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CHAPTER 3
SYNTHESIS, GROWTH, OPTICAL, THERMAL AND
MECHANICAL PROPERTIES OF 2-AMINOPYRIDINIUM
4-METHYLBENZOATE DIHYDRATE
3.1 INTRODUCTION
Organic crystals show large nonlinear optical (NLO) properties
(Williams 1983, Chemla and Zyss 1987). 2-Aminopyridinium
4-methylbenzoate Dihydrate 5 7 2 8 7 2 2( . .2 )C H N C H O H O+ − (2A4M) is an organic
NLO compound. The crystal structure of 2A4M was reported by Yun Liu and
Jie Li (2008). The title compound is composed of 4-methylbenzoate anion,
one 2-amino pyridinium cation and two water molecules. Carboxyl group of
4-methylbenzoic acid is deprotonated. In the crystal, 2-amino pyridinium
cation and 4-methylbenzoic acid anion together with water molecules are
linked into a three-dimensional supramolecular framework by multiple
N—H···O and O—H···O hydrogen bonds (Yun Liu and Jie Li 2008).
Pyridine and their organic complexes exhibit a large amount of fluorescence
in crystalline environment (Arivanandhan et al 2006). As such, pyridine
compounds are considered to be an optical material. The optical properties of
the derivatives of 2-Aminopyridine and their suitability for the fabrication of
optical devices were studied and reported (Periyasamy et al 2007).
This chapter presents the synthesis, growth and characterization of
2-Aminopyridinium 4-methylbenzoate Dihydrate (2A4M) by the slow
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evaporation solution growth technique at room temperature using water as a
solvent.
3.2 SYNTHESIS AND GROWTH OF 2-AMINOPYRIDINUM
4-METHYLBENZOATE DIHYDRATE
4-Methylbenzoic acid (1 mmol) and 2-aminopyridine (1 mmol)
were dissolved in 20 ml of double distilled water at room temperature and
stirred using a teflon coated magnetic stirrer for one hour. The solution was
refluxed at 353 K to avoid evaporation of 2-aminopyridine. The product of
2-Aminopyridinum 4-methylbenzoate Dihydrate was obtained. The reaction
mechanism of 2-Aminopyridinium 4-methylbenzoate Dihydrate is depicted in
Figure 3.1.
N-H
NH2
+
OO
_
.2H2ON-H
NH2
OO
.2H2O+
Figure 3.1 Reaction mechanism of 2-Aminopyridinium
4-methylbenzoate Dihydrate
The synthesised solution was cooled to room temperature. The
prepared solution was filtered using Whatman filter paper in a degreased
beaker and optimally covered by a thin polythin paper to avoid fast
evaporation of solvent. The solution was housed in the constant temperature
bath at room temperature for slow evaporation and closely observed every
day. After two weeks nucleation was observed in the solution and bulk
colourless crystals of size 11 × 4 × 3 mm3 were obtained in 25 days. The as
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grown 2A4M crystals are shown in Figure 3.2. The harvested crystals were
cut and polished for various characterization studies.
Figure 3.2 As grown single crystals of 2-Aminopyridinium
4-methylbenzoate Dihydrate
3.3 CHARACTERIZATION
3.3.1 Single Crystal XRD
The single crystal X-ray diffraction data of 2A4M crystal were
collected using Nonius CAD4/MACH 3 single crystal X-ray diffractometer
using MoKα radiation (λ = 0.71073 Å). It is observed that the crystal belongs
to the monoclinic system with noncentrosymmetric space group Cc. The
obtained lattice parameters are presented in Table 3.1. These values are in
good agreement with the literature (Yun Liu and Jie Li 2008). Crystal
morphologies are predicted from data stored in files in the CIF format
(crystallographic information file standard of the International Union of
Crystallography), (Bravais et al 1866, Friedel et al 1907, Donnay and Harker
et al 1935). CIF data are given as input in the Win X Morph software package
and the morphology diagram of 2A4M crystal is drawn and shown in the
Figure 3.3. As grown crystal of 2A4M from solution and the morphology of
the grown crystal resembles each other.
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Table 3.1 Crystallographic data of 2A4M
Lattice Parameter Present study Reported values
(Yun Liu and Jie Li 2008)
a 12.179(2)Å 12.2059 (14) Å
b 13.128(2) Å 13.1531 (16)Å
c 8.981(2) Å 8.9937 (11)Å
β 96.62(1)° 96.617 (2)°
V 1426.5(4) Å3 1434.3 (3) Å3
System Monoclinic Monoclinic
space group Cc Cc
Figure 3.3 Morphology diagram of 2A4M single crystal
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3.3.2 Optical Studies
3.3.2.1 Transmittance
The UV–Vis transmittance spectrum was recorded for the grown
2A4M crystal using a SHIMADZU UV-2501 PC, UV–Vis spectrophotometer
in the range 250 – 700 nm. Figure 3.4 shows the transmittance spectrum of
the 2A4M crystal, which indicates a good transmittance in the visible region
and there is no significant absorption between 349 and 700 nm. The presence
of lone electron pairs of O atoms in carbonyl groups in 4-methylbenzoic acid
is favourable to the promotion of an electron to an unoccupied orbital giving
an (n, π*) excited state. The lower cut off at 350 nm combined with very
good transparency attest the usefulness of this material for optoelectronics
applications (Bhat 1994, Sethuraman et al 2008).
