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CHAPTER 2
GROWTH AND CHARACTERIZATION OF
1H-IMIDAZOLINIUM HYDROGEN L-TARTRATE
SINGLE CRYSTALS
2.1 INTRODUCTION
Nonlinear Optical (NLO) materials are of current research interest
in materials science for their applications in second and third harmonic
generation, optical bistability, laser remote sensing, optical disk data storage,
laser driven fusion, medical and spectroscopic laser (Santhanu Bhattacharya
et al 1994). Organic molecules possess large second order molecular
polarizability (β) and more favorable physical properties like large optical
damage threshold and large birefringence. L-tartaric acid is a chiral
dihydroxycarboxylic acid and it is capable of initiating multidirectional
hydrogen bonding (Aakeröy et al 1994). The salts of tartaric acid belong to an
important class of materials because of their interesting physical properties
such as ferroelectricity, piezoelectricity and nonlinear optical properties
(Second Harmonic Generation).
The L-tartaric acid analogs were incorporated into organic salts and
their NLO properties were widely studied in recent experiments (Renuka
Kadirvelraj et al 1998). The nonlinear optical (NLO) properties of some
complexes of L-tartaric acid nicotinamide have attracted significant attention
because organic components contribute specifically to the process of second
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harmonic generation (Haja Hameed et al 2004). The 1H-Imidazolinium
Hydrogen L-tartrate single crystals are grown by slow evaporation solution
growth technique. Single crystal XRD and the theoretical factor group
analysis were carried out. The factor group analysis reveals the vibrational
modes. The IR spectrum has been recorded to confirm the functional groups
present in the material and the Second Harmonic Generation behaviour of
grown crystal was studied.
2.2 Growth of 1H-Imidazolinium Hydrogen L-tartrate single
crystals
The 1H-Imidazolinium Hydrogen L-tartrate was synthesized and
grown using two different solvents. Firstly the Imidazole and L(+)-tartaric
acid (equimolar ratio) were dissolved separately in ethanol and deionized
water, respectively and they were mixed together. The mixture of solutions
was found to be turbid and ethanol was added and stirred well for an hour by
using a motorized magnetic stirrer till a clear solution was obtained. The
solution was filtered using Whatman (grade no.1) filter paper in clean vessels
and the vessels containing the solution were closed with perforated polythene
cover and housed in the constant temperature bath (CTB) for growth at 32°C.
Single crystals were obtained within 12 days and one of the harvested crystals
is shown in Figure 2.1.
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Figure 2.1 As grown 1H-Imidazolinium Hydrogen L-tartrate single
crystal using ethanol as solvent
Secondly, the equimolar (1:1) ratio of Imidazole (C3N2H4)
(SRL-extra pure) and L (+) tartaric acid (C4H6O6) (Merck-extra pure) were
dissolved separately in deionised water. Then the solutions of the individually
prepared raw materials were mixed together and continuously stirred for six
hours. The solution was filtered using Whatman (grade No.1) filter paper in
clean vessels and the vessels containing the solution were closed with
perforated polythene covers and housed in the constant temperature (CTB)
bath at 32°C. The nucleation was observed in seven days and allowed to grow
for four weeks. The reaction is shown in the Figure 2.2. The crystal of size
15mm × 10mm × 5mm is obtained after four weeks (Figure 2.3).
N
NH
H
H
H
COOH
COOH
OH
OH
+N
NH
H
H
H
COOH
COOH
OH
OH
C3N2H4 + C4H6O6 C7N2H10O6
Imidazole L(+)-tartaric acid Imidazolinium Hydrogen L-tartrate
Figure 2.2 Reaction Scheme of 1H-Imidazolinium Hydrogen L-tartrate
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Figure 2.3 As grown 1H-imidazolinium Hydrogen L-tartrate single
crystal using water as solvent
2.3 CHARACTERIZATION OF 1H-IMIDAZOLINIUM
HYDROGEN L-TARTRATE SINGLE CRYSTALS
2.3.1 Single crystal XRD analysis
From the X-ray diffraction results, it has been found that IH-
imidazolinium Hydrogen L-tartrate belongs to the monoclinic crystal system
with space group P21 having two molecules in the unit cell. The single crystal
X-ray diffractometer (model Nonius CAD-4/MACH) with MoKα (0.71073Å)
radiation was used to obtain the accurate cell parameters of the grown
1H-Imidazolinium Hydrogen L-tartarate crystals at room temperature by the
least square refinement of the setting angles of 25 reflections. The obtained
lattice parameters are presented in Table 2.1, which are in good agreement
with the reported values (Aakeröy and Hitchcock 1993).
