Download CHAPTER 4 GROWTH OF SEMIORGANIC TRIGLYCINE CALCIUM

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CHAPTER 4
GROWTH OF SEMIORGANIC TRIGLYCINE CALCIUM
DIBROMIDE CRYSTAL AND ITS CHARACTERIZATION
4.1
INTRODUCTION
Amino acids with inorganic compounds are promising materials for
nonlinear optical applications, as the high optical nonlinearity of the purely
organic amino acid tends to combine with the favorable mechanical and
thermal properties of the inorganic salt. Nonlinear optical (NLO) materials are
used in optical computing, optical communication, harmonic generators,
medical diagnostics, frequency mixing and optical switching (Prasad and
Williams 1991, Iwai et al 1997, Lin et al 1999). The potential development of
optoelectronic devices based on the nonlinear polarization of molecular
materials has aroused much recent interest (Eaton 1991, Dorn et al 1992). The
search for large second-order electric susceptibilities (that is, proportional to
the square of an applied electric field) has concentrated on acentric organic or
organometallic chromophores with an organic
-electron system coupling
electron donor and acceptor groups (Laidlaw et al 1993). Theoretical studies
also show that hydrogen bonds can strongly enhance the nonlinear optical
responses of bulk materials (Dongfeng Xue and Siyuan Zhang 1998,
Dongfeng Xue and Siyuan Zhang 1999, Dongfeng Xue and Henryk Ratajczak
2003, Dongfeng Xue and Henryk Ratajczak 2005), which widely exist in
semiorganic crystals. Many semiorganic nonlinear optical materials have been
grown by slow solvent evaporation technique, which are attracting a great
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deal of attention in the nonlinear optical field from application point of view.
L-Hystidine Tetra-Fluoro-Borate (L-HFB) is a semiorganic nonlinear optical
material whose single crystal exhibits more NLO properties than that of
inorganic crystals like KDP, BBO, LBO and many other semiorganic
materials (Sinha et al 2009).
4.2
PRESENT WORK
The crystal structures of many addition compounds of glycine with
inorganic acids and metallic salts and also many metal complexes of glycine
are known. Because of the biological and chemical importance of both
glycine and calcium, a series of glycine complexes with calcium halides, viz
CaC12, CaBr2 and CaI2, were crystallized and their structures were studied
(Mohana Rao and Natarajan 1980). In this chapter we are presenting growth
and characterization of triglycine calcium dibromide such as XRD, optical,
thermal, dielectric and mechanical studies.
4.3
GROWTH OF TRIGLYCINE CALCIUM DIBROMIDE
Single crystals of triglycine calcium dibromide were grown from a
saturated aqueous solution containing glycine (Merck) and calcium dibromide
(Lobo) in the molar ratio 1:1. A necessary quantity of glycine is taken in a
beaker and dissolved in Millipore water of resistivity 18.2M
cm at room
temperature until it attains saturated condition. After preparing saturated
solution of glycine, the proportionate amount of calcium dibromide was
added little by little with continuous stirring of the solution for bringing a
homogeneous mixture. Due to exothermic reaction, the temperature of the
solution increases to 50 oC. The prepared solution of pH value 5.4 was filtered
and allowed to evaporate slowly at ambient temperature. Colorless good
optical single crystals of triglycine calcium dibromide of size 20 × 12 × 4
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mm3 were harvested in forty days. The photograph of the as grown single
crystals of the same is shown in the Figure 4.1. The title compound triglycine
calcium dibromide is formed from the starting 1:1 molar ratio solution of
glycine and calcium dibromide due to the more lattice energy benefit for 3:1 than
1:1 coordination of glycine and calcium dibromide. The expected chemical
reaction for triglycine calcium dibromide is as follows:
3NH2CH2COOH + 3CaBr2
[Ca(C2H5NO2)3]2+.2Br¯
+ 2CaBr2 (in solution)
(4.1)
Figure 4.1 Photograph of as grown triglycine calcium dibromide crystal
4.4
CHARACTERIZATION STUDIES
4.4.1
Single Crystal X-ray Diffraction
X-ray diffraction studies were carried out to reveal the crystal
structure of the grown crystal. The unit cell parameters obtained are
a = 9.095(6)Å, b = 14.752(3)Å, c = 20.217(20)Å,
= 90.34(6)°,
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= 90.46(7)°,
= 90.07(4)° and volume = 2713(3) Å3. Triglycine calcium
dibromide belongs to orthorhombic crystal structure with the space group
Pbc21. When the obtained cell parameters are compared, the result is in good
agreement with the reported values (Mohana Rao and Natarajan 1980).