Figure 3.4 Transmittance spectrum of 2-Aminopyridinium
4-methylbenzoate Dihydrate
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3.3.2.2 Optical band gap
Optical band gap of the title compound was calculated from the
transmittance spectrum. The measured transmittance (T) was used to calculate
the absorption coefficient (α) using the relation 2.1 and 2.2 given in the
second chapter. The band gap was calculated from the plot between hν and
(αhν) 1/2 as shown in the Figure 3.5 and the optical band gap is found to be
2.9 eV.
Figure 3.5 Plot between photon energy and (αhν) 1/2
of 2A4M
3.3.2.3 Refractive index (n)
The refractive index of the material can be measured from the
reflectance. The reflectance (R) in terms of the absorption coefficient can be
obtained using the equations 2.3 and 2.4 (Presented in second chapter).
Figure 3.6 shows the wavelength dependent refractive index (n) of 2A4M
crystal. The refractive index (n) is 1.40 at 1200 nm for 2A4M crystal. The
optical behaviour of crystal can be correlated with dielectric behaviour. The
complex dielectric constant is fundamental intrinsic material property. The
real part of it is associated with the term that how much it will slow down the
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speed of light in the material and imaginary part gives that how a dielectric
absorb energy from electric field due to dipole motion. The real part dielectric
constant εr and imaginary part dielectric constant εi can be calculated using
following relations
2( )
r ii n ikε ε ε= + = + (3.1)
where εr and εi are the real and imaginary parts of the dielectric constant
respectively and are given by
εr = n2-K2 and εi = 2nK. (3.2)
The value of real (εr) and imaginary (εi) dielectric constants, at
λ = 1200 nm are 1.38 and 3.82 × 10-4 respectively.
Figure 3.6 Wavelength dependent refractive index of 2A4M crystal
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3.3.2.4 Photoluminescence (PL)
Photoluminescence (PL) measurement is a prominent tool for
determining the crystalline quality of a system as well as its exciton fine
structure. The PL spectrum was recorded using a Perkin Elmer (LS 45) PL
unit at room temperature with slit width 10 nm in the wavelength range of
340–450 nm. The recorded PL spectrum of 2A4M crystal is shown in
Figure 3.7 with the excitation wavelength of 240 nm. From the PL spectrum,
a strong and broad emission peak centered at 386 nm was observed. A higher
intensity ratio specifies better purity and crystallinity of 2A4M. The
broadening of emission band is due to lattice vibrations in the 2A4M crystal.
The emission is assigned to electronic transition from π* antibonding
molecular orbital to π bonding molecular orbital of 2A4M.
Figure 3.7 The PL spectrum of 2A4M crystal
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3.3.3 FT-IR Spectral Study
The FT-IR spectrum of 2-Aminopyridinium 4-methylbenzoate
Dihydrate was recorded using Perkin Elmer spectrophotometer. Figure 3.8
shows the FT-IR spectrum recorded in the range 4000-500 cm-1. The peak due
to O-H stretch of water appears as a shoulder in the higher energy portion of
the N-H stretch vibration occurring at 3266 cm-1. The aromatic
C-H vibration gives the peak at 3021 cm-1. The aliphatic C-H vibration occurs
as a shoulder to the peak just below 3000 cm-1. The peaks at 2029 cm-1 and
1974 cm-1 are due to combination bands of 2-Aminopyridinium and
4-methylbenzoic acid. The peak at 2029 cm-1 is due to combination of the
peak at 1260 and 764 cm-1. The peak at 1974 cm-1 is the combination of the
peak at 1383 and 608 cm-1. The C═O vibration of the carboxylate group gives
its peak at 1684 cm-1. The ring skeletal vibrations of aromatic rings give peaks
at 1605, 1515 and 1436 cm-1 (Silverstein and Webster 1998).
The bending vibration of water occurs at 1639 cm-1 indicating the
presence 2 moles of water which can also be verified from the TGA analysis.
The intense sharp peak at 1383 cm-1 is due to COO– vibration. The groups of
peaks between 1660 and 850 cm-1 are due to C-N vibration. The
1, 4-disubsituted aromatic ring gives its C-H bend at 842 cm-1. All other
vibration below this is due to C - H bending modes of other aromatic rings.