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Table 2.1 Crystallographic data of 1H-Imidazolinium Hydrogen
L-tartarate
Lattice
parametersPresent work
Reported
(Aakeröy and Hitchcock 1993)
a 7.555(4)Å 7.569(1) Å
b 8.989(4) Å 8.993(1) Å
c 6.961(4) Å 6.953(1) Å
α 90° 90°
β 101.43°(4) 101.55°(1)
γ 90° 90°
V 463.4(4) Å3 463.7 Å3
2.3.2 High Resolution X-ray Diffraction Studies on ImiLT Crystal
Good quality single crystals are much needed for device
fabrication. The defects are discrete entities and their location and degree of
disturbance produced in a lattice can be determined experimentally. The high
resolution X-ray diffraction technique (multicrystal X-ray diffractometer) is a
non-destructive analysis and can be used for direct observation of boundaries
and dislocations.
The high resolution X-ray diffraction analysis was carried out to
study the structural perfection of 1H-Imidazolinium Hydrogen L-tartrate. A
multicrystal crystal X-ray diffractometer (MCD) designed and developed at
National Physical Laboratory has been used to study the crystalline perfection
of the single crystal(s). Figure 2.4 shows the schematic diagram of the
multicrystal X-ray diffractometer. In this system a fine focus X-ray source
(Philips X-ray Generator; 0.4 mm × 8 mm; 2kWMo) energized by a well
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collimated and monochromated MoKα1 beam obtained from the three
monochromator Si crystals set in dispersive (+, -, -) configuration has been
used as the exploring X-ray beam. The specimen crystal is aligned in the
(+, -, -, +) configuration. Due to dispersive configuration, though the lattice
constants of the monochromator crystal(s) and the specimen are different, the
unwanted dispersion broadening in the diffraction curve of the specimen
crystal is insignificant.
Figure 2.4 Schematic of the Multicrystal X-ray diffractometer set up
Before recording the diffraction curve, the specimen surface was
prepared by lapping and polishing and then chemically etched by a non
preferential chemical etching using the etchant of the mixture of water and
acetone in 1:2 ratio. Figure 2.5 shows the high resolution X-ray diffraction
curve (rocking curve) recorded with high resolution X-ray diffractometer
using (100) diffracting planes for 1H-Imidazolinium Hydrogen L-tartrate
single crystal. Figure 2.5 shows the DC is quite sharp without any satellite
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-100 -50 0 50 1000
100
200
300
400
500
600
Dif
frac
ted
X-r
ay
inte
nsit
y[c
/s]
Glancing angle [arc s]
22"
peaks. The full width at half maximum (FWHM) of the diffraction curves is
22 arc sec, which is close to that expected from the plane wave theory of
dynamical X-ray diffraction. The single sharp diffraction curve with low
FWHM indicates that the crystalline perfection is quite good. The 1H-
Imidzolinium Hydrogen L-tartrate is a nearly perfect single crystal without
having any internal structural boundaries.
Figure 2.5 Rocking curve of ImiLT
2.3.3 Spectral Analysis
2.3.3.1 Factor group analysis
The factor group and the site group are important in the application
of group theoretical methods for the analysis of spectra of solids. Symmetry
analysis is made by applying all the symmetry operations of the factor group
to each atom in the unit cell, and reducing the representation thereby obtained
in order to determine the number of normal modes belonging to each
irreducible representation. An additional advantage of the factor group
method is that it provides a basis for the prediction of the IR and Raman
spectra of lattice vibration (Rousseau et al 1981).
