivity of KDP crystal was found to be increased with the addition

of Kl / Nal and with gold doping, as well as upon rise in temperature. Anne Assencia

and Mahadevan [45] have grown the pure and impurity added (with urea and

thiourea) ADP single crystals by the free evaporation method and have reported the

DC electrical conductivity measurements made on the grown crystal. This study

gives evidence to prove that the conduction in ADP also is protonic and mainly due to

the anions and not the cations. Meena and Mahadevan [46] have recently found that

L-arginine doping leads to reduction in dielectric constants in the case of KDP and

ADP single crystals.

Zhang et al [47] have grown Ga and Ce doped KTP (potassium titanyl

phosphate) crystals by the flux method. KTP has wide applications as wave guides

and electro-optical and periodic poling structures. In this context, KTP crystals

should possess low conductivity. By doping the KTP with Ga or Ce, it was found that

the conductivity of KTP crystal is reduced. Feigelson [48] has predicted the

enhancement of optical transparency in CdGeAS2 single crystals by controlling

crystalline defects. A nonlinear optical crystal of calcium fluoroborate (Ca5(BO3)3F)

was grown by Guojun Chen et al [49] using LiF as a flux. Transmission spectrum

showed that the UV cut-off for Ca5(BO3)3F crystal was about 190 nm. Successful

growth of new nonlinear LiKB4O7 single crystal was achieved by Adamiv et al [50]

using Czochralski technique. The Mohs hardness of the crystal was found to be equal

to 5. Xin Yuan et al [51] have obtained a high optical quality cesium lithium borate

40

(CLBO) crystal with dimensions of 146 × 132 × 118 mm3 by Kyropoulos method.

Centimeter – sized single crystals of Tl3PbBr5 were grown by Alban Ferrier et al [52]

using the Bridgman-Stockbarger method. This compound undergoes a phase

transition at 237ºC. The spectroscopic properties and second harmonic generation

tests suggest that it is a potential material for middle infrared nonlinear optics.

Enhancement of crystalline perfection by organic dopants in ZTS, ADP and KDP

crystals were investigated using HRXRD and SEM by Bhagavannarayana et al [53].

Zhoubin Lin et al [54] have found that the SHG efficiency of YCa9 (VO4)7

single crystals is 4.7 times as large as that of KDP crystal. The absorption edge of the

crystal was found at 360 nm. The structures of the non-centrosymmetric borate

chlorides Ba2TB4O9Cl (T = Al, Ga) have been determined by Jacques Barbier [55].

The second harmonic generation (SHG) efficiency [deff] for a powder sample of

Ba2GaB4O9Cl was found to be 0.95 relative to a KH2PO4 standard.

Shirsat et al [56] have reported the influence of lithium ions on the NLO

properties of KDP single crystals. The SHG efficiency of KDP increases by 1.33

times after addition of lithium ion. Nonlinear optical crystals of thiourea mixed

cadmium – lead chloride dihydrate Cd [(PbCl3) (NH2CSNH2)].2H2O (TCCPC) have

been grown in solution by the slow evaporation technique at room temperature by

Nagarajan et al [57]. Thiourea mixed cadmium chloride (TCCPC) crystal is roughly

three times more efficient than the second harmonic generation of ADP. Novel

morphologies and phase transformation of CaCO3 crystals formed in calcium dodecyl

sulfate (CDS) and urea aqueous solution were reported by Zhaodong Nan et al [58].

In view of unique nonlinear optical responses and vast applications in the field of high

41

power laser, Karan and Sen Gupta [59] have studied the effect of urea addition to

magnesium sulphate heptahydrate single crystals.

3.2 Organic Materials Organic compounds are often formed by weak Van der Waals and hydrogen

bonds and hence possess high degree of delocalization. Thus they are expected to be

optically more nonlinear than inorganic compounds. Some of the advantages of

organic materials include ease of varied synthesis, scope for altering the properties by

functional substitutions, inherently high nonlinearity, high damage resistance, etc.

The prototype organic NLO material contains one or more delocalized bonds,

typically a ring structure like benzene. When substituted with donor and acceptor at

the para position (e.g. p-nitroaniline), they have large induced dipole moments under

the influence of electromagnetic fields. However such structures, when packed as

crystals, tend to be mostly centrosymmetric, thus leading to vanishing dipole moment.

A suitable addition at another site, as in the case of 3-methyl-4-nitroaniline, can

ensure a macroscopically non-vanishing dipole moment for the donor-acceptor

substituted systems. The above example typifies the strategy of molecular

engineering towards achieving efficient nonlinear materials [3]. But this type of

organic crystals have several unfavourable physical parameters. Large dipole

moment which leads to the large χ(2) is also responsible for increased absorption at

higher frequencies. Hence most of these molecular materials have poor transparence

and small transparency windows. Consequently, the generated harmonic wave gets

absorbed in the crystal leading to poor efficiency. Also, organic NLO materials are

inherently poor in mechanical hardness and have low melting points and poor

42

chemical inertness. Owing to the high polar nature of the molecules they often tend to

crystallize as long needles or this platelets [33].

The low-temperature solution growth technique is widely used for the growth

of organic compounds to get good quality single crystals. Vijayan et al [60] have

grown p-hydroxy acetophenone (C8H8O) (one of the potential organic NLO

materials). It has been grown by the slow evaporation technique. Nagaraja et al [61]

showed that benzoyl glycine (BG), an organic nonlinear optical crystal grown by slow

evaporation from DMF solution has the advantages of both the organic and inorganic

NLO materials and is nondeliquescent. Owing to high nonlinear efficiency, high

melting point, good chemical stability, less sublimation problems and improved

hardness and cleavage properties (unlike other organic materials) benzoyl glycine is

found to be a promising material for NLO applications. Lakshmana Perumal et al

[62] further extended the effort in synthesizing 4-methoxy benzaldehyde-N-methyl-4-

stilbazolium tosylate (MBST), which is a derivative of stilbazolium tosylate, and a

new material having high NLO property. The Kurtz powder SHG measuremets on

MBST showed that the peak intensity is 17 times more than that of urea. Methyl

p-hydroxybenzoate (p-MHB) is a para-substituted aromatic compound with a

molecular formula C8H8O3 which is also a potential NLO material.

Urea has been used in optical parametric oscillator to generate tunable

radiation throughout the visible region but intrinsic absorption and phase matchability

considerations make it unsuitable for wavelengths longer than 1000 nm [63]. The

efforts made to resolve the problems associated with urea have not been successful.