From the above discussion it is verified that 2-Aminopyridinium and
4-methylbenzoic acid remains the salt of dihydrate.
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Figure 3.8 FT-IR spectrum of 2-Aminopyridinium 4-methylbenzoate
Dihydrate
3.3.4 Thermal Analysis
The thermal stability of 2A4M was identified by the
thermogravimetric (TG) and differential thermal analyses (DTA). The thermal
analyses were carried out using a NETZSCH STA 409 C/CD system between
the temperatures 30°C and 500°C at a heating rate of 10°C/ minute in nitrogen
atmosphere in the alumina crucible. The TG and DTG of the compound are
shown in the Figure 3.9. The initial weight loss up to 125°C corresponds to
13.2%, illustrating the presence of 2 moles of water in the lattice. Desorption
of water is immediately followed by decomposition of 2-Aminopyridinium
4-methylbenzoate Dihydrate. The decomposition extends up to 300°C.
The DTA trace is shown in the Figure 3.10, the initial endotherm
below 100°C is due to desorption of water. The sharp clear endothermic peak
at 125°C is due to melting point of complex. It is followed by endothermic
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decomposition of the salt. Comparison of DTA and TGA results illustrates the
NLO applicability of the material is limited up to 125°C.
Figure 3.9 TGA of 2-Aminopyridinium 4-methylbenzoate Dihydrate
Figure 3.10 DTA of 2-Aminopyridinium 4-methylbenzoate Dihydrate
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3.3.5 Microhardness Measurements
The mechanical behaviour of the grown 2A4M crystal was studied
Using Reichert MD 4000E ultra microhardness tester fitted with a Vickers
diamond pyramidal indenter in the plane (3 1 1). The indentations were made
at room temperature with a constant indentation time of 2 second. The as
grown 2A4M crystal was cut with dimension of 6 mm × 4 mm × 2 mm,
polished and subjected to Vicker’s hardness testing. The (3 1 1) plane of
prepared crystal was properly mounted on the base of the microhardness
tester attached with microscope and indented gently by applying the loads
10 to 70 g in steps of 10 g with a dwell time of 2 sec. The indented
impressions for various loads 10 g, 40 g (initial stage of crack formed) and
80 g (deformed stage) are shown in the Figure 3.11.
(a) 10 g (b) 40 g (c) 80 g
Figure 3.11 Indentation impression of 2-Aminopyridinium
4-methylbenzoate Dihydrate for (a) 10 g, (b) 40 g
and (c) 80 g
From the Figure 3.11 it can be seen that radial cracks increase in
length with increasing load and are accompanied at higher loads by severe
in-plane fracture to form lateral cracks. The lateral cracks appear as light
halos and lie parallel to the plane. The lengths of the two diagonals of the
indentations were measured by a calibrated micrometer attached to the eye
piece of the microscope after unloading. The Vickers’s hardness number was
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calculated using the relation given in equation 1.11 (in chapter 1).
Figure 3.12 (a) shows the plot between load (P) and (Hv) of 2A4M single
crystal. It is very clear that Hv increases with the increase of load. The
increase in the hardness number can be attributed to the electrostatic attraction
between the zwitterions present in the molecule. Above 30 g, cracks were
observed, owing to the release of internal stress generated locally by
indentation.
(a) (b)
Figure 3.12 (a) Variation of (Hv) with Load (P) of 2A4M single crystal,
(b) Plot of log P with log d
The Meyer's index number was calculated from the Meyer's law,
which relates the load and indentation diagonal length using the relation given
in equations 1.12 and 1.13 (presented in chapter 1). The plot between log P
and log d, (Figure 3.12 b) is a straight line and the slope of this straight line
gives the value of n. The calculated value of n is 5, from the expression Hv
should increase with the increase of P if n > 2 and decrease if n < 2. The value
of n agrees well with the experiment. According to Onitsch (1947) n should
lie between 1 and 1.6 for harder materials and above 1.6 for softer materials.
Thus 2A4M belongs to the soft material category.
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For a crystal with well-defined cracks, the resistance to fracture
indicates the toughness of a material. According to Ponton and Rawling
(1989) fracture toughness Kc is dependent on the ratio of c/a, where c is the
crack length and a is the half-diagonal length of the square indentation as
shown in Figure 1.10 in the first chapter.
For c/a ≥ 2.5, the cracks are developed with median crack system
and the fracture toughness c/a is calculated using the equation (1.14), where
(l = c–a) is the mean Palmqvist crack length. From the Figure 3.13 (a) it can
be observed that as the load increases the fracture toughness also increases for
2A4M crystal.