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2.3.3.1.1 Vibrational analysis of 1H-imidazolinium hydrogen L-tartrate
1H-Imidazolinium Hydrogen L-tartrate crystallizes in the
monoclinic crystal system with the non-centrosymmetric space group P21 and
factor group symmetry22C . The factor group analysis of the unit cell of
1H-Imidazolinium Hydrogen L-tartrate is carried out using the character table
for the site symmetry group C1(2). The two molecules of the primitive unit
cell of 1H-Imidazolinium Hydrogen L-tartrate occupy general sites of C1 (2)
symmetry. A single molecule of 1H-Imidazolinium Hydrogen L-tartrate
crystal contains 25 atoms which in turn gives rise to 150 modes. Group
theoretical analysis of 1H-Imidazolinium Hydrogen L-tartrate gives 150
vibrational optical modes which decompose into Γ150 = 74A + 73B apart from
three acoustic modes (A + 2B). In monoclinic crystals like 1H-Imidazolinium
Hydrogen L-tartrate, the modes have associated polarizability tensors of the
form
0
0 0
0 0
xx xy
yy
zz
A
α α
α
α
� �� �
= � �� �� �
0 0
0 0
0 0 0
xz
yzB
α
α
� �� �
= � �� �� �
Here the polarizability tensors are depicted along the crystallographic
X-, Y- and Z-axes. Both phonon A and B are Raman and IR active. The
summary of the factor group analysis of 1H-Imidazolinium Hydrogen
L-tartrate is presented in Table 2.2.
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Table 2.2 Factor group analysis - Summary
Site symmetry
C1(2)
Factor
group
symmetry
22C Ext Int
C H N O Optical Acoustic Total
A 1T 3R 69 21 30 6 18 75 1 74
B 2T 3R 69 21 30 6 18 75 2 73
Total 3T 6R 138 42 60 12 36 150 3 147
Analysis of the vibrational spectra reveals the information
regarding the nature of bonding, structure of co-ordination compounds and
material confirmation. The molecular structure of 1H-Imidazolinium
Hydrogen L-tartrate enumerates that the title compound consist of C-H, N-H,
and O-H groups etc. The observed vibrations of 1H-Imidazolinium Hydrogen
L (+) tartrate could be due to lattice vibrations and internal vibration. The
bands observed between 4000 cm-1 and 400 cm-1 in Figure 2.6 arise from the
internal modes of 1H-Imidazolinium Hydrogen L-tartrate. The bands obtained
below 400 cm-1 arise from the deformational vibrations and the vibrational
and translational modes of anions and cations. Table 2.3 presents the
correlation scheme obtained by following the procedures of Fately et al
(1972). Each internal mode of 1H-Imidazolinium Hydrogen L-tartrate ions
split into two components of (A(Z), B(X) and B(Y)) are IR active and
A(αxx, αyy, αzz, αxy) and B(αxz, αyz) are Raman active.
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Table 2.3 Correlation Scheme of 1H-Imidazolinium Hydrogen L-tartrate
ActivitySite symmetry Factor group
symmetry Raman IR
74A αxx, αyy, αzz, αxy Z
A150
73B αxz, αyz X,Y
.
2.3.3.1.2 Internal Vibrations
As the 1H-Imidazolinium Hydrogen L-tartarate molecules do not
have any symmetry, the internal vibrations exhibited are of both IR and
Raman active exclusive of acoustic mode. The internal vibrations of
1H-Imidazolinium Hydrogen L-tartrate may be arising from the C-H, N-H
and O-H functional groups. These vibrations are strongly coupled between
themselves.
2.3.3.1.3 External vibrations
The bands observed below 400 cm-1 are mainly due to external
modes. The rotational modes are expected to have higher frequency and
intensity than translational modes in the Raman spectra. However, the
translational modes are more intense in IR spectra (Bhatacharjee 1990,
Hanuja and Fomitsev 1980) of 1H-Imidazolinium Hydrogen L-tartrate. It is
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found to have (A+2B) translational and (A+2B) rotational vibrations in the
title compound which can be achieved experimentally by polarized Raman
measurements.