The binary UNBA crystal [64] is thermally and mechanically harder than the crystal

43

of the parent components. It is quite transparent almost in the entire UV region and

hence it can be used for producing green / blue laser light. Lin et al [65] have

synthesized two component urea - mNBA systems and urea-L-malic acid systems

with different urea compositions. Jan Shen et al [66] have grown single crystals of

L-tartaric acid - nicotinamide and D-tartaric acid - nicotinamide by the temperature

lowering method from aqueous solution. Single crystal of 3-methyl-4-nitropyridine-

1-oxide (POM) was grown by Boomadevi et al [67]. Manivannan and Dhanushkodi

[68] have grown 3-[(IE)-N-ethylethanimidoyl]-4 hydroxy-6-methyl-2H-pyran 2-one,

by the slow evaporation technique and found that the SHG efficiency is close to that

of urea. A chiral mixed carboxylate, [Nd4(H2O)2 (OOC(CH2)COO)4 (C2O4)2] was

grown by Vaidhyanathan et al [69]. It was found to possess about 1% the SHG

activity of urea. Lakshmanaperumal et al [70] have reported single crystals of

4-hydroxy-benzaldehyde-N-methyl-4-stilbazoliumtosylate grown using the solution

growth technique and confirmed the irregular cut chunk and irregular pyramid

morphology. Single crystals of N - methyllutidone trihydrate [C8H11NO . 3H2O)

(NM) were grown by the slow evaporation technique [71] and their SHG was found to

be 0.51 times that of urea. NLO single crystals of benzimidazole have been reported

by Vijayan et al [72]. Solution grown single crystals of bis-2, 7- diethylaminohepta-

2, 5-dien-4-one (BEDO) have produced SHG efficiency of 0.51 times that of urea

[73]. However, the shortcomings of aromatic crystals, such as poor physico-chemical

stability, low hardness and cleavage tendency hinder their device application.

A new ligand N-(3-fluorophenyl)naphthaldimine has been synthesized by

Unver et al [74]. The electric dipole moment (µ) and the first hyperpolarizability

values of the N-(3-fluorophenyl)naphthaldimine have been computed and the results

44

reveal that the synthesized molecule might have microscopic nonlinear optical (NLO)

behaviour with non-zero value.

L-arginine acetate (LAA) single crystal was grown from its aqueous solution

with pH of 6 [75]. Morphological analysis reveals that LAA is a polyhedron with 16

developed faces with major face forms (100), (001) and (102) (pinacoids) parallel to

the polar axis. LAA is an organic nonlinear optical material which has a wide optical

transmission window between 220 and 1500 nm. Its laser damage threshold and SHG

efficiency are comparable with that of KDP. Vickers microhardness measurement was

done for different crystallographic planes of LAA [76]. Single crystals of sodium -

substituted lithium paranitrophenolate trihydrate (Na - NPLi.3H2O) with dimensions

upto (16×8×4) mm3 have been successfully grown by the slow evaporation technique

by Milton Boaz and Jerome Das [77]. The N-(3-nitrophenyl)phthalimide (N 3NP) is a

phase matchable NLO crystal and can be used as an efficient frequency doubler and

optical parametric oscillator due to its high SHG conversion efficiency, which was

grown by the slow evaporation technique using DMF solvent [78]

Single crystals of pure, benzophenone and iodine doped benzoyl glycine (BG)

were grown and characterized by Prem Anand et al [79]. Its hardness anisotropy is

confirmed by the microhardness study. N-(4-nitrophenyl)-N-methyl-2-

aminoacetonitrile (NPAN) material was synthesized and their single crystals were

grown with dimensions 36 ×8 ×8 mm3 using 2 - butanone and 21×15×15 mm3 using

nitromethane as the solvents. Second - harmonic generation (SHG) in the NPAN

crystal was observed using Nd : YAG laser with a fundamental wave length of 1064

nm. Haja Hameed et al [81] have obtained DAST crystals by the two – zone growth

45

technique and the crystal surfaces were analysed with the help of optical and scanning

electron microscopic results. Ramachandran et al [82] have employed photo acoustic

spectroscopy (PAS) method to determine the thermal diffusivity and conductivity of

the gel-grown nonlinear optical single crystals of hipparic acid. Optical absorption of

the specimen was studied using its PA spectrum and compared with UV-Visible

absorption spectrum. An organic NLO material, 4 - OCH3 - 41 nitrochalcone (MNC),

has been synthesized and single crystals grown by Patel et al [83] which has NLO

efficiency 5 times more than that of KDP.

A new organic crystal of semicarbazone of 2 - amino - chloro - benzophenone

(S2A5 CB) has been grown and characterized by proton nuclear magnetic resonance

by Sethuraman et al [84] and its second harmonic generation property was confirmed

by Kurtz powder method. Vibrational spectral analysis of the non-linear optical

material L-prolinium tartrate (LPT) was carried out using NIR - FT - Raman and FT -

IR spectroscopy by Padmaja et al [85]. Also the single crystals of LPT were grown

by Martin Britto Dhas and Natarajan [86] using submerged seed solution growth

method. An organic electro-optic and nonlinear optical (NLO) crystal, L-alaninium

oxalate (LAIO), was grown and its physicochemical properties were studied [87].

Justin Raj et al [88] have grown bulk single crystals of L-alanine formate of

10 mm diameter and 50mm length with an aid of modified Sankaranarayana –

Ramasamy (SR) uniaxial crystal growth method within a period of 10 days. The SHG

efficiency of the output signal was found to be 0.75 times of KDP.

Jagannathan et al [89] have synthesized the organic material 4 –

ethoxybenzaldehyde-N-methyl-4-stilbazolium tosylate (EBST), a new derivative in

46

stilbazolium tosylate family. Its NLO efficiency is 11 times greater than that of urea.

Studies on the nucleation kinetics of sulphanilic acid (SAA) single crystals was

reported by Mythili et al [90]. The laser damage threshold values of the SAA crystals

are found to be 7.6 and 6.6 Gw/cm2 for single and multiple shots respectively. Single

crystals of pure and Cu2+ and Mg2+ doped L-arginine acetate (LAA) were grown by

Gulam Mohamed et al [91] using the slow evaporation method. It is observed that

both Cu2+ and Mg2+ dopants have increased the percentage of transmission in LAA.

