Crystal growth and vibrational spectroscopic studies of the semiorganic non-linear optical crystal - Bisthiourea zinc chloride

Article abstract of DOI:10.1016/j.saa.2004.04.014

The semiorganic non-linear optical crystal bisthiourea zinc chloride (BTZC) was grown by mixed solvent slow evaporation technique. The solubilities under various solvents in different proportions have been studied. Vibrational spectra were recorded to det

Full text of DOI:10.1016/j.saa.2004.04.014

Spectrochimica Acta Part A 61 (2005) 499–507
Crystal growth and vibrational spectroscopic studies of the semiorganic
non-linear optical crystal—bisthiourea zinc chloride
∗
V. Krishnakumar , R. Nagalakshmi
Department of Physics, Nehru Memorial College, Puthanampatti, Tiruchirappalli, 621 007, India
Received 9 February 2004; accepted 2 April 2004
Abstract
The semiorganic non-linear optical crystal bisthiourea zinc chloride (BTZC) was grown by mixed solvent slow evaporation technique. The
solubilities under various solvents in different proportions have been studied. Vibrational spectra were recorded to determine the symmetries
of molecular vibrations. The observed Raman and infrared bands were also assigned and discussed. The optical transmission spectral study
was also carried out. The property of second harmonic generation of BTZC was also verified.
©
2004 Elsevier B.V. All rights reserved.
Keywords: Bisthiourea zinc chloride; Infrared; Polarized Raman; Optical transmission; Second harmonic generation
1
. Introduction
In continuation of our work on non-linear optical mate-
rials [3], an attempt has been made in this study to grow
and characterize a single crystal of bisthiourea zinc chloride
(BTZC).
The organic compounds possess a high degree of opti-
cal non-linearity than their inorganic counterparts. Semior-
ganic non-linear optical (NLO) crystals of high efficiency
and optical quality can be obtained by combining the organic
molecules of high polarizability with an inorganic host. Ac-
centric molecules, consisting of high delocalized π electron
systems interacting with suitably substituted electron donor
and acceptor groups exhibit a high value of second-order
polarizability (␤) [1].
An alternative and closely related strategy is to form
metal-coordination complexes of highly polarizable organic
molecules. These materials are known as semiorganics. In
addition to retaining the high optical non-linearities of the
organic molecules, they also possess favourable physical
properties. An added advantage is that large single crystals
can be grown from slow evaporation solution growth [2].
Thiourea as such is a centrosymmetric, but when it is
co-ordinated with metal ions, it becomes a non-centrosym-
metric material, which is an essential property for a crystal
to exhibit non-linear optical activity.
2. Experimental
In the present investigation, we tried to grow single crys-
tals of BTZC using deionized millipore water and mixture
of acetone and water as solvents by slow evaporation solu-
tion growth. Bisthiourea zinc chloride (Zn[CS(NH2)2]2Cl2)
was synthesized according to the reaction
2
CS(NH2)2 + ZnCl2 → Zn[CS(NH2)2]2Cl2
2.1. Solubility
As a first step towards crystallization, the solubilities of
BTZC in different solvents were tried. The component salts
were very well stirred using magnetic stirrer. The solvents,
which dissolve zinc chloride and thiourea, were examined
including water, methanol and mixed solvents such as ace-
tone and water, acetone and methanol in various proportions.
The solubilities of BTZC in the solvents were measured by
a variable composition method where an excess amount of
BTZC was added to the solvent at room temperature and the
saturated concentration was determined by evaporating the
∗
Corresponding author. Tel.: +91-431-2769224;
fax: +91-431-2791300.
E-mail address: vkrishna kumar@yahoo.com (V. Krishnakumar).
1
386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.04.014