The brittleness index Bi is calculated using the relation (1.16) from
the hardness values, the yield strength σy can be calculated and it is defined as
the stress at which the material begins to deform plastically and depends on
Meyer's index n. For n > 2, the yield strength σy may be calculated using the
expression (1.17). For n < 2, the yield strength is calculated using the relation
(1.18). It is seen from the Figure 3.13 (b) that the yield strength also increases
as load increases. The obtained data are tabulated in Table 3.2.
Table 3.2 The data of Mechanical properties on the (311) face of 2A4M
crystal
P(g) Hv
(kg/mm2)
c (µm) K (MPa.m1/2
) Bi (m1/2
)
30 38.5 46 0.425 886.6
40 44.6 24 1.448 301.6
60 57.8 26 1.901 297.9
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The elastic stiffness constant (C11) for different loads calculated
using Wooster's empirical formula C11 = Hv7/4 (Wooster 1953).
Figure 3.13 (c) shows the plot of Hv7/4
with C11, stiffness constant increases
with hardness value which gives an idea about the tightness of bonding
between neighbouring atoms. The increase in hardness accompanied by
increase in fracture toughness, brittleness index and stiffness constant are
clearly observed.
(a) (b)
(c)
Figure 3.13 (a) Fracture toughness with load of 2A4M single crystal,
(b) Plot between load and yield strength and (c) The plot
of Load with C11
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3.3.6 Etching Studies
The etching studies were carried out on the (3 1 1) plane of the
as-grown single crystal of 2A4M using distilled water as an etchant at room
temperature for the etching time of 15s, 30s, 45s, 60s, 4 minutes and
6 minutes. In the present experimental work, transparent crystal free from
inclusions and cracks was selected. Etching of the crystal surface was carried
out by dipping the crystal in water at room temperature. The etched surfaces
were dried by gently pressing them between two filter papers and then
immediately examined and their microstructures were analyzed using an
optical microscope in the reflection mode. Some well defined and
crystallographically aligned etch pits were observed on the as grown surface.
Figure 3.14 (a) represents the photomicrograph of as grown crystal
before etching. Figures 3.14 (b) shows some rectangular growth hillocks. The
shape of the etch pits indicates the direction of the dislocation lines.
Figures 3.14 (c) (d) (e) show the bunch of rectangular growth hillocks of
2A4M crystal and there is no change in etch pits with varying etching time
(15–60 s). As etching time increases the rectangular hillock got elongated in
their size. This observation may also be associated with a decreasing
undersaturation at the dislocation source with increasing time. Figures 3.14 (f)
and (g) show elongated rectangular hillocks increases with the increase of
etching time while the pit pattern remains same. The observed etch pits, due
to layer growth, confirmed the two-dimensional nucleation (2D) mechanism
with less dislocations (Mukerji and Kar 1999).
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(a) As grown crystal surface (b) 15 sec (c) 30 sec
(d) 45 sec (e) 60 sec (f) 4 minutes
(g) 6 minutes
Figure 3.14 Etch pit patterns of 2A4M single crystal on (3 1 1) face with
water as an etchant (a) As grown crystal surface, (b) 15 s,
(c) 30 s, (d) 45 s, (e) 60s, (f) 4 min and (g) 6 min
3.3.7 Second Harmonic Generation Test
The second harmonic generation behaviour was tested by the
Kurtz-Perry powder technique using Nd: YAG laser as a source
(λ = 1064 nm). The powdered material of the crystal was packed in the
capillary tube and beam energy of 4.9 mJ/pulse was given as input. The
powdered sample of 2A4M was illuminated by the laser source. The second
harmonic signal generated in the sample was collected and detected by the
monochromator, which is coupled with the photomultiplier tube. The bright
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green emission was observed from the output of the powder of the 2A4M.
KDP sample was used as the reference material and the output power
intensity of 2A4M was compared with the output power of KDP. A second
harmonic signal of 82 mV was obtained from 2A4M and 27 mV for reference
material KDP. The output power of 2A4M is 3.03 times that of standard KDP.
3.4 CONCLUSION
A good quality bulk single crystal of 2A4M was successfully
grown by slow evaporation solution growth method. The lattice parameters
are evaluated by single crystal X-ray diffraction analysis. It revealed that the
crystal belongs to monoclinic system. The optical band gap (2.9 eV) and the
refractive index (n) were obtained from the optical transmittance data. The
functional groups were confirmed by FT-IR. The thermal behaviour of the
grown crystal was studied by TG-DTA. From the Vickers hardness studies
hardness (Hv), Fracture toughness (Kc), Brittleness index (Bi), Yield strength
(σy) and Elastic stiffness constant (C11) were calculated. The work hardening
coefficient was evaluated as 3. The etching study revealed rectangular
hillocks and one layer growth mechanism. Kurtz-Perry powder method was
used to confirm the SHG of the material. SHG relative efficiency of 2A4M is
3.03 times that of KDP.