Investigation on the nucleation studies of L-arginine acetate single crystals were

reported by Selvaraju et al [92]. Modified hippuric acid (HA) single crystals have

been grown from aqueous solution of acetone by doping with NaCl and KCl with the

vision to improve the physicochemical properties of the sample. It was noted that the

dopants have increased the thermal stability and mechanical strength of the crystal

[93]. The influence of isolectric pH (Pl) on the growth, linear and nonlinear and

dielectric properties of L-threonine single crystals has been studied. The crystalline

powder SHG efficiency of L-threonine crystals grown at Pl (Isoelectric pH) was

found to be 1.2 times that of KDP. High quality bulk crystals of L-threonine grown at

different pH values were tested using high-resolution X-ray diffractometry [94]. Melt

grown ethyl-p-aminobenzoate (EPAB) single crystal was recently identified as new

organic nonlinear optical material, with nearly six times higher SHG efficiency than

that of KDP [95]. Good optical quality single crystals of organic nonlinear material

1-chloro-2, 4-dinitrobenzene (CDNB) were successfully grown at low temperature by

the solution growth technique [96]. Bulk single crystal of L-arginine maleate

dihydrate (LAMD) of size 48 ×33 ×7 mm3 was grown by the slow cooling technique

in a period of 3 weeks, whose SHG efficiency was found to be 1.4 times that of KDP

47

crystal and its physicochemical properties were investigated [97]. Single crystals of

L-arginine trifluoroacetate (LATF) were grown by employing the low temperature

solution growth method and its thermal and nonlinear properties were studied [98].

L-arginine trifluoracetate (LATF) crystals have been grown from aqueous solution

using the micro-crystallization method by Liu et al [99]. It has been reported that

growth properties could be improved by adding appropriate amount of HCl. Etch

patterns on the (101) face of LATF crystal were also described. Haja Hameed and

Rohani [100] have obtained pure and additive mixed LAP single crystals by the slow

cooling technique. The surface second harmonic generation (SHG) analysis was done

on (100) face of the grown crystals and the SHG intensity on (100) face of the crystals

were measured. L-nitroarginine and its salts can show better nonlinear optical

properties in comparison with L-arginine and its salts because of the presence of

electron acceptor nitro group (NO2) in addition to existing electron donor amino

group (NH2) [101]. An organic second-order nonlinear optical single crystal 2, 4, 5-

trimethoxy-41-chlorochalcone was grown and characterized by powder X-ray

diffraction, and UV-Vis-NIR analyses. The second harmonic generation of the crystal

was confirmed by using the Kurtz powder technique [102]. A new class of nonlinear

optical (NLO) chromophores of which 2-dicyanomethylene-3-cyano-4-{-2-[2-[4-

(N,N-di(2-dydroxyethyl)-amino)phenylazo)-thein-5]-E-vinyl} - 5, 5 - dimethyl-2, 5

dihydrofuran (2a) is the prototype with high thermal stability and large optical

nonlinearity [103]. Single crystals of organic nonlinear optical (NLO) materials

L-histidine nitrate, L-cysteine tartrate monohydrate were grown by submerged seed

solution method [104].

48

Pure and deuterated L-alanine crystals have been grown by the slow

evaporation as well as slow cooling techniques [105].

Experimental determination of stability, meta-stable zone width and induction

period for an organic nonlinear optical L-arginine trifluoroacetate (LATF) was

reported by Arjunan et al [106]. It was also reported that the meta-stable zone width

becomes narrower with increasing solute concentration. Nonlinear optical properties

of LATF crystal was confirmed by the Kurtz powder test. Single crystals of N,

N-dimethylanilinium picrate (DMAP) were grown by the slow evaporation solution

growth technique at room temperature by Chandramohan et al [107]. SHG

conversion efficiency of DMAP crystal was found to be 1.29 times that of urea.

Single crystals of acenaphthene picrate (ACP) were also grown by the slow

evaporation solution growth technique by Chandramohan et al [108]. SHG efficiency

of the grown crystals were found to be 0.39 times that of urea. Bharathikannan et al

[109] have grown crystals of 2-nitroaniline picric acid (2NAP) by the slow

evaporation solution growth technique. The second harmonic generation (SHG)

efficiency of this crystal was estimated using Nd:YAG laser as the source.

Ferroic crystals of tetra(methyl)ammonium tetrachlorozincate (TMA-ZnCl)

were grown by the slow evaporation technique and the variation of morphology of the

grown crystals with different pH values was reported by Devashankar et al [110]. The

laser SHG efficiency of the grown crystal was found to be 1.3 times that of KDP

crystal. With a vision to improve the properties of the L-alanine crystals, a new

organic nonlinear optical crystal urea-L-alanine acetate (ULAA) has been grown by

the solution growth - slow cooling technique by Jaikumar et al [111]. SHG efficiency

49

of this crystal is found to be nearly equal to that of pure KDP. The structural,

mechanical, optical, dielectric and SHG studies of undoped and urea doped γ-glycine

crystal were reported by Selvarajan et al [112].

In order to retain the merits and overcome the shortcomings of organic

materials, some new classes of NLO crystals such as metal organic or semiorganic

complex crystals have been developed. The relatively strong metal ligand bond

permits complex crystals to combine the advantages of inorganic crystals such as

good stability with the advantages of organic crystals such as high nonlinearity and

molecular engineering features.

3.3 Semiorganic Materials In the recent years to search for new non-lineair optical materials included

semi-organic and coordination compounds due to their advantages over traditional

inorganic and organic compounds. There are three main types of interest in the study

of NLO semi-organic and coordination compounds thus far. The first interest is to

find transparent second harmonic generation materials to be used for the frequency

doubling of a laser. For this purpose, materials should exhibit a large SHG effect and

optical transparency both at second harmonic and fundamental wavelengths.

Secondly, semi-organic and coordination compounds may offer some excellent

chromophores. The third purpose is to find new, third order NLO materials [113].

The search for new frequency conversion materials over the past decade has

led to the discovery of many organic NLO materials with high nonlinear

susceptibilities. The approach of combining the high nonlinear optical coefficients of

the organic molecules with the excellent physical properties of the inorganics has

50

been found to be overwhelmingly successful in the recent past. Hence, recent search

is concentrated on semiorganic materials due to these large nonlinearity, high

resistence to laser included damage, low angular sensitivity and good mechanical

hardness [114]. Initially, metal complexes of urea and urea analogues have been

explored [115]. Examples of these complexes are zinctris(thiourea) sulphate (ZTS),

zincbis(thiourea) chloride (ZTC), bis(thiourea)cadmium chloride (BTCC) and

copper(thiourea) chloride (CTC). These crystals have better nonlinear optical

properties than KDP. Bis(thiourea)zinc chloride (BTZC) single crystals have been

grown by the slow evaporation technique at room temperature. The metal

thiocyanates and their Lewis-base adducts are one of the interesting themes of

structural chemistry [116]. As SONLO materials, bimetallic thiocyanates:

ZnCd(SCN)4, ZnHg(SCN)4, CdHg(SCN)4 and MnHg(SCN)4 (abbreviated as ZCTC,

ZMTC, CMTC and MMTC respectively), exhibit efficient SHG at short wavelengths.

