Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.6, pp.517-526, 2011
jmmce.org Printed in the USA. All rights reserved
517
Synthesis, Optical and Dielectric Properties of Tris-Glycine Zinc Chloride
(TGZC) Single Crystals
S. Suresh and D. Arivuoli*
*Crystal Growth Centre, Anna University, Chennai-600 025, India.
Corresponding Author:
ABSTRACT
Non-linear optical materials find wide range of applications in the fields of
opto-electronics, fiber optic communication, computer memory devices etc. Tris-Glycine Zinc
Chloride (TGZC) is one of the NLO materials exhibiting more efficiency. In the present study
Tris-Glycine Zinc Chloride were grown is single crystal form using slow evaporation
technique. Single crystal X-ray diffraction analysis reveals that the crystal belongs to
orthorhombic system with the space group Pbn21. The optical absorption studies show that
the crystal is transparent in the entire visible region with a cut off wavelength of 250 nm. The
optical band gap is found to be 4.60 eV. The dependence of extinction coefficient (K) and
refractive index (n) on the wavelength has also been reported. Force constants (k) were
calculated using FTIR spectral analysis which shows higher values of k for COO and C=O
stretching vibrations. The dielectric studies show that the dielectric constant and dielectric
loss decrease exponentially with frequency at different temperatures (35◦C, 55◦C, 75◦C and
95◦C).
Key words: Single Crystal, Growth from solution, X-ray diffraction, FTIR Spectroscopy,
Dielectric constant.
1. INTRODUCTION
Nonlinear optics (NLO) is at the forefront of current research because of its importance in
providing the key functions of frequency shifting, optical modulation, optical switching,
optical logic, and optical memory for the emerging technologies in areas such as
telecommunications, signal processing, and optical interconnections [1]. Organic materials
have been of particular interest because the nonlinear optical response in this broad class of
materials is microscopic in origin, offering an opportunity to use theoretical modeling
coupled with synthetic flexibility to design and produce novel materials [2]. Also, organic

518  S. Suresh and D. Arivuoli  Vol.10, No.6
nonlinear optical materials are attracting a great deal of attention, as they have large
optical susceptibilities, inherent ultrafast response times, and high optical thresholds for
laser power as compared with inorganic materials. Hariharan et al [3] and Fleck et al [4]
have studied the crystal structure and phase-matching studies on Tris-glycine zinc chloride
(TGZC). Non-linear laser properties of crystals of non- centro symmetric orthorhombic semi-
organic Tris-glycine zinc chloride have been reported by Kaminskil et al [5].The growth and
characterization of TGZC have been reported by Suresh et al [6]. In the present work, the
band gap energy was calculated using optical absorption spectrum for the grown crystals.
The force constant for the TGZC crystal has been evaluated by using FTIR analysis. An
attempt was made to study the dielectric properties of the grown crystals.
2. EXPERIMENTAL PROCEDURE
A solution of glycine and zinc chloride was prepared in 3:1 molar ratio and stirred
continuously using magnetic stirrer for homogenization and tiny seed crystals were obtained
by spontaneous nucleation. The chemical reaction is represented as
3NH2-CH2-COOH +ZnCl2  [(NH2-CH2-COOH) 3] ZnCl2 (1)
Recrystallisation process was carried out two times and finally the crystals with dimensions
(10 × 8 × 6 mm3) were obtained over a period of 45 days. Fig.1 shows single crystals of
TGZC with high degree of transparency.
Fig .1.The grown single crystal of TGZC
3. RESULTS AND DISCUSSION
3.1 Single-crystal X-ray Diffraction
Single crystal X-ray analysis was carried out for the grown crystals using ENRAF NONIUS
CAD 4 automatic X-ray diffractometer. The lattice parameter values are found to be a =
11.26 Å, b = 15.26 Å and c = 15.65 Å, α= β = γ = 90 and volume of the unit cell is 2688 Å3.

Vol.10, No.6 Synthesis, Optical and Dielectric Properties 519
The XRD data prove that the crystal is orthorhombic in structure with the space group Pbn21.
The results are found to be in good agreement with the results predicted by Fleck et al [4].
3.2. CHN Test
CHN analysis for the grown crystals has been carried out using ELEMENTAR VARIO
EL111-GERMANY and the results are presented in Table 1.The presence of carbon,
hydrogen and nitrogen has been confirmed in the grown crystals.
Table 1. Chemical analysis of TGZC (Molecular weight = 360 gm)
3.3 Optical Absorption
Fig.2 shows optical absorption spectrum of Tris-glycine Zinc Chloride (TGZC) single crystal
recorded in the wavelength region ranging from 200 nm to 800 nm using PERKIN-ELMER
LAMBDA 25 spectrophotometer. For optical fabrication, the crystal should be highly
transparent over a considerable region of wavelength [7-8]. The UV cut off wavelength for
the grown crystal was found to be 250 nm which makes it a potential material for optical
device fabrications. The optical absorption coefficient (α) was calculated using the following
relation,
11 log
dT

 

(2)
where T is the transmittance and d is the thickness of the crystal. As a direct band gap
material, the crystal under study has an absorption coefficient (α) obeying the following
relation for high photon energies (hν)
1/2
()
g
Ah E
h

  (3)
where Eg is the optical band gap of the crystal and A is a constant. The plot of (αhν)2 versus
hν is shown in Figure 3. Eg was evaluated by the extrapolation of the linear part [9]. The band
gap is found to be 4 .60 eV. As a consequence of wide band gap, the grown crystal has large
transmittance in the visible region [10].
Element Composition%
Theoretical
Measured
Carbon 20.00 20.52
Hydrogen 4.16 4.22
Nitrogen 11.66 11.47

