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CHAPTER - IV
SYNTHESIS, GROWTH AND CHARACTERIZATION OF PICRATE
SINGLE CRYSTALS
4.1 INTRODUCTION
ngineering of new nonlinear optical (NLO) materials, structures and
devices with enhanced figure of merit has developed over the last two
decades as a major force to help drive nonlinear optics from the laboratory to real
applications. Because of their potential applications in photonic devices, the NLO
properties of molecules and their hyperpolarizabilities have become an important area
of extensive research and a lot of experimental [1,2] and theoretical efforts [3,4] are
focused on the bulk NLO properties as well as their dependence on the
hyperpolarizabilities of molecules. An organic molecule should have large second-
order hyperpolarizability to exhibit good nonlinear optical properties [5]. The
hyperpolarizability can be enhanced by increasing intramolecular charge transfer
interaction by extending π-conjugated system [6]. The term charge transfer gives a
certain type of complex resulting from interactions of donor and acceptor with the
formation of weak bonds [7, 8].
Molecular complexes in which extensive charge transfer interactions between
electron donors and acceptors molecules are generally expected to have high NLO
properties. Picric acid forms crystalline picrates of various organic molecules through
ionic and hydrogen bonding and π-π interactions. Bonding of electron donor/acceptor
E
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
picric acid molecules strongly depends on the nature of the partners. Picric acid
derivatives are interesting candidates, as the presence of phenolic OH favors the
formation of salts with various organic bases. The conjugated base, picrate formed has
increased molecular hyperpolarizability because of the proton transfer.
In the present investigation, we attempted to synthesis the molecular complex
adduct of triethylamine with picric acid involving charge transfer from donor to
acceptor followed by proton transfer from the acceptor. The solubility of complex
salts in methanol has been determined gravimetrically. Single crystals were grown by
low temperature solution growth technique. Material formation and purity was
confirmed by CHN analysis. The structural properties of the grown crystals were
characterized by single crystal and powder X-ray diffraction techniques. Fourier
transform infrared spectroscopic analysis, UV-Vis-NIR analysis, TG/DTA and second
harmonic generation measurements were also carried out.
4.2 REVIEW OF LITERATURE
Graham Smith et al (2004) have reported that the monoclinic polymorph of
anilinium picrate shows a three dimensional hydrogen bonded polymer with strong
primary interspecies interactions involving the proximal phenolate and adjacent nitro
group O-atom acceptors and separate anilinium H-atom donors in two cyclic
associations [9]. P.Srinivasan et al (2006) have been grown good quality single
crystals of L-asparaginium picrate by a low temperature solution growth technique
[6].
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
A comparative study of infrared and Raman spectra of DL-valine DL-valinium
picrate (DL-VVP) and DL-methionine DL-methioninium picrate (DL-MMP) at the
room temperature in 4000 - 50 cm−1 range helps to determine the effect of hydrogen
bonds in these crystals. The existence of the zwitterion and the protonated form in
both the crystals have been observed by M.Briget Mary et al (2006) [10].
A.Chandramohan et al (2007) have synthesized the crystalline substance of N,N-
dimethyl anilinium picrate (DMAP) and the single crystals were grown by slow
evaporation solution growth technique at room temperature [11].
R.Bharathikannan et al (2008) have been investigated the charge transfer
complex adduct of 2 nitro aniline with picric acid. The needle shaped crystals were
grown by slow evaporation method [12]. A.Chandramohan et al (2008) have been
synthesized the acenaphthene picrate material and single crystals were grown and
fundamental studies were characterized [13]. T.Uma Devi et al (2008) have
investigated the synthesis of glycine picrate material and have grown single crystals.
The cell parameters and functional groups present in the material were studied [14].
A.Chandramohan et al (2008) reported the synthesis, growth and
characterization of caffeinium picrate (CAFP) material [15]. P.Srinivasan et al
(2008) reported the Z scan determination of the asparaginium picrate crystals. The
magnitude of third order susceptibility and non-linear refractive index were also
determined [16]. S.A.Martin Britto Dhas et al (2008) characterized the vallinium
picrate single crystals. Structural, functional and mechanical studies were performed
for the grown crystals [17].
