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].