5
00
V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
Table 1
Influence of solvents and solvent mixtures on growth forms of BTZC
Solubility (g 100 ml 1)
−
Habit
Visual quality
Solvents
Water
15
Plate like
Transparent
Methanol
20
Plate like
Transparent
Acetone + methanol (1:1)
Acetone + methanol (1:2)
Acetone + methanol (2:1)
Acetone + water (1:1)
Acetone + water (1:2)
Acetone + water (2:1)
13
11.5
12
17
16
Plate like
Plate like
Plate like
Bipyramidal
Bipyramidal
Bipyramidal
Transparent
Transparent
Transparent
Transparent and bulky
Transparent
16.5
Transparent
solvent after the equilibrium is reached. A summary high-
lighting the extremes of the results obtained in some solvent
mixtures are presented in Table 1. The solubilities are given
tals of BTZC were harvested within 12–15 days of size
17 mm × 5 mm × 1 mm as shown in Fig. 1a.
◦
at 30 C.
2.2.2. Mixed solvent
In order to probe the solvent effects on the growth mech-
anism of BTZC, we have also tried the growth of BTZC
in mixed solvents. We have made an attempt to grow the
single crystals of BTZC in the mixture of acetone and water
for three proportions (1:1, 1:2 and 2:1). In that, the ratio 1:1
yielded us colourless transparent good quality crystals hav-
ing dimensions 18 mm × 4 mm × 4 mm within a week. The
morphology of the crystals grown in mixed solvent shows
that the crystals are elongated along the crystallographic
a-axis. The crystals obtained by this method are shown
in Fig. 1b. Inspite of having high solubility in methanol
2
2
.2. Crystal growth
.2.1. Water as solvent
Bisthiourea zinc chloride (BTZC) single crystals were
grown using water as solvent by slow evaporation technique
by Dhanuskodi and Angelimary [4]. In the present study,
we have also tried the growth of BTZC in deionized milli-
pore water at an optimized pH condition. The single crys-
(Table 1), the growth rate was found to be very slow and
also we could not obtain the bulky crystals.
Owing to the faster growth rate, high transparency and
good yield, the mixed solvent (acetone+water) slow evapo-
ration technique is found to be more suitable for the growth
of semiorganic bisthiourea zinc chloride (BTZC) crystals.
3. Recording of spectra
3.1. IR measurements
The room temperature infrared spectrum of BTZC was
−
1
measured in the region 400–4000 cm at a resolution of
±
−
1
1 cm using a BRUKER IFS 66V vacuum Fourier trans-
form spectrophotometer equipped with an MCT detector, a
KBr beam splitter and globar source; boxcar apodization
was used for the 250 averaged interferogrames collected for
the sample.
3.2. Polarized Raman measurements
A finely polished crystal washed in distilled water is prop-
erly aligned to record Raman spectra. The crystallographic
axes were determined using a polarizing microscope. The
polarized Raman spectra in various scattering geometries
such as a(bb)a, c(aa)c, b(cc) b, c(ba) c, a(bc) a and b(ac) b
were recorded with a Nicolet 950 FT–Raman instrument
Fig. 1. (a) Water-grown BTZC crystal. (b) Mixed solvent (acetone:water
(1:1))-grown BTZC crystal.
2
equipped with a liquid-N -cooled Ge detector. Nd: YAG