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
P.Srinivasan et al (2008) reported the synthesis, growth, crystal structure
determination, and hyperpolarizability studies of L-argininium-4-nitro phenolate
monohydrate (LARP) single crystals. First order hyperpolarizabilty of LARP has been
computed using density functional theory [18]. T.Uma Devi et al (2008) have
investigated the synthesis and growth of prolinium picrate crystals. The cell
dimensions were obtained by single crystal X-ray diffraction study. FTIR, UV-Vis-
NIR and fluorescence spectral analyses were carried out for the grown crystals.
Thermo gravimetric study was also carried out to determine the thermal properties of
the grown crystal [19].
A.Chandramohan et al (2008) have synthesized the crystalline substance of
naphthalene picrate (NP) and single crystals were grown using slow evaporation
solution growth technique [20]. G.Anandha Babu et al (2010) have reported the
growth of single crystal of dimethylammonium picrate (DMAP). High resolution X -
ray diffraction study was carried out for the grown crystals. The optical, dielectric and
mechanical studies were also performed [21]. S.Natarajan et al (2010) have been
grown the organic nonlinear optical (NLO) crystal from the amino acid family, viz., l-
threoninium picrate (LTHP) by solvent evaporation technique from aqueous solution
[22].
B.Dhanalakshmi et al (2010) have been grown the bulk single crystals of L-
histidine-4-nitrophenolate 4-nitrophenol (LHPP) using slow evaporation solution
growth technique at room temperature [23]. M.Magesh et al (2011) have grown
single crystal of dimethyl ammonium picrate (DMAP) by slow evaporation solution
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
technique (SEST) and subsequently by Sankaranarayanan Ramasamy (SR) method
using acetone as solvent [24]. G.Anandha Babu et al (2011) reported the growth of
1,3-dimethylurea dimethylammonium picrate crystal. The crystal structure of the
grown material has been determined [25].
S.Anandhi et al (2011) have reported the synthesis and growth of organic
crystal of imidazolium picrate (IP) by the slow cooling solution growth method using
ethanol and acetone as solvents. The structural, thermal, optical and mechanical
properties were studied for the grown crystal [26]. A.Antony Joseph et al (2011)
have grown glycine mixed L-valine picrate (GVP) from saturated aqueous solution by
slow evaporation method [27]. S.Gowri et al (2011) have investigated the spectral,
thermal and optical properties of L-tryptophanium picrate [28]. K.Muthu et al (2011)
have studied the proton transfer complex of 2,4,6-trinitrophenol as an electron
acceptor with p-toluidine as electron donor [29].
G.Bhagavannarayana et al (2011) have grown the single crystals of L-
leucine L-leucinium picrate (LLLLP) by the slow evaporation solution technique and
fundamental characterizations were analysed [30]. S.Gowri et al (2012) have reported
the synthesis and growth of adenosinium picrate crystals by solution growth
technique. Fundamental characterizations of the grown crystals have been studied
[31]. Mohd.Shkir et al (2012) have studied the growth, spectroscopic, relative second
harmonic generation (SHG) efficiency and thermal analysis of 2-aminopyridinium
picrate (2APP) [32]. T.Chen et al (2012) have grown the good-quality single crystals
of L-histidinium-4-nitrophenolate 4-nitrophenol (LHPP) by slow cooling method [33].
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
N.Sudharsana et al (2012) have been determined the centrosymmetric crystal
structure of hydroxyethylammonium picrate (HEAP) crystals by single crystal X-ray
diffraction analysis [34]. Preparation, crystal growth and molecular structure as well
as vibrational spectra of the crystal L-alanine L-alaninium picrate monohydrate were
described by V.V.Ghazaryan et al (2012) [35].