4.4.2
Fourier Transform Infrared Analysis
Theoretical studies carried out by both Phillips-Van Vechten-
Levine-Xue bond theory and chemical bonding theory of single crystal
growth indicate that some important functional groups such as hydrogen
bonds and ammonia group play a very important role in crystallization and
nonlinear optical performances (Dongli Xu and Dongfeng Xue 2008,
Xiue Ren et al 2008, Zdzislaw Latajka et al 2009). The Fourier transform
infrared analysis of triglycine calcium dibromide single crystal was carried
out between 4000 and 450 cm-1 using Perkin Elmer spectrum one FT-IR
spectrometer. Figure 4.2 shows the resulting spectrum of that analysis, in
which the functional groups present in the molecules, can be identified by
stretching vibration. In this spectrum the broad and strong band obtaining
between 3049 and 2598 cm-1 is due to the NH3+ stretching vibration (Robert
Silverstein and Francis Webstar 1998). A weak asymmetrical NH3+ bending
band occurs at 1627 cm-1 and a fairly strong symmetrical bending band occurs
at 1522 cm-1. The carboxylate ion group COO- absorbs strongly at 1591 cm-1
and more weakly at 1426 cm-1 (John Coats 2000, Sharma 2007). These bands
result from asymmetrical and symmetrical C( O)2 stretching respectively.
This observation confirms that glycine molecules exist in zwitterionic form.
The absorption at 515 cm-1 is due to the presence of oxygen-calcium bond
(Bharat Parekh and Mihir Joshi 2007). Absorption peaks characterizing
different functional groups are shown in the Table 4.1.
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515 504
670
528
897
40
1125
2689
2598
60
3431
Transmittance %
1040
595
1830
80
2017
2386
2266
100
3049
1653
1627
15911522
142614801415
1331
20
0
4000
3500
3000
2500
2000
1500
1000
500
-1
Wavenumber (cm )
Figure 4.2 FTIR spectrum of triglycine calcium dibromide crystal
Table 4.1 Frequencies of the fundamental vibrations of TGCB crystal
Frequency in
wavenumber (cm-1)
3049
Assignment of
vibrations
NH3+ stretching
1653
N-H bending
1627
NH3+ weak asymmetrical bending
1591
COO- asymmetric stretch
1522
NH3+ strong symmetrical bending
1426
COO- symmetric stretch
1331
C-H bending
1040
C-N stretching
897
O-H out of plane bending
670
C-Br stretching
515
Ca-O bond
504
torsional oscillations
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4.4.3
UV-Vis-NIR Studies
In order to reveal optical properties of the triglycine calcium
dibromide single crystal, UV-Vis-NIR transmission spectrum was recorded in
the range of 200 to 1100 nm using Perkin Elmer Lamda 35 UV/VIS
spectrometer. The as grown crystal of 2mm thickness was used for recording
the spectrum. Figure 4.3 shows the transmittance curve, in which the lower
cut off region is obtained at 240 nm. Further it is found that the maximum
transmittance of the grown triglycine calcium dibromide single crystal is 52%
and it has almost more than 45% transmittance from 400 to 1100 nm.