520  S. Suresh and D. Arivuoli  Vol.10, No.6
Fig.2. Optical absorption spectrum of TGZC
Fig.3. Spectral dependence hν vs (αhν)2

Vol.10, No.6 Synthesis, Optical and Dielectric Properties 521
3.4 Optical Constants
The optical constants (n, K) were determined from the transmission (T) and reflection (R)
spectrum. The transmittance of the crystal [11] is
2
2
(1) exp()
1Rexp(2)
R
t
Tt

 (4)
where t is the thickness and α is related to extinction coefficient K by
4
K


 (5)
The Reflectance (R) is the written in terms of refractive index (n) [12] as


2
2
1
1
n
R
n

 (6)
It can also be written in terms of absorption coefficient as
11 exp(exp()
1 exp()
tt
Rt
  
 (7)
From the above equation, the refractive index n can be derived as
2
(1)3 103
2( 1)
RRR
nR
 
  (8)
Figure 4 shows the plot of extinction coefficient (K) as a function of wavelength. From the
graph, it is clear that extinction coefficient (K) value increases with increase in the photon
energy. The dependence of refractive index (n) on the wavelength is shown in Figure 5 and it
is seen that the refractive index decreases as the photon energy decreases. Thus, the
extinction coefficient (K) and refractive index (n) depend on the photon energy. It is
understood that the higher value of photon energy will enhance the optical efficiency of the
material. Hence, by tailoring the photon energy, one can achieve the desired material for
optical device fabrication.

522  S. Suresh and D. Arivuoli  Vol.10, No.6
Fig.4. Wavelength vs extinction coefficient (K)
Fig.5. Wavelength vs Refractive index (n)

Vol.10, No.6 Synthesis, Optical and Dielectric Properties 523
3.5 FTIR Spectrum
The FTIR spectral analysis for the grown crystal has been recorded in the range 400-4000
cm-¹ using KBr pellet technique and the resultant spectrum is shown in Fig.6. The carbonyl
stretching C=O is found to be near 1083 cm-1.A peak at 2694 cm-1 corresponds to NH2
deformation. The C-N stretching and the O-H stretching bands are found to be near 1125 and
3186 cm-1 respectively. A peak at 2694 cm-1 corresponds to CH2 stretching. The band at 1580
cm-1 indicates the presence of symmetric stretching of carboxylate (COO-) ion. The force
constant (k) calculations were made for the vibration frequencies of NH2 deformation, COO-
stretching, CH2 stretching, O-H stretching, C-N stretching, C=O stretching, by using the
formula,

222
4e Ck  (9)
where ωe2 is the fundamental vibrational frequency, μ is the reduced mass 









ba
ba
MM
MM

Fig.6. FTIR spectrum of TGZC crystal
The force constants for different vibrations are presented in Table 2. It is seen that the values
are higher for C=O stretching and COO groups. The objective of this investigation is to find
out the stretching, deformation and interaction force constants corresponding to each
substituent and its position in the molecule. The value of force constant k was found to be
1282 N/m and 1083 N/m for COO stretching and C=O stretching respectively. The set of
force constants reported here predicts the observed frequencies for COO stretching and C=O
stretching.

524  S. Suresh and D. Arivuoli  Vol.10, No.6
Table 2 Force Constant (k) for different vibrations of TGZC single crystal
Frequency (cm-1) Vibration
assignment
Force Constant(k)
N/m
1610 NH2 deformation 269
1580 COO stretching 1282
2694 CH2 stretching 737
3186 O-H stretching 566
1125 C-N stretching 481
1638 C=O stretching 1083
3.6 Dielectric Property
The dielectric studies were carried out using silver coated samples placed between the two
copper electrodes which form a parallel plate capacitor. The capacitance of the sample was
noted for the applied frequency that varies from 50 Hz to 5 MHz at different temperatures
(35◦C, 55◦C, 75◦C and 95◦C). Fig.7 shows the plot of dielectric constant (εr) versus applied
frequency for different temperatures. The applied frequency is represented by logarithmic
values in the plot. The dielectric constant decreases with the applied frequency and it is also
observed that εr increases with increasing temperature. The very high value of εr at low
frequencies may be due to the presence of all the four polarizations namely: space charge,
orientation, electronic and ionic polarization and its low value at higher frequencies may be
due to the loss of significance of these polarizations gradually. The high value of dielectric
constant at lower frequencies may be attributed to space charge and ionic polarizations. The
low value of dielectric loss at high frequencies suggests that the sample possesses enhanced
optical quality with lesser defects and this parameter is of vital importance for NLO
applications [13].Fig. 8 represents the dielectric loss versus frequency at different
temperatures. It is observed that the dielectric loss decreases with increase of frequency.

Vol.10, No.6 Synthesis, Optical and Dielectric Properties 525
Fig.7. Frequency vs dielectric constant
Fig.8. Frequency vs dielectric loss
4. CONCLUSION
Single crystals of Tris-glycine Zinc Chloride (TGZC) were grown from aqueous solution by
slow evaporation technique under room temperature. The grown crystals were characterized
by single crystal XRD and it is confirmed that the crystal belongs to the orthorhombic system

526  S. Suresh and D. Arivuoli  Vol.10, No.6
with space group Pbn21. The band gap for the grown crystal is found to be 4.60 eV. The
optical investigations show a high value of both extinction coefficient (K) and refractive
index (n) indicating high transparency of the crystal which confirms its suitability for optical
switch device fabrication. The functional groups and the force constant (k) values were
predicted by using FTIR analysis. Dielectric measurements were carried to analyse the
dielectric constant and dielectric loss at different frequencies and temperatures.
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