4.3 EXPERIMENTAL DETAILS
4.3.1 Material synthesis
Analar grade of picric acid, triethylamine and methanol were used for the
synthesis process. The picric acid is less soluble in water. For the synthesis of
complex salt triethylamine picrate (TEAP), equimolar quantities of the parent
compounds picric acid and triethylamine were dissolved in methanol separately and
mixed together, then stirred well for about half an hour. When a proton is transferred
from the electron-donor group of an acid to the electron acceptor group of a base, it
results in increase of hyperpolarizability of the resultant compound. The picric acid
necessarily protonates the amino group of the triethylamine resulting in the formation
of the yellow colored precipitation of the charge transfer complex salt TEAP. The
yellow precipitation of the complex salt was filtered off and then further purified
using methanol by recrystallization process. The purified material was then used as a
raw material for the growth process. The reaction involved in the synthesis process is
illustrated in figure 4.1.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.1 Synthesis Scheme of TEAP material
4.3.2 Solubility measurement
The equilibrium solubility and its temperature dependence are essential for
solution growth. The data from the solubility curve will suffice to start growing fair
quality single crystals. The solubility of TEAP in methanol was assessed as a function
of temperature in the temperature range 30 - 45 °C in steps of 5 °C. Synthesized
TEAP salt was dissolved in methanol in an airtight container and kept in the constant
temperature bath (CTB). On reaching saturation, the equilibrium concentration of the
solute was determined by gravimetric method. Figure 4.2 shows the solubility curve
of TEAP. We infer that the TEAP exhibits good solubility compared to other organic
materials and a positive solubility temperature gradient (direct solubility) in methanol
solvent. Hence this material is appropriate for bulk crystal growth by solution growth
method.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.2 Solubility curve of TEAP in methanol
4.3.3 Crystal growth
Saturated solutions of TEAP in methanol at 40 °C were prepared in accordance
with the determined solubility data using the recrystallized salt and then stirred for
half an hour for the homogenous solution mixture. The solution was filtered by
Whatman filter sheet in order to remove the suspended impurities from the solution.
The filtered solution was transferred into 100 ml vessel having uniform perforated
closure and this vessel was placed in a constant temperature bath (CTB) having a
controlling accuracy of ± 0.02 °C for solvent evaporation. By employing solvent
evaporation method, the nucleated crystals were allowed to grow for a definite period
and then harvested from the mother solution. The grown single crystal of TEAP from
methanol is shown in figure 4.3.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.3 As grown TEAP single crystal
4.4 CHARACTERIZATION OF TEAP CRYSTAL
4.4.1 CHNS analysis
The purity and percentage compositions of the constituent elements present in
the synthesized compound were examined by CHN analysis using Elemental vario
micro CHNS analyzer. The percentage of elements present in the synthesized TEAP
material is given in table 4.1. It shows that the C, H and N values are fairly in good
agreement with the theoretically calculated values. The result further indicates that
TEAP is free from impurities and devoid of the water molecules in any form. From
the result, the stoichiometry and hence the molecular formula of the synthesized
material is confirmed.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Table 4.1 Elemental composition of the TEAP material
TEAP
Elements
Theoretical Experimental
C 43.59 % 43.34 %
H 5.45 % 4.79 %
N 16.95 % 17.22 %
4.4.2 Single crystal X-ray diffraction (SXRD) analysis
Single crystal X-ray diffraction is by far the most popular method for the
identification of substances for the investigation of crystal structure and degree of
crystalline perfection. In this study, good quality crystal of TEAP was chosen and the
cell parameters of the grown crystal was estimated using MACH 3 Nonius CAD - 4 X-
ray diffractometer with Mo Kα radiation (λ = 0.7107 Å). From the results, TEAP
crystallizes into orthorhombic crystal system. Cell parameter values of TEAP
determined from the single crystal X-ray diffraction analysis are given as;
a = 6.947 Å, b = 20.735 Å, c = 21.941 Å and β = 90° and cell volume V = 3161 Å3.
4.4.3 Powder X-ray diffraction (PXRD) analysis
Powder X-ray diffraction studies were also carried out for the complex salt
TEAP to demonstrate the crystallinity using PANalytical model X’PERT PRO X-ray
diffractometer system. The Kα radiations from a copper target (λ = 1.5406 Ǻ) was
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
used. The single crystals of TEAP were ground into fine powder and then powdered
samples were spread over a square centimeter area and placed in a beam of
monochromatic X-rays. The mass of powder was rotated about all possible axes. From
the θ value for each peak, the spacing d was obtained. The diffraction peaks were
indexed by least square fitting and X-ray diffraction pattern of TEAP is shown in
figure 4.4. Appearance of sharp and strong peaks confirm the good crystalline nature
of the crystals. The lattice parameters of TEAP crystal were calculated theoretically
using the powder XRD data (table 4.2) and it is in good agreement with the values
obtained from single crystal XRD.