2.3.4 Infrared Spectral Analysis
The mid Fourier Transform Infrared spectrum of 1H-Imidazolinium
Hydrogen L-tartrate was recorded at room temperature in the region
4000–400 cm-1 by JESCO 416 PLUS FT-IR spectrophotometer equipped
with LiTaO3 detector, KBr beam splitter and He-Ne Laser source boxcar
apodization used for 250 averaged interferogram collections for both the
sample and background using KBr pellet technique. The recorded FT-IR
spectrum of the title compound is shown in Figure 2.6 and the functional
group assignments were made using the standards (Martin Britto Dhas et al
2007a, Vijayan et al. 2004). The strong broad peak at 3491cm-1 is due to the
presence of O-H stretching in the carboxyl group. The N-H stretches of
Imidazole ring produce broad intense signals between 2000 and 3000 cm-1.
In the present case a very strong peak occurs at 3319 cm-1 which indicates the
functional groups of the title compound. The peak observed at 1726 cm-1
indicates the presence of C=O bond. The aromatic ring vibrations produce
their characteristic peaks at 1584 cm-1 and 1410 cm-1. The sharp peak at
1211 cm-1 is due to C-H bending vibrations of the aromatic ring. The weak
intensity peak at 842 cm-1 is assigned to the symmetric stretch, further it is
attributed to five membered Imidazole ring. The strong peak at 1262 cm-1 is
assigned to the breathing mode of imidazole ring in plane C-H deformation
(Juan Antonio Asensio et al 2002).
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Figure 2.6 FT-IR Spectrum of 1H - Imidazolinium Hydrogen L-tartrate
2.4 DIELECTRIC STUDIES
Dielectric properties are correlated with the electro-optic property
of the crystals (Boomadevi and Dhanasekaran 2004). The dielectric
measurements of the 1H-Imidazolinium Hydrogen L-tartrate were made using
HIOKI 3532 HiTESTER LCR meter. Good quality single crystals of 1H-
Imidazolinium Hydrogen L-tartrate were polished on soft tissue papers with
fine grade alumina powder. The sample was electroded on either side with
silver paste to make it behaves like a parallel plate capacitor. The studies were
carried out and the capacitance, dielectric loss (tanδ) and ac conductivity of
the sample were measured as a function of frequency (50 Hz to 5 MHz) and
temperature (in the range 35°C, 50°C and 100°C). A small cylindrical furnace
with dimensions 20 cm × 20 cm × 20 cm was used for the experiment and the
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temperature was controlled by Eurotherm temperature controller (±0.01°C).
The dielectric constant was calculated using the relation
0r
Cd
Aε
ε= (2.1)
where ε0 is the permittivity of dielectric region, C is the capacitance, d is the
thickness of the grown 1H-Imidazolinium Hydrogen L-tartrate crystal and A
is the area of cross section of the crystal used for experiment.
The frequency dependent dielectric constant is shown in Figure 2.7.
The dielectric constant decreases with increasing frequency and becomes
almost saturated beyond 10 kHz for all temperatures (35°C, 50°C and
100°C). The higher value of dielectric constant is due to higher space charge
polarization at lower frequency region. This may be explained on the basis of
the mechanism of polarization similar to the conduction process. The
electronic exchange of the number of ions in the crystal lattice gives local
displacement of the applied field, which gives the polarization. As the
frequency increases, at which the space charge cannot sustain and comply the
external field. Therefore the polarization decreases and exhibiting the
reduction in the value of dielectric constant with increasing frequency. The
magnitude of the dielectric constant depends on the degree of polarization
charge displacement in the crystals. The dielectric constant of materials is due
to the contribution of electronic, ionic, dipolar and space charge polarization
which depends on the frequencies (Dharmaprakash et al 1989). At low
frequencies, all these polarizations are active. The space charge polarization is
generally active at lower frequencies and high temperatures.