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50
Transmittance (%)
40
30
20
10
0
200
400
600
800
1000
1200
Wavelength (nm)
Figure 4.3 UV-Vis-NIR spectrum of triglycine calcium dibromide crystal
4.4.4
Laser Damage Threshold Studies
Laser damage threshold studies were made on the as grown
triglycine calcium dibromide crystal of 2 mm thickness using Nd:YAG laser
of 532 nm wavelength and spot size about 140 m in the method of multiple
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shots mode. A lens of focal length 8 cm was used to focus the light spot on
the crystal. The pulse width and repetition rate of the laser pulse was adjusted
to 7 ns and 10 pulse/sec respectively. When the laser beam of energies 10 mJ
and 20 mJ were made to be incident on the crystal for 30 seconds
respectively, there were no remarkable changes. But, when the beam energy
was adjusted to 30 mJ for 32 seconds, the crystal got damaged.
4.4.5
NLO Studies
Since the process of second-harmonic generation (SHG) is relevant
to new laser technology, and the electro-optic effect, SHG efficiency of the
grown crystal was determined by Kurtz powder technique. The crystalline
sample was ground into very fine powder and tightly packed in a
microcapillary tube. Then the tube was placed in the path of Nd:YAG laser
beam of energy 1.95 mJ/pulse. When potassium dihydrogem phosphate
crystal in the form of powder was used as a reference material, the transmitted
laser beam voltage was 62 mV. But it was observed that the output voltage
was 32.8 mV for the triglycine calcium dibromide crystal. Hence the SHG
efficiency of triglycine calcium dibromide crystal is half of that of KDP
crystal.
4.4.6
Thermal Studies
Simultaneous thermogravimetric analysis (TGA) and differential
thermal analysis (DTA) were carried out between 30 °C and 1100 °C in
nitrogen atmosphere at a heating rate of 10 °C/min using NETZSCH STA
409 C/CD TG/DTA instrument for the as grown crystals to determine the
melting point and the thermal stability of the crystal. Figure 4.4 shows the
resulting traces of TG/DTA for the triglycine calcium dibromide crystal. In
the spectrum of thermogravimetric analysis, there is no weight loss upto
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305 oC. Hence the triglycine calcium dibromide crystal is devoid of any
physically adsorbed water on it and also it is observed that there is a sudden
weight loss occurring at the temperature 305 °C. The DTA response curve too
shows a sharp endothermic peak at the same temperature. Further a small
quantity of the fine powdered sample was taken in a microcapillary tube and
heated up using Monatch melting point apparatus for confirming the thermal
stability.
2
305.128
80
Weight %
0
60
-2
40
Microvolt Exo Down (µv)
TGA
DTA
100
20
0
200
400
600
800
1000
-4
1200
o
Temperature ( C)
Figure 4.4 TG/DTA curves of triglycine calcium dibromide crystal
From the thermal analysis, it is observed that triglycine calcium
dibromide decomposes with melting at 305 °C temperature. Hence the grown
triglycine calcium dibromide crystal is stable upto the temperature 305 °C and
can be designed for device application upto the limitation of that temperature.
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4.4.7
Dielectric Studies
Dielectric studies of triglycine calcium dibromide single crystal
were carried out as a function of temperature for various frequencies using
Precision LCR meter AGILENT 4284A model. The as grown transparent
crystal of 2 mm thickness was selected as a sample for studying dielectric
constant. Typical area of the sample for the experiment was 63 mm2. In order
to make a contact with the electrodes, both the crystal surfaces were coated
with fine graphite powder. The prepared sample was placed between the two
electrodes and heated from room temperature to 160 °C using thermostat.
Then the crystal was annealed at thermostat itself. Capacitance and dielectric
loss measurements were carried out with the sample as a function of
temperature for the range of frequencies from 100 Hz to 1 MHz. Dielectric
constant ‘ r’ was calculated directly using the formula
r
= Ccrystal / Cair as the
area of the crystal is equal to area of the electrode.
Figures 4.5 and 4.6 show the plots of dielectric constant and
dielectric loss factor respectively. The current investigations showed that
dielectric constant is maximum at 100 Hz since all types of polarization such
as electronic, ionic, orientation and space charge polarizations occur at lower
frequency. Dielectric loss factor too corresponds to the dielectric constant
plots at the given experimental frequencies ranging from 100 Hz to 1 MHz.