Figure 4.4 Powder X-ray diffraction pattern of TEAP
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Table 4.2 Lattice parameter values of TEAP crystal
Sample a (Å) b (Å) c (Å) β Cell Volume (Å3)
TEAP
(SXRD) 6.947 20.735 21.941 90° 3161
TEAP
(PXRD) 6.894 20.797 21.973 89° 3160
4.4.4 Fourier transform infrared (FTIR) spectral analysis
Infrared spectroscopy is used to identify the functional groups and modes of
vibration of the synthesized complex salts. In charge transfer complex, a proton
transfer from donor to the acceptor is expected to take place which is strongly
supported by the appearance of a new band of medium intensity in the spectrum of
TEAP. However, the bands of donor and acceptor were shifted and this shift owes to
the changes in the electronic structure on the formation of charge transfer complex. In
order to analyze qualitatively the presence of functional groups in TEAP, the FTIR
spectrum was recorded using a Thermo Nicolet 380 FTIR spectrometer by the KBr
pellet technique in the range of 400 - 4000 cm-1. The FT-IR spectrum of TEAP is
shown in Figure 4.5. The bands observed in the spectra of the complex salt TEAP
arises from the internal vibrations of the picric acid (comprises nitro group vibration
and OH vibration), triethylamine (encompass the methyl group and ethyl group
vibrations).
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Vibration of N+- H group
In TEAP, the amino N atom of triethylamine cation form N-H bond with the
picrate anion. Intermolecular hydrogen bonding between the donor and the acceptor
molecules is the root cause for the NLO property of the picrate materials [36]. This
intermolecular hydrogen bonding exist in the charge transfer complex is expected at
around 3400 cm-1. The peak observed at around 3408 cm-1 is the evidence for
hydrogen bonding in the TEAP. In the IR spectrum of TEAP, asymmetric N+-H
deformation modes are observed at 1629 cm-1 and the bending vibrations of N-H are
found at 714 cm-1 respectively.
Figure 4.5 FTIR spectrum of TEAP
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Vibration of NO2 group
The asymmetric vibration of NO2 group is observed at 1553 cm-1 for TEAP.
The absorption at 1331 cm-1 is due to the NO2 symmetric vibration of TEAP. Usually
for the free picric acid NO2 vibration occurs at 1607 cm-1 [37]. Charge transfer
interaction in the complex salt TEAP, NO2 vibration is shifted to lower frequency at
1553 cm-1 due to the increased electron density of the picric acid. The rocking modes
of NO2 group are identified at 529 cm-1 for the TEAP. The NO2 scissoring vibrational
modes appear in the spectra of TEAP at 787 cm-1. The band observed at 913 cm-1 is
due to the C-NO2 stretching vibration in TEAP.
Vibration of Phenolic and Phenoxcy groups
In the charge transfer interaction of TEAP, picric acid necessarily protonates
the phenolic O vibration which produces peaks at 1156 cm-1 [38]. The absorption peak
at 1271 cm-1 can be ascribed to the C-O vibration of the TEAP complex salt.
Vibration of CH3 group
Internal vibration of the cations in the TEAP arises from the functional group
of CH3. The peak at 3031 cm-1 for TEAP is attributed to asymmetric stretching
vibration of C-H in methyl group. The symmetric stretching vibration of C-H in the
methyl group is observed at 2746 cm-1 for TEAP. The asymmetric C-H deformation of
methyl group occurs near 1497 cm-1 for TEAP. The peaks at 1073 cm-1 and 1082 cm-1
indicate the rocking vibrations of methyl group.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Vibration of CH2 group
The CH2 deformation mode in TEAP appears at 1443 cm-1. The
observed vibrational frequencies and their corresponding assignments are presented in
table 4.3.