Single crystals of cadmium bis(thiourea) acetate (CTA) have been grown by the slow

evaporation technique at ambient temperature. CTA has good optical transmission in

the entire visible region [117]. Kannan et al [113] have grown bis(thiourea)zinc

acetate (BTZA) by the slow evaporation technique and observed that BTZA has better

SHG efficiency than CTA. Spectroscopic and thermal properties of iron mercury

thiocyanate (FMTC) crystals were studied by Wang et al [118]. Its SHG efficiency

was found to be 0.6 times that of urea. A new metal-organic co-ordination nonlinear

optical crystal, tri-allyl(thiourea)zinc chloride (ATZC) was synthesized in water and

recrystallized in ethanol. The powder SHG efficiency of ATZC was found to be

comparable with urea [119]. Single metal (Cd2+ and Cu2+) substituted single crystals

of bis(thiourea)zinc chloride were grown by the slow evaporation technique [120].

51

Selvakumar et al [121] have grown yet another variety of organometallic nonlinear

optical crystal bis(thiourea)cadmium formate (BTCF) and characterized by optical

and mechanical studies. Vicker’s hardness measurements show that BTCF has a high

VHN value of 109.7 kg-mm-2. Bis(dimethyl/sulfoxide) tetrathiocyanato-

cadmium(II)mercury(II) (CMTD), a promising organometallic NLO crystal was

grown and characterized by Rajarajan et al [122] and the SHG efficiency of the

crystal was found to be 15 times that of urea. A highly efficient nonlinear optical

crystal of tetrathioureamercury(II) tetrathiocyanatozinc(II) (TMTZ) was grown by

Rajarajan et al [123]. The lower cut-off wavelength of TMTZ is at 330 nm. A

spectroscopic investigation on the single crystal of thiocyanatomanganesemercury-N,

N-dimethylacetamide (MMTWD) reveals that MMTWD belongs to the two

dimensional layer net structure. The water and DMA molecules present in the layers

induce large nonlinearity and higher environmental stability [124].

Optically clear manganesemercury thiocyanate (MMTC) crystals have been

grown by Joseph Arul Pragasam et al [125]. The high second harmonic efficiency of

nearly 18 times that of urea and the wide transmittance (373-2250 nm) of MMTC

indicate that this material is an excellent candidate for photonics device fabrications.

The growth mechanism of MMTC crystal was studied using Atomic Force

Microscopy (AFM) and it was found that the crystal grows by 2D nucleation

mechamism [126]. Siddheswaran et al [127] have grown ATMC crystal which is a

nonlinear optical material (NLO) having high optical quality and its second harmonic

generation (SHG) efficiency is thrice that of urea. Complex degradation of ATMC

compound takes place at above 230°C. Single crystals of BTZA have been grown

[128] by the low temperature solution growth method using slow cooling process at

52

an optimized pH of 3.5. Transmission spectrum reveals that the crystal has a low UV

cut off at 435 nm and has a transmittance of 100%. The Vicker’s hardness value was

estimated to be 108.36 kg/mm2.

The influence of metallic substitution (Mg2+ and Cd2+) on the physical

properties of MMTC was studied by Joseph et al [129] and it was found that metallic

substitution has improved the physico chemical properties. Highly efficient single

crystals of zinccadmium thiocyanate (ZCTC) with SHG efficiency 12 times that of

urea was grown by Joseph et al [130]. ZCTC has a UV cut-off wavelength of 296 nm

and a high thermal stability up to 350°C.

Tetrathioureacadmium(II) tetrathiocyanatozinc(II) (TCTZ) single crystals

were grown and optical and dielectric properties were studied [131]. The UV cut-off

wavelength of TCTZ was found to be 245nm. Single crystals of

tetrathioureamercury(II) tetrathiocyanatomanganese(II) (TMTM) were grown from its

aqueous solution by Rajarajan et al [132] using the slow evaporation technique. The

thermal stability of TMTM was found to be upto 199.06°C. Effect of different metal

ions on the physical properties of tri-allythioureacadmium chloride (ATCC) and tri-

allylthioureamercury chloride (ATMC) single crystals were studied by Perumal and

Moorth Babu [133]. It was reported that the presence of different central metal (Cd

and Hg) atoms have changed the thermal properties of the materials when formed

with the common ligand allylthiourea.

A novel second order nonlinear optical co-ordination complex crystal tris

allylthioureazinc bromide (ATZB) was synthesized and grown from ethanol (used as

the solvent). It was found that the crystal exhibited no temporal degradation due to the

53

oxylic, hygroscopic or efflorescent effects at room temperature [134]. Growth and

characteristic of pure and Zn2+ doped bis(thiourea)cadmium acetate (BTCA), a

nonlinear optical single crystal, was reported by Selvakumar et al [135]. Metal

complex of thiourea such as zinctris(thiourea) sulphat(ZTS) have been grown by the

slow cooling technique [136]. Grown crystals show good optical transmission in the

entire visible region. The optical transmission studies and second harmonic generation

(SHG) efficiency studies justified the device quality of the grown crystals.

Semiorganic nonlinear optical thiosemicarbazidecadmium chloride

monohydrate (TSCCCM) single crystals were grown from aqueous solution by the

slow evaporation method by Sankar et al [137]. The SHG conversion efficiency of

TSCCCM crystal was found to be 14 times higher than that of KDP crystal. In

TSCCCM crystal structure, the planar π-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 [137].

Single crystals of nonlinear optical L-arginine iodate were successfully grown

for the first time by the temperature - lowering method and also by the slow

evaporation method at a constant temperature (30°C) from its aqueous solution at pH

value of 6 [138].

Single crystals of nonlinear optical material bis(thiourea)zinc chloride (BTZC)

were successfully grown by the temperature lowering method and also by the slow

evaporation method at a constant temperature of 28.5° from aqueous solutions having

various pH values. The best quality crystal was obtained when the pH value was 3.13.

Studies on structural and thermal properties of the crystals have been carried out on

54

the basis of X-ray diffraction (XRD), infrared spectroscopy (IR), differential thermal

analysis (DTA) and thermo-gravemetric analysis (TGA). DTA study indicates the

possibility of structural changes without weight loss of BTZC [139]. A new semi-

organic nonlinear optical rubidium bis-dl-malato borate (RBMP) has been synthesized

and single crystals were grown by the slow cooling technique from aqueous solution.