Further it is found that dielectric constant is 6.6 at 160 °C temperature. The
orientation and ionic contributions are small at high frequencies due to the
inertia of the molecules and ions (Charles Kittel 1993). So, the resultant
amount of polarization increases with the decrease of frequencies.
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7.0
6.5
100 Hz
1 KHz
10 KHz
100 KHz
1 MHz
Dielectric Constant ( r)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
40
60
80
100
120
140
160
o
Temperature ( C)
Figure 4.5 Dielectric constant of triglycine calcium dibromide crystal
0.7
100 Hz
1 KHz
10 KHz
100 KHz
1 MHz
0.6
Dielectric Loss
0.5
0.4
0.3
0.2
0.1
0.0
40
60
80
100
120
140
160
o
Temperature ( C)
Figure 4.6 Dielectric loss of triglycine calcium dibromide crystal
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4.4.8
Mechanical Studies
Mechanical properties of the grown triglycine calcium dibromide
crystals were studied using HMV2T Microhardness tester. The Vickers
hardness number of the crystal was calculated using the relation
Hv = 1.8544 P/d2 kg/mm2, where Hv is the Vickers microhardness number, P
is the applied load in kg and d is the average diagonal length of the
impression in mm. The corresponding trace is shown in the Figure 4.7, from
which it is observed that the hardness increases with the increase of load up to
100 g and crack occurs at that load. The maximum hardness obtained in this
material is 90 kg/mm2.
In order to find work hardening coefficient (n) of the grown crystal,
another graph (Figure 4.8) was drawn between logarithmic values of load and
diagonal length of indentation. Work hardening coefficient or Meyer index
was calculated from the relation P = adn, where ‘a’ is the constant for the
given material. The work hardening coefficient “n” was calculated as 3.3.
According to Onitsch (Onitsch 1947), if n lies between 1 and 1.6, then the
grown crystal will be a harder material and it is more than 1.6 for soft
materials (Ramesh Babu et al 2003, Rajesh et al 2004). Since the calculated
work hardening coefficient ‘n’ is more than 1.6, the grown crystal is
suggested that it comes under the category of soft material. Yield strength of
the grown triglycine calcium dibromide single crystalline material was also
calculated using the formula (Ramesh Babu et al 2003)
where
y
y
= ( Hv /3)(0.1)n-2,
is the yield strength, Hv is the Vicker’s microhardness and n is the
logarithmic exponent. It was found to be 1.5 MPa from the relation and hence
the grown triglycine calcium dibromide single crystal has low mechanical
strength.
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2
Vicker's Hardness (kg/mm )
90
80
70
60
50
40
20
30
40
50
60
70
80
90
100
110
Load (gram)
Figure 4.7
Hardness Vs load graph of triglycine calcium dibromide
crystal
1.66
1.64
Log d (mm)
1.62
1.60
1.58
1.56
1.54
1.52
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
Log P (gram)
Figure 4.8 Plot of log P and log d of triglycine calcium dibromide crystal
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4.5
CONCLUSION
Semiorganic nonlinear optical crystal triglycine calcium dibromide
has been grown by slow solvent evaporation technique from aqueous solution
of glycine and calcium dibromide at room temperature. The unit cell
parameters estimated by single crystal X-ray diffraction analysis agree with
the reported values. Various functional groups present in the crystal were
identified using FTIR spectrum. Optical studies reveal that the maximum
transmittance under UV-Vis-NIR radiation is 52% and the material has good
transparency in the entire visible region. The crystal gets damaged at 30 mJ
laser beam energy when it was subjected to Laser damage threshold testing
using multiple shots mode method. The grown crystal has NLO efficiency
half of that of KDP crystal and has good thermal and mechanical stabilities.
Its decomposition and melting temperature is at 305 °C suggesting that it has
higher thermal stability. From the microhardness investigations made on the
grown crystalline material, the crystal has maximum surface hardness about
90 kg/mm2 at 100 g load and is soft material and having low yield strength.