V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
501
Table 2
Various illumination observation geometries and the associated polarizability tensor components contributing to the Raman spectrum
Axis along
incident light
Axis along
polarization of
incident light
Axis along
polarization of
scattered light
Axis along
scattered light
Contributing
polarizability
tensors
Symmetry
a
c
b
c
a
b
b
a
c
b
b
a
b
a
c
a
c
c
a
c
b
c
a
b
αyy
αxx
αzz
αyx
αyz
αxz
A1(Z)
A1(Z)
A1(Z)
A2
B2(Y)
B1(X)
laser operating at 150 mW power exciting at 1064 nm line
was used as a source to record the spectra in the stokes re-
−
1
gion at 2 cm resolution.
4
. Results and discussion
4
.1. Spectral analysis
The present vibrational spectroscopic study of BTZC was
carried out with a view of obtaining an insight into the
structural basis of the optical non-linearity of thiourea-based
semiorganics.
In BTZC, there are four molecules in the unit cell. This
gives rise to 76 atoms in the unit cell in which all the atoms
occupy general positions. A group theoretical analysis tak-
ing all the atoms into account results in 225 (56A1 +57A2 +
Fig. 2. (a) BTZC molecular unit. (b) Crystal packing diagram of BTZC.
5
6B1 +56B2) vibrational degrees of freedom apart from the
three acoustic modes (A1 + B1 + B2). In an orthorhombic
crystal like BTZC, the modes have an associated polariz-
ability tensors of the form
able scattering geometries. The geometries employed and
the associated polarizability tensor components contributing
to the Raman spectrum are given in Table 2. As the vibra-
tional spectroscopic analysis requires a knowledge of crystal
structure, we discuss it briefly here.
BTZC belongs to orthorhombic crystal class with space
group Pn21a. The zinc ion is at the tetrahedral coordination
site with two Cl atoms and two NH2 groups at its apices
as given in Fig. 2a. This results in a three-dimensional
bonding network. All thiourea molecules are planar and
are equidistant from the central zinc atom. The packing
diagram of BTZC crystal is shown in Fig. 2b. This en-
dows a polymeric character to the BTZC molecule, with
asymmetric units contributing additively to the effective
non-linearity. This overweighs the reduction in non-linearity
by the three-dimensional bonding network and typifies the








αxx
0
0
0
αxy
0
0
A1(Z) =  0
αyy 0  A2 =  αyx
0 
0
0
αzz
0


0
0




0
0
0
0
αxz
0
0
0
0
0


B1(X) =  0

B2(Y) =  0
αyz 
αzx
0
0
αzy
0
Here, the polarizability tensors are defined along the crys-
tallographic X, Y and Z axes. Only the A2 mode is IR inactive.
All the other modes are both Raman and IR active. Hence,
it is possible to determine the symmetry species of vari-
ous vibrations by recording the Raman spectra using suit-
Table 3
Lattice parameters of TU, BTCC and BTZC
◦
◦
◦
Unit cell volume (Å3)
Sl. No.
Samples
A (Å)
B (Å)
C (Å)
α ( )
β ( )
γ ( )
1
2
3
Thiourea (TU)a (orthorhombic)
14.82925
5.80
5.9010
8.901418
13.07
12.7514
7.862325
6.48
12.9778
90
90
90
90
90
90
90
90
90
1037.84
491.2228
976.5138
b
Cd(tu)2Cl2 (orthorhombic)
c
Zn(tu)2Cl2 (orthorhombic)
a
Ref. [8].
Ref. [7].
This work.
b
c

5
02
V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
approach in engineering semiorganic molecules for NLO
applications [5].
In order to understand the role of metal ions in the crystal-
lographic properties of divalent metal thiourea complexes,
a comparison is made for free ligand (TU), bisthiourea
cadmium chloride (BTCC) and bisthiourea zinc chloride
b(ac) b), we have not obtained any useful information. So,
they are not discussed here. The IR spectrum of BTZC
−1
recorded in the region 400–4000 cm is shown in Fig. 6.
The assignments of the Raman and IR bands to various
molecular vibrations of BTZC along with their symmetry
species are summarized in Table 4. The analysis of the vi-
brational spectra particularly in the internal mode region is
carried out in terms of vibrations of thiourea and those of
zinc tetrahedron.
(BTZC). The crystallographic data are depicted in Table 3.
It is understandable that the addition of metal ions and
halogen ions are responsible for the reduction in the unit
cell volume of BTCC and BTZC when compared to that
of the free ligand (TU). However, between Zn(tu)2Cl2 and
Cd(tu)2Cl2, the unit cell parameters and volume are found
to be different, even though they belongs to the same or-
thorhombic crystal system. This may be ascribed due to the
differences between the structural properties of Zn and Cd
metal ions.
4.2. Lattice vibrations
The lattice vibrations of thiourea-based complexes usually
−
1
appear in the region 10–300 cm . In the present study, the
low-frequency lattice vibrations are due to the rotational and
translational vibrations of thiourea and Zn ion-co-ordinated
tetrahedral bonds of BTZC. The low wave number hydro-
gen bonds expected in this region are usually weak. Further,
the lattice vibrations arising due to the translatory or rota-
tory motions of the Zn(tu) Cl (tu, thiourea) tetrahedral will
The Raman spectra recorded under six different set-
tings are classified in three regions viz., 0–300, 300–1700
−
1
and 3100–3500 cm . Fig. 3 shows the Raman spectra
recorded for six different settings (a(bb)a, c(aa) c, b(cc) b,
2
2
−
1
c(ba) c, a(bc) a and b(ac) b) in the region 0–300 cm . The
spectra recorded under three different geometries (a(bb)a,
be very weak on account of the heavy zinc ion. The simi-
lar observations reported by the earlier investigators on zinc
tristhiourea sulphate (ZTS) and bisthiourea cadmium chlo-
ride (BTCC) leads to confirm the lattice vibrations of BTZC
proposed in this study [6,7].
−
1
c(aa) c and b(cc) b) for the regions 300–1700 cm
3
and
100–3500 cm are shown in Figs. 4 and 5, respectively.
In the other scattering geometries (c(ba) c, a(bc) a and
−
1
Fig. 3. Raman spectra of BTZC in the region 0–300 cm 1.
−