The emission of SHG using Nd:YAG laser was confirmed by a modified Kurtz and

Perry powder setup [140].

Tri-allylthioureacadmium chloride (ATCC) was synthesized in deionized

water by the solvent evaporation method. Its (powder) SHG efficiency is higher than

that of urea. Single crystals of the co-ordination complex nonlinear optical crystal

ATCC with dimensions 8 ×8 ×4 mm3 were grown by Perumal and Moorthy Babu

[141]. Tri-allylthiourea complex nonlinear optical single crystals were grown by the

slow evaporation technique [142]. Single crystals of nickelmercury thiocyanate

(NMTC) was successfully grown by the slow evaporation technique and reported by

Ramachandra Raja et al [143]. The SHG efficiency of NMTC is 0.66 times higher

than that of KDP. Growth of glycine doped ZTS crystals was achieved by the slow

evaporation technique [144]. The SHG efficiency of 1 mol % glycine doped ZTS is

4.14 times higher than that of pure ZTS.

Ravi Kumar et al [145] have reported the growth of a novel organometallic

single crystal of bis(thiourea)cadmium formate (BTCF). A new semiorganic nonlinear

optical crystal, bis(thiourea)cadmiumzinc chloride (BTCZC) crystal, was synthesized

by Kirubavathi et al [146]. The crystals were grown from aqueous solution by the

slow evaporation technique. The crystals are thermally stable upto 201°C.

55

Single crystals of pure and potassium iodide-doped zinctris(thiourea) sulphate

(ZTS) were grown from aqueous solutions by the slow evaporation technique by

Krishnan et al [147]. The SHG efficiency of the grown crystal was observed about 1.2

times as that of KDP. They also reported the growth and characterization of pure and

potassium chloride doped zinctris(thiourea) sulphate (ZTS) single crystals [148].

Dielectric properties and phase transition of zinctris(thiourea) sulphate single crystal

was reported by Moitra et al [149]. The dielectric properties and ferrelectric to

paraelectric phase transition of zinctris(thiourea) sulphate single crystal in a wide

range of temperatures and frequencies are reported. In ZTS, prominent first-order

ferroelectric to paraelectric phase transition occurs at 323 K.

Single crystals of new semiorganic nonlinear optical zincguanidinium sulphate

have been grown from solution by the slow evaporation technique. As grown crystals

were characterized structurally, chemically and optically by making PXRD and FTIR

and UV-Vis-NIR spectral measurements. The nonlinear optical property of the grown

crystal was confirmed by the Kurtz-powder SHG test [150].

Growth and characterization of tetramethylammonium tetrachlorozincate(II),

whose SHG efficiency is 1.3 times that of potassium dihydrogen orthophosphate

(KDP) crystal, have been reported by Devashankar et al [151]. The metastable zone

width studies were carried out for various temperatures for supersaturated aqueous

solutions of zincbis(thiourea) chloride added with 1 mol % of L-arginine. The SHG

efficiency measurements carried out with different doping concentrations of

L-arginine revealed that NLO property was enhanced by L-arginine dopant [152].

Spectroscopic properties of metal complexes of thiourea single crystals

56

[tris(thiourea)zinc acetate, bis(thiourea)cadmiumzinc acetate and bis(thiourea)

ammonium chloride] which are non-linear optic materials were investigated by

Raman scattering spectroscopy [153].

Synthesis and characterization of bis(thiourea)zinc chloride doped with

L-arginine have been reported by Sweta Moitra and Tanusree Kar [154]. A drastic

change in morphology was observed due to doping. The SHG efficiency of pure and

doped samples was found to be almost the same, which was equivalent to that of

potassium dihydrogen phosphate. Karthick et al [155] have reported the synthesis,

growth and characterization of semi-organic nonlinear optical bis(thiourea)antimony

tribromide (BTAB) single crystals. Lydia Caroline and Vasudevan (2009) [156] have

reported the growth and characterization of pure and Cd2+ doped bis(thiourea)zinc

acetate (BTZA). This semiorganic nonlinear optical single crystal’s laser damage

threshold value was determined to be 12.44 MW/cm2. Krishnan et al [157] have

reported the growth and characterization of pure and potassium iodide - doped

zinctris(thiourea) sulphate (ZTS) single crystals.

A semiorganic nonlinear optical material thiosemicarbazidecadmium chloride

monohydrate (TCCM) was synthesized and single crystals were grown from aqueous

solution by the slow evaporation method at ambient temperature. The second

harmonic generation (SHG) from this material was confirmed using Nd:YAG laser

[158]. Effect of K+ ion on the dielectric properties of metalorganic L-alaninecadmium

chloride (LACC) single crystals was reported by Bright and Freeda [159]. The

dielectric constant and dielectric loss of both pure and potassium doped LACC

crystals decrease with increase of frequencies and decrease of temperatures. The same

57

authors have also reported the growth and characterization of organometallic

L-alaninecadmium chloride single crystal by the slow evaporation technique.

L-alanine doped KDP crystals were grown by the slow aqueous solvent

evaporation technique and reported by Parikh et al [160]. SHG efficiency and optical

transmission percentage of pure crystal [L-alanine] has increased by adding the

dopant. Selvarajan et al (2010) [161] have reported the structural, mechanical, optical,

dielectric and SHG studies of undoped and urea doped γ-glycine crystals.

Morphological changes were noticed in γ-glycine crystals when urea was added as the

dopant. Effect of strontium chloride on the optical and mechanical properties of

γ-glycine has been reported by Anbuchezhiyan et al [162]. The SHG efficiency of γ-

glycine was observed to be greater than that of standard potassium dihydrogen

phosphate (KDP). The value of the damage threshold intensities for the grown

compound in single shot mode was found to be 4.58 GW/cm2.

Dinakaran et al [163] have reported the optical imaging of the growth kinetics

and polar morphology of zinctris(thiourea) sulphate single crystals. The grown

crystals were imaged in two different growth geometries using laser shadowgraphy

technique. The anisoptropy in the growth rates of the (001) and (001) faces was very

high resulting in polar morphology.

A novel organometallic nonlinear optical crystal, namely, thiourea complex of

tetrakisthiourea potassium iodide (TTPI) has been grown by the slow evaporation

solution growth technique by Thomas Joseph Prakash et al [164]. The harvested

crystal was large in size. The grown crystals were characterized by taking single

crystal and powder X-ray diffraction, FTIR spectral, UV-Vis-NIR spectral, thermal,

58

etching, etc studies. The SHG efficiency of TTPI was found to be higher than that of

KDP. It is a potential material for frequency conversion.