4.4.5. Laser Raman study
The grown single crystal of TEAP was subjected to laser Raman spectral study
using a laser Raman Spectrometer model (R3000) with 532 nm as the operating
source in the region 3500 - 400 cm-1. The recorded laser Raman spectrum is shown in
figure 4.6. The sharp and broad peaks obtained are due to hydrogen bonding. The
peak at 1356 cm-1 is assigned to symmetric stretching of NO2. The peak at
1271.09 cm-1 confirms C-O vibration of the crystal. C-NO2 stretching is assigned at
915.99 cm-1. The peaks at 753.64 and 541.96 cm-1 are attributed to N+-H bending and
NO2 rocking respectively. The absorption peak obtained at 1156 cm-1 in the spectrum
representing phenolic ‘O’ vibration of the crystal. The presence of this band in both
FTIR and Raman spectra confirms the formation of TEAP salt. The laser Raman
spectral assignments are given in table 4.4.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Table 4.3 Frequency assignments of TEAP
Vibration TEAP
(cm-1)
Assignments
3408 Intermolecular hydrogen bonding N+-H asymmetric
Stretching
1629 R2 N+- H deformation mode
N+- H group
714 Bending of N+- H
1553 Asymmetric stretching vibration of NO2 group
1331 Symmetric stretching vibration of NO2 group
787 Scissoring of NO2
529 Rocking of NO2
NO2 group
913 Stretching vibration of C- NO2
Phenolic group 1156 Phenolic O vibration
Phenoxcy
group 1271 C-O vibration
2746 Symmetric C-H stretching vibration of methyl group
1497 Asymmetric C-H deformation of methyl group
1073 CH3 rocking Methyl group
3031 Asymmetric stretching vibration of methyl group
Ethyl group 1443 CH2 deformation
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.6 Laser Raman spectrum of TEAP
Table 4.4 Assignments of Raman spectrum of TEAP
Raman (cm-1) Assignments
1356 Symmetric stretching of NO2
1271 C-O vibration
1141 Phenolic O vibration
1022 CH3 rocking
915 Stretching of C-NO2
753 Bending N+-H
541 Rocking of NO2
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
4.4.6 UV-Vis-NIR analysis
The optical absorption spectrum of TEAP was recorded in the range 200 - 1100
nm which is shown in figure 4.7. Strong absorption was observed at 362 nm for
TEAP, which is attributed to π- π* transition of picrate ion. It is seen from the
spectrum that the crystal is transparent in the range 450 to 1100 nm without any
intermediate absorption peak due to the charge transfer of the electron from the donor
to the acceptor. This is an essential parameter for NLO crystals and can be used as
SHG material in the visible range.
Figure 4.7 Absorption spectrum of TEAP
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.8 Transmittance spectrum of TEAP
To determine the transmission range, the UV-Vis transmittance spectrum was
recorded for the grown crystal in the range 200 - 1100 nm (Figure 4.8). The UV
cutoff wavelength of TEAP was observed at 361 nm. This spectrum again confirms
the suitability of this crystal for optoelectronic applications and second- order
harmonic generation of the Nd:YAG laser (1064 nm). In order to determine the band
gap of the grown crystals, extrapolation of the straight line in the plot of (αhν) 2 versus
hν, has been done for TEAP (Figure 4.9) where α is the absorption co efficient and hν
is the photon energy. The band gap energy of TEAP was calculated as 3.55 eV.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.9 Plot of energy versus (αhν)2 for TEAP
4.4.7 Second harmonic generation measurement
The relative second harmonic generation behaviour of the charge transfer
complex salt TEAP was tested using the Kurtz and Perry method [39]. The grown
single crystal of TEAP was grounded into fine powder with uniform particle size and
then filled into the micro capillary tube. Then high-intensity Nd:YAG laser (λ =1064
nm) with a pulse duration of 10 ns was passed through the micro capillary tube. The
emission of bright green radiation (λ = 532 nm) from the samples confirm the
generation of second harmonics. The second harmonic signal of 30 mV for TEAP was
obtained for an input energy of 5.3 mJ/pulse. The SHG value of reference KDP
samples gives a signal of 18.5 mV/pulse for the same input energy. Thus, it is
observed that the SHG efficiency of the title compound TEAP was 1.62 times than
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
that of the standard KDP crystal. The extent of charge transfer across the NLO
chromophore determines the level of SHG output of the material, the greater the
charge transfer, the larger the SHG output. The presence of strong intermolecular
interactions can extend the level of charge transfer into the supramolecular realm,
thereby enhancing the SHG response [40, 41].