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Figure 2.7 Variation of Dielectric constant of ImiLT with frequency
The change of dielectric loss (tanδ) with frequency is represented
for the as grown crystal in Figure 2.8. It is observed that the dielectric loss
decreases with increasing frequency. The low value of dielectric loss indicates
good quality (Benet Charles and Gnanam 1994) of the crystal. The larger
value of dielectric loss (tanδ) at lower frequencies may be attributed to space
charge polarization owing to charged lattice defects (Smyth 1965). The low
values of dielectric loss indicate that the grown crystal contains minimum
defects.
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Figure 2.8 Variation of dielectric loss of ImiLT with frequency
The conductivity of 1H-Imidazolinium Hydrogen L-tartrate
increases with increase in temperature. The electrical conduction in dielectrics
is mainly a defect controlled process in the low temperature region. It is
inferred from Figure 2.9 that the electrical conductivity of 1H-Imidazolinium
Hydrogen L-tartrate is low at low temperature owing to trapping of some
carriers at defect sites. At any particular temperature, however the Gibb’s free
energy of a crystal is minimal when a certain fraction of ions leaves the
normal lattice. As the temperature increases, more and more defects are
created, and as a result, the conductivity, which is predominantly due to the
movement of defects produced by thermal activation, increases (Jain et al.
1964).
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1 2 3 4 5 6 7
0.00000
0.00002
0.00004
0.00006
0.00008
0.00010co
nd
uc
tiv
ity
(S/c
m)
log f
(35oC)
(50oC)
(100oC)
Figure 2.9 Variation of conductivity of ImiLT with frequency
2.5 OPTICAL STUDIES OF ImiLT Crystal
To determine the absorption range and hence to know the
suitability of 1H-Imidazolinium Hydrogen L-tartrate single crystals for optical
applications, UV-Vis spectrum was recorded with 2 mm thick crystal
between 200 – 800 nm using UV-VIS-NIR (PERKIN ELMER LAMBDA
35) Spectrometer which covers ultra violet (200-400 nm) and visible
(400-800 nm) region. The spectrum obtained is attributed to the promotion of
electrons in σ, π and n- orbital from the ground state to higher state. The
recorded UV-Vis spectrum of the title compound is shown in Figure 2.10.
The spectrum indicates the absorbance due to electronic transition between
338 nm and 800 nm. The cut off wavelength 338 nm may be assigned to the
electronic transitions in the aromatic ring of 1H-Imidazolinium Hydrogen
L-tartrate single crystals. Absence of absorbance in the region between
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400 and 800 nm is an advantage as it is a key requirement for materials
having NLO properties.
Figure 2.10 UV-Vis Spectrum of 1H-Imidazolinium Hydrogen
L-tartrate
2.6 TGA-DTA Analysis of ImiLT
Thermogravimetric and differential thermal analyses give
information regarding phase transition, water of crystallization and different
stages of decomposition of the crystal (Meng et al 1998). The
thermogravimetric analysis (TGA) was carried out on the 1H-Imidazolinium
Hydrogen L-tartrate crystals and TGA spectrum was recorded in Nitrogen
atmosphere between 50 and 500°C using NETZSCH STA 409 C/CD TGA
unit. The recorded 1H-Imidazolinium Hydrogen L-tartrate is shown in
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Figure 2.11. There is no weight loss between 50°C and 204°C. This indicates
that there is no inclusion of water in the crystal lattice, which was used as the
solvent for crystallization. The thermogram spectrum reveals that the major
weight loss (around 92%) starts at 204.4°C and it continues up to 250°C. The
nature of weight loss indicates the decomposition point of the material.
However, below this temperature no weight loss is observed. In the DTA
spectrum an irreversible exothermic peak observed around 204.4°C
corresponds to the decomposition temperature of the material.