The UV-Vis NIR transmission spectrum of TTPI had a wide optical

transmission window (200-1100) nm[164]. TTPI has good transparency 65% and

lower cutoff wavelenth of the crystal is found to be 240nm and thus to ascertain the

fact that the crystal can be used for laser application.

The thermogravimetric analysis of TTPI was carried out between 27°C and

800°C and recorded the spectrum[164]. The DTA trace indicates a strong endothermic

starting at 183.3°C due to its melting. From the DSC study it was found that the

crystal was stable up to its melting point (183°C).

3.4 BTCC and BTCI Crystals The quest for new frequency conversion materials is presently concentrated on

semiorganic crystals due to their large nonlinearity, high resistance to laser induced

damage, low angular sensitivity and good mechanical hardness. Recently metal-

complexes of urea and urea analogs like thiourea have been formed to be excellent

nonconcentrosymmetric materials when they are stoichiometrically incorporated into

the respective inorganic salt. Examples of these salt complexes are

bis(thiourea)cadmium chloride (BTCC), zinctris(thiourea) sulphate (ZTS),

copper(thiourea) chloride (CTC) [165].

BTCC is a thiourea - complex of divalent cadmium chloride. It crystallizes in

the noncentrosymmetric orthorhombic space group Pmn21 The crystal structure

consists of two molecular units in a unit cell. The cell parameter values are

59

a - 5.812 Å, b = 6.485 Å, c = 13.106Å and cell volume, V = 494.092 Å3 [166]. In

BTCC, all thiourea molecules are planar and perpendicular to the crystallographic

c-axis. Figures 8 and 9 show the crystal packing and molecular structure of BTCC

single crystals.

Figure 8: Crystal packing diagram of BTCC

Figure 9: Molecular structure of BTCC

A preliminary report on optical transmission and measurement of second and

third order nonlinear optical (NLO) co-efficients was made by Newman et al [167].

60

To be economically viable, the laser fusion experiments demand for harmonic

generators that are very efficient and cost-effective. BTCC is one such material which

is more efficient and less expensive than KDP. Among the solution grown crystals,

BTCC is one among the crystals that has the highest damage threshold. The single

shot damage threshold has been reported to be 32 GW/cm2 and the multiple shot

damage threshold has been reported to be 6 GW/cm2 [27].

Venkataraman et al [26] have reported that low variation of solubility with

temperature was serious handicap in growing large BTCC crystals by the slow

cooling technique. Hence, they had adopted solvent evaporation method for the

growth. These crystals can be successfully grown in bulk form at optimized growth

conditions, enabling it to be applicable for laser fusion experiments and second

harmonic generation. Vibrational spectroscopic characterization has also been carried

out on BTCC crystals and the effect of metal complexation on thiourea vibrations was

studied [168].

Ushasree et al [166] have grown single crystals of BTCC by the slow cooling

technique for different values of pH. The results indicated that as the pH of the

solution is decreased, the growth rate along the a-direction, R [100] increased and at a

pH below 1, the morphology of the crystal changed from hexagonal to rectangular.

This increase in the growth rate enables the bulk BTCC crystals with favourable

growth rate along all the three directions, making it more suitable for laser fusion

experiments and SHG device applications.

Growth and micromorphology of as grown and etched bis(thiourea)cadmium

chloride (BTCC) single crystals have been reported by Ushasree and Jayavel [165].

61

The micromorphology studies show that the growth usually takes place by spreading

of layers.

The effect of EDTA on the growth of BTCC crystal has been reported by

Pricilla Jeyakumari et al [169]. The growth rate of the BTCC crystals has been

improved by the addition of EDTA. The powder SHG efficiency was found to be 0.73

times that of urea.

Selvakumar et al [170] have reported the microhardness, FTIR and

transmission spectral studies of Mg2+ and Zn2+ doped nonlinear BTCC single crystals.

Doped samples showed an increase in percentage of transmission in comparison to

pure BTCC crystals. The SHG efficiency of the metal doped crystals are improved

due to the metallic (Mg2+ and Zn2+) substitution. Thermal, dielectric and

photoconductivity studies on pure, Mg2+ and Zn2+ doped BTCC single crystals have

also been reported [171].

Bis(thiourea)cadmium chloride crystals have been crystallized by the slow

evaporation technique and high pressure electrical resistivity study has been carried

out on this crystal [172]. This study gives the hints for possible pressure induced

phase transition.

Electron paramagentic resonance (EPR) and optical studies have been carried

out on Cu2+ doped bis(thiourea)cadmium chloride single crystal by Ravi and

Subramanian [173]. The transmission spectrum for copper doped BTCC shows very

high transmission in the entire visible region than pure BTCC gives cut-off

wavelength below 200 nm, which gives wide scope in NLO applications.

62

Good quality single crystals of Ni2+ and Co2+ ions doped

bis(thiourea)cadmium chloride (BTCC) are some of the excellent and efficient

nonlinear optical materials grown from aquous solutions by the slow evaporation

method. Optical and dielectric studies on pure and Ni2+ and Co2+ doped single crystals

of BTCC were carried out by Uthrakumar et al [174]. These crystals have a good

transmission in the entire visible region, which is an essential property of materials for

NLO applications. The morphology of BTCC and BTCI crystals are given in

Figure 10.

Figure 10: Morphology of BTCC crystal and BTCI crystal

Single crystals of bis(thiourea) cadmium iodide (BTCI), a semiorganic

material has been successfully grown by both the slow evaporation and slow cooling

methods by Lydia Caroline and Vasudevan [5]. Grown crystals were characterized

structurally, chemically, optically and thenmcelly. Studies on dielectric properties

indicate that BTCI crystal is a good candidate for electrooptic modulators.

Transmission spectra reveals that the crystal has low UV cutoff at 324 nm and has a

good transmittance in the entire visible region enabling its use in optical applications.

(Len

gth

of th

e cr

ysta

l a-

dire

ctio

n)

63

3.5 Studies Made on BTCC and BTCI All the physical properties of crystals are governed by the nature of the atomic

arrangement within the crystal structure, and their chemical composition. The

physical properties can be directional or non-direction dependent. Optical properties

are an integral part of crystallography, because of their direct relation to the symmetry

and structure. Material scientists and device engineers need to know the degree of

perfection and purity of crystals to interpret structure dependent properties in order to

determine whether the material can be successfully employed in the equipments or

device fabrication.