4.4.8 Dielectric study
In order to carry out the dielectric measurements, carefully selected samples of
TEAP single crystal were cut and later polished to obtain a good surface finish.
Dielectric study was carried out from 35-50 °C at different frequencies range from
100 Hz to 100 kHz. The capacitance and the dielectric loss were measured at different
temperatures for TEAP crystal and then subsequently the dielectric constant (εr) was
calculated. Frequency dependence of dielectric constant (εr) and dielectric loss of
TEAP crystals at different temperatures are shown in figure 4.10 and 4.11
respectively.
Both the dielectric constant (εr) and the dielectric loss (D), are inversely
proportional to the frequency. This can be understood on the basis that the mechanism
of polarization was similar to that of the conduction process. The electronic exchange
of number of ions in the crystal give local displacement of electrons in the direction of
the applied field, which in turn gives rise to polarization. As the frequency increases, a
point will be reached where the space charge cannot sustain and comply with the
external field and hence the polarization decreases, giving rise to diminishing values
of (εr) and D.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.10 Frequency dependence of dielectric constant of TEAP crystal
Figure 4.11 Frequency dependence of dielectric loss of TEAP crystal
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Continuous gradual decrease in D as well as (εr) suggests that TEAP crystal is
like any normal dielectric, may have domains of different sizes and varying relaxation
times. The high value of (εr) at lower frequencies may be due to the presence of all the
four polarizations, namely space charge, orientational, electronic and ionic
polarizations and its low value at higher frequencies may be due to the loss of
significance of these polarizations gradually. The low value of dielectric loss with
high frequency for these samples suggests that the samples possess enhanced optical
quality with lesser defects and this parameter is of vital importance.
4.4.9 Thermal analysis
Thermal stability and physiochemical changes of the grown TEAP crystal has
been identified in powder form by recording TG/DTA curve in the temperature range
0 and 600 °C using NETZSCH STA 449 F3 analyzer under nitrogen atmosphere at a
rate of 10 °C/min. Figure 4.12 shows the thermal properties of the TEAP crystal
carried out by TG/DTA. In the differential thermogram, sharp exothermic peak was
found at 155.3 °C. This exothermic is assigned to the melting point at which no
weight loss from TG has been noticed. The sharp endothermic reaction observed at
around 252.3 °C may be possibly due to some complex formation. There is steep loss
of weight starting around 252.3 °C and after complex formation the weight loss is
gradually decreased.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
Figure 4.12 TG and DTA spectra of TEAP
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
4.5 CONCLUSION
The organic charge transfer molecular complex salt triethylaminium picrate
was synthesized and purified by recrystallization process using methanol. Solubility of
TEAP in methanol was determined by gravimetric method. The single crystals of
TEAP were grown by slow evaporation method using methanol as a solvent.
Elemental analysis data confirm the purity, stoichiometry and molecular formula of
TEAP crystal. As grown single crystal of TEAP was characterized by single crystal
X-ray diffractogram, which reveals that TEAP crystallizes into orthorhombic crystal
system. From the powder XRD pattern the various planes of reflections have been
identified and reconfirmed the lattice parameters and crystal system of TEAP. FTIR
and laser Raman spectral studies established the molecular structure of TEAP and also
bring forth the evidence for the prevalent charge transfer activity in the complex salt.
The UV-Vis-NIR spectrum of TEAP in solution mode exhibits a wide transparency in
the visible region between 450 and 1100 nm due to the π- π* transition of picrate ion
in the complex salt. The band gap energy of TEAP was estimated from the UV -Vis
spectrum. The relative SHG activity in the complex salt was confirmed by employing
Kurtz and Perry method. The result reveals that SHG efficiency of TEAP is 1.62 times
greater than that of KDP. Dielectric constant and loss of TEAP decreases with
increase in frequency. The very high value of dielectric constant at lower frequencies
may be due to the presence of all the four polarizations and its low value at higher
frequencies may be due to the loss of these polarizations gradually. Thermal stability
of the grown TEAP was confirmed by TGA and DTA analyses.
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CHAPTER-IV | Synthesis, Growth and Characterization of Picrate Single Crystals
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