Figure 2.11 TG and DTA curves of 1H-Imidazolinium Hydrogen
L-tartrate
2.7 SECOND HARMONIC GENERATION
A quantitative measurement of the Second Harmonic Generation
(SHG) conversion efficiency of 1H-Imidazolinium Hydrogen L-tartrate was
made by the Kurtz and Perry powder technique (1968). The schematic of the
experimental set up is shown in Figure 2.12. The finely powdered sample of
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1H-Imidazolinium Hydrogen L-tartrate was densely packed between two
transparent glass slides. A fundamental Laser beam of 1064 nm wavelength
from an Nd: YAG (DCR11) laser was made to fall normally on the sample
cell. The power of the incident beam was measured using a power meter. The
transmitted fundamental wave was passed over a monochromator (Czerny
turner monochromator) which separates 532 nm (second harmonic signal)
from 1064 nm, and absorbed by a CuSO4 solution F1 which removes the
1064 nm light. F2 is a BG-38 filter, which also removes the residual 1064 nm
light. F3 is an interference filter with bandwidth of 4 nm and central
wavelength 532 nm. The green light was detected by a photomultiplier tube
(Hamamatsu R5 109, a visible PMT) and displayed on a storage oscilloscope
(TDS 3052 B 500 MHz phosphor digital oscilloscope). KDP and Urea
crystals were separately powdered to identical particle size and were used as
reference materials in the SHG measurement. A bright green flash emission
from the title sample was observed which indicates the NLO behavior of the
material. The SHG of 1H-Imidazolinium Hydrogen L-tartrate crystal was
found to be 75 mV and that of 95 mV for KDP.
Figure 2.12 Experimental set up used for measuring the relative SHG
efficiency
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2.8 VICKER’S MICROHARDNESS STUDIES
The mechanical characterization of 1H- Imidazolinium Hydrogen
L-tartrate crystals has been done by microhardness testing at room
temperature. Transparent 1H-Imidazolinium Hydrogen L-tartrate crystals free
from cracks having the dimension of 3mm × 3 mm × 2 mm, with flat and
smooth faces are chosen for the static indentation tests. The crystal was
mounted properly on the base of the microscope. Now the selected faces have
been indented gently by applying loads varying from 10 to 50 g for a dwell
period of 3 second using Vickers diamond pyramid indenter attached to an
incident researcher microscope. The indented impressions are pyramidal in
shape. The shape of the impression is structure dependent, face dependent and
also material dependent. The length of the two diagonals has been measured
by a calibrated micrometer attached to the eyepiece of the microscope after
unloading and the average is found out. For a particular load five
well defined impressions were considered and the average of all the diagonals
(d) was considered. The Vickers hardness numbers (Hv) have been calculated
using the standard formula
2
1.8544v
PH
d= kg/mm2 (2.2)
where P is the applied load in kg and d in mm. Crack initiation and materials
clipping become significant beyond 50 g of the applied load. Hence hardness
test could not be carried out above this load. Figure 2.13 shows the variation
of Hv as a function of applied load ranging from 10 to 50 g. It is clear from
the Figure 2.13 that Hv increases with increase in load (5 to 50 g). This is
known as load dependent hardness and here its value is found to be
approximately 50 kg/mm2. Such a phenomenon of dependence of
microhardness of a solid on the applied load at low level of testing load is
58
known as indentation size effect (ISE). The observed increase in hardness
with increasing load is usually termed as reverse indentation size effect
(Ramesh Babu et al 2006).
Figure 2.13 Plot between load and Hardness number
2.9 CONCLUSION
Bulk single crystals of 1H-Imidazolinium Hydrogen L-tartrate were
grown by slow evaporation solution growth technique. The optical studies
show the absence of absorption above 338 nm. The SHG efficiency is
comparable to that of the standard KDP crystal. From FT-IR spectrum, the
functional groups were identified. The occurrence of π-π* transition in the
carboxyl group accounts for the nonlinearity in the title compound. The
dielectric behaviour of 1H-Imidazolinium Hydrogen L-tartrate was analysed.
Group theoretical analysis of 1H-Imidazolinium Hydrogen L-tartrate reveals
59
that there are 150 vibrational optical modes which are seen to decompose into
Γ150 = 74A + 73B apart from three acoustic modes (A + 2B). The thermogram
of 1H-Imidazolinium Hydrogen L-tartrate crystal recorded in the present
work, reveals that the incipient melting occurs at 204.4°C. The hardness
study enumerates that grown crystals are moderately harder substance. Based
on these facts, it could be proposed that this material can be better
accommodated for optical applications.