3.5.1 Single crystal X-ray diffraction measurements The crystals were subjected to single crystal X-ray diffraction (SXRD) to

determine the unit cell dimensions and morphology. Summary of reports on SXRD

measurements carried out on BTCC and BTCI crystals already available in various

literature are given in Table 2.

3.5.2 Powder X-ray diffraction X-ray diffraction is an important technique in the field of materials

characterization to obtain structural information on an atomic scale from both

crystallaine and noncrystalline (amorphous) materials. X-ray powder diffraction is an

instrumental technique that is usually used to study crystallaine materials. For

comparison purpose, the list of d-spacing with their experimental and calculated

X-ray intensities for BTCC crystals [26] and powder X-ray diffraction (PXRD)

patterns of BTCI crystals [5] are given in Table 3 and Figure 11 respectively.

64

Table 2: Summary of reports on SXRD of BTCC and BTCI

Sl. No. Type of crystal

Lattice parameters Volume

(Å)3 Reference

No. a(Å) b(Å) c(Å)

1 BTCC 5.80 6.48 13.07 491.22 [168]

2 BTCC 5.812 6.485 13.106 494.092 [166]

3 BTCC 5.9488 6.5059 13.235 512.1974 [169]

4 BTCC 5.812 6.485 13.106 494.09 [165]

5 BTCC 5.834 6.501 13.148 498.7 [170]

6 BTCC+Mg2+ 5.816 6.473 13.110 493.65 [170]

7 BTCC+Zn2+ 5.817 6.479 13.127 494.77 [170]

8 BTCC 5.8269

± 0.0145

6.5207 ±

0.0145

12.8945 ±

0.0212 489.916 [172]

9 BTCC 5.834 6.501 13.148 - [173]

10 BTCC 5.834 6.501 13.148 498.7 [174]

11 BTCC+Ni2+ 5.769 6.456 13.169 493.82 [174]

12 BTCC+Co2+ 5.813 6.482 15.086 494.50 [174]

13 BTCI 10.520 7.600 15.086 1205.75

[5] β = 91.50°

65

Table 3: Diffracting planes and their relative intensities for BTCC [26]

Sl. No. d Observed (Å) d Calculated (Å) I/I0 (Observed) hkl

1 6.47 6.48 29.11 1 1 0

2 5.77 5.80 78.38 0 0 1

3 5.27 5.30 76.65 0 1 1

4 4.31 4.32 78.31 1 0 1

5 3.60 3.60 37.87 1 2 1

6 3.47 3.48 26.82 0 3 1

7 3.26 3.26 27.86 0 4 0

8 3.23 3.24 100 2 0 0

9 2.89 2.90 29.88 0 0 2

10 2.75 2.76 21.89 2 1 1

11 2.65 2.65 33.98 0 2 2

12 2.59 2.59 20.10 1 1 2

13 2.42 2.42 19.31 1 5 0

14 2.37 2.37 17.79 2 3 1

15 2.30 2.30 14.31 2 4 0

16 2.26 2.26 19.24 1 3 2

17 2.17 2.16 19.17 0 4 2

18 2.16 2.16 50.17 2 0 2

19 2.16 2.16 50.17 3 0 0

20 2.05 2.05 22.44 2 2 2

21 2.05 2.05 22.44 3 2 0

22 2.05 2.05 22.44 1 4 2

23 1.93 1.93 25.50 0 0 3

24 1.93 1.93 25.50 3 2 1

66

Figure 11: Powder X-ray diffraction pattern of BTCI crystal 3.5.3 Fourier transform infra-red spectral analysis The infrared (IR) spectroscopy is mainly concerned with the absorption of

energy by a molecule, ion of radical from a continum or with the study of emission of

infrared radiation by species of excited states. The infrared spectrum is the simplest,

most rapid and often most reliable means for assigning a compound to its class. It can

also provide a variety of information on structure, symmetry, purity, structural and

geometrical isomers and hydrogen bonding.

The observed bands along with their vibrational assignments for some of the

thiourea complexes already reported are given in Table 4. The absorption bands

provided are in cm-1.

67

Land table 4

68

3.5.4 UV-Vis-NIR spectral measurements Venkataramanan et al [26] have found that the transmission spectrum for a

BTCC crystal shows a UV cut-off below 300 nm. The transmission range extends

from 285 nm in the UV to 1900 nm with significant absorption around 1500 nm. A

notable feature in the spectrum is the reduction in absorption at the Nd:YAG laser

fundamental, when compared to ZTS [182]. This is due to the fact that the number of

N-H bonds, which causes absorption around 1040 nm by vibrational overtones is

lesser in BTCC than in ZTS. BTCC has only two thiourea units whereas ZTS has

three. This reduction in absorption at around 1064 nm has a significant contribution

towards an improvement in the laser damage resistance of the crystal.

Ushasree and Jayavel [165] have recorded the absorption spectra of BTCC

crystals. The spectra revealed that the crystal has a low UV cut off at 285 nm with

significant absorption around 1040 nm. They found that the spectrum is in good

agreement with the previous work of Venkataramanan et al for the wavelength range

200-1000 nm [26].

Precilla Jeyakumari et al [169] have studied the transmission spectrum of

BTCC crystal and EDTA added BTCC crystals. The UV transparency lower cut off

wavelength for pure EDTA added BTCC crystal occurs at 330 nm. The wide

transmission range in the entire visible region (330 - 800 nm) enables it to be a

potential candidate for opto electronic applications.

The UV-Vis-NIR transmission spectra of pure, Mg2+ and Zn2+ doped BTCC

single crystals were recorded by Selvakumar et al [170]. In all the three spectra,

69

overtone bands are observed in the near infrared region below 750 nm. The

percentage of transmission is very high for all the three samples, which is a required

property for NLO materials. In addition, the higher percentage of transmission for

doped ones in comparison to pure BTCC, is likely to improve the NLO property.

Ravi and Subramanian [173] have found that transmission spectrum for Cu2+

doped BTCC shows very high transmission in the entire visible region than for pure

BTCC and other metal ions doped BTCC. The cutoff wavelength for pure BTCC is

300 nm, consistent with the previous report [26]. Copper doped BTCC gives cutoff

wavelength at below 200 nm, which gives wide scope in NLO application.

UV spectral analyses on pure and Ni2+, Co2+ doped single crystals of BTCC

revealed the improved transparency of the doped crystals ascertaining the inclusion of

metal ion in the lattice [174]. The UV cut-off wavelength seems to be different for

different ion dopants.

Transmission spectra of BTCI crystals reveal that the crystal has low UV cut

off at 324 nm and has transmittance in the entire visible region enabling its use in

optical application[5].

3.5.5 Second harmonic generation Venkataramanan et al [27] have reported the laser induced damage threshold

values of zinctris(thiourea) sulphate and bis(thiourea)cadmium chloride, which the

highest among the solution grown crystals. The single and multiple shot damage

thresholds for BTCC are 33 and 6 GW/cm2 respectively and that for ZTS were 40 and

7.8 GW/cm2.

70

Pricilla Jayakumari et al [169] measured the SHG efficiency of BTCC crystal

using Q. switched Nd:YAG laser having a wavelength 1064 nm and a pulse width of

8ns. The powder SHG efficiency of BTCC was found to be 0.73 times that of urea.

Selvakumar et al [170] have studied the NLO property of the pure, Mg2+ and Zn2+

doped BTCC single crystals by passing the output of Nd:YAG Quanta ray laser of pulses

width of 8ns on the samples. The observed result is, that the efficiency of frequency

doubling in Mg2+ - doped (68%) and Zn2+ - doped (65%) BTCC crystal is better than the

pure BTCC (60%) and the KDP (21%) crystals. Comparison of SHG efficiency for

various semiorganic crystals with respect to KDP are listed in Table 5 [88].

Table 5: Comparison of SHG efficiencies for various semiorganic crystals

with respect to KDP [88].

Sl. No. Compounds SHG –efficiency related to KDP

1 Na.NPLi.3H2O 2.00

2 NPLi.3H2O 1.70

3 NPNa.2H2O 1.85

4 Pottassiumaluminium borate 2.30

5 L-histidine bromide 1.20

6 L-arginine hydrochloride 1.29

7 L-alanine formate 0.75

8 L-alanine 0.33

9 L-alanine fluoroborate 0.35

10 BTCC 2.80

11 ZTS 1.20

12 L-hystidine fluroborate 0.46

13 LLTN.DTN Equivalent to KDP

14 L-arginine tetrafluroborate Comparable to KDP

15 L-arginine diphosphate 0.98

71

3.5.6 Thermogravimetric measurements Venkataramanan et al [26] have made thermal analysis on the grown BTCC

crystals. Thermogravimetric (TG) analysis of BTCC showed that the compound does

not sublime nor does it undergo any phase transition. DSC studies showed that BTCC

crystals melts at 215° and does not lose weight at melting. It turns yellowish after

melting, ruling out the possibility of melt growth.

Ushasree and Jayavel [165] have reported that TG trace of dried powder of

single crystals of BTCC exhibit a single stage decomposition at 200°C directly

without producing any detectable weight loss dependent intermediate compounds.

Though BTCC has high melting point, crystal cannot be grown from the melt, as it

turns yellow after melting due to decomposition. The high melting point of BTCC

compared with other organic crystals is attributed to the existence of stronger bonding

between the thiourea molecules and metal ion.

The thermograms of the pure, Mg2+ and Zn2+ doped BTCC crystals were

reported in the literature [171]. The thermograms appear nearly similar for the three

samples with four stages of decomposition between 200 and 750°C. The first and

fourth stages of decompositions have produced maximum weight loss compared to

the second and third stages. The maximum decomposition temperature for parent

sample was found to be 244°C, whereas for the Mg2+ and Zn2+ doped BTCC the

values are 246 and 249°C respectively. The slight increment in temperature is evident

for the doped crystals, suggesting that the substitution of Cd2+ by Mg2+ / Zn2+ can

enhance the thermal stability of the BTCC crystals. The increment in temperature for

the doped crystals correlates with the lessor ionic radius of the Mg2+ (0.65Å) and Zn2+

72

(0.69 Å) than Cd2+ (0.92 Å). The lessor ionic radius of Mg2+ and Zn2+ can give more

bonding interaction with thiourea, thus giving more thermal stability to the crystal.

Both pure and doped BTCC crystals are thermally more stable than that of some

organometallic complex NLO crystals such as CMTC (203°C), LACC (110°C),

ATCC (101°C), CMTD (150°C), ATCB (133°C), ATMB (125°C) and CMTC

(100°C) [183].

The TG trace of the BTCI compound showed a weight loss starting close to

(200°C) [5]. It is due to the decomposition of the crystal. But below 200°C, there was

no weight loss, and hence the crystal was free from any lattice entrapped water. The

residue after the first stage of decomposition, was also seemed to decompose at higher

temperature. The DTA showed a sharp endothermic peak at 117.09°C which was

assigned to the melting of the compound. Thus thermal studies revealed that BTCI

was stable up to its melting point (117.09°C).

3.5.7 Electrical measurements No reports are available on d.c. conductivity measurements on BTCC and

BTCI crystals. But various reports are available on dielectric measurements.

The dielectric permittivity is maximum at low frequencies and increases with

increasing frequency. The increase in the dielectric constant at low frequency is

attributed to space charge polarization. From a value of 0.99 at 0.1 kHz, the dielectric

loss decreases to 0.09 at 100 kHz for BTCC single crystals [165].

Variation of dielectric constant (∈′) of pure and doped (Mg2+ and Zn2+) BTCC

crystal as a function of frequency along [100] orientation was also reported [171]. It

73

was found that dielectric constant of these three samples decreases with increase in

frequency. The trend of the dielectric constant of both pure, Mg2+ and Zn2+ doped

BTCC crystals was almost the same. But for a fixed frequency, the dielectric constant

of doped BTCC crystal was more than that of pure one, which may be due to the

lighter mass of dopants than pure BTCC. The low value of dielectric loss indicates

that the grown BTCC crystals have lesser defects.

The Ni2+ doped BTCC crystals have high dielectric constants compared to

pure crystal; and Co2+ doped BTCC crystal has low dielectric constant compared to

pure crystals [174]. The variation of dielectric constant is due to incorporation of

metal ions inside the BTCC lattices and also, the characteristic of low dielectric loss

with high frequency for the sample suggests that the crystal possess enhanced optical

quality with lesser defects and this parameter plays a vital role for the construction of

devices from nonlinear optical materials.

The dielectric study on BTCI showed that the maximum dielectric constant

560 appeared in the lower frequency region (100 Hz) and minimum value (310) in the

high frequency region (5 MHz). Crystals with high dielectric constant lead to power

dissipation. The material having low dielectric constant will have less number of

dipoles per unit volume. As a result it will have minimum losses when compared to

the material having high dielectric constant. Dielectric loss strongly depends on the

frequency of the applied field which is similar to the dielectric constant in the ionic

system [5].