V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
503
Fig. 4. Raman spectra of BTZC in the region 300–1700 cm 1.
−
4.3. Analysis of the IR and Raman spectra of thiourea
vibrations in BTZC
The IR and Raman spectra of the free ligand and its
transition metal complexes with Cu(tu) Cl2, Cu(tu) Br2,
6
6
Co(tu) Cl2, Co(tu) Br2, Ni(tu) Cl2 and Ni(tu) Br2 have
6
6
6
6
been reported earlier [8]. In the present study, an attempt has
been made to reveal the effect of coordination on the vibra-
tional bands of the ligand in the zinc metal complex with Cl
ion as halogen. One of the important features in the Raman
spectra of BTZC is the pronounced variation in the intensity
of Raman lines with the crystal orientation. This essentially
arises because of the anisotropic nature of the polarizability
with the normal coordinates of molecular vibrations, which
must be due to the planar nature of the thiourea molecules.
A study of the Raman intensities reveals that Raman lines
belonging to A1 symmetry types are more intense than the
lines due to the other symmetry types. Hence, one would
expect Raman lines due to αxx, αyy and αzz components to
be strong compared with the off-diagonal elements. This is
what was observed experimentally in the present study also.
The most important features of the spectra were investigated
in the following two regions.
.3.1. 3100–3465 cm 1 region
−
4
In the FTIR spectrum of BTZC, the NH-stretching vibra-
tional bands of NH2 were observed and shifted to higher
Fig. 5. Raman spectra of BTZC in the region 3100–3500 cm 1.
−

5
04
V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
Table 4
Wave numbers (cm 1) of Raman and infrared bands of BTZC
−
a(bb)a
c(aa)c
b(cc)b
c(ba)c
a(bc)a
b(ac)b
Symmetry
IR
Assignments
–
–
–
–
–
–
54
–
75
–
–
–
–
–
135
–
176
208
223
478
–
–
–
–
–
–
–
–
64
74
–
–
–
107
–
–
149
–
–
223
–
501
–
609
–
–
47
–
–
–
81
–
94
–
–
–
146
–
–
227
478
–
–
–
–
–
–
–
–
73
–
–
93
114
–
–
149
173
–
228
478
30
–
–
62
78
–
88
–
104
129
–
157
174
–
227
–
–
B1(X)
A2
A1(Z)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Lattice
Lattice
Lattice
Lattice
Lattice
Lattice
Lattice
Lattice
A1(Z) + B1(X)
7
4.5
A1(Z) + B1(X) + B2(Y)
–
–
–
–
–
–
48
–
08
23
–
10
–
10
–
A2
B1(X)
B2(Y) + A2
A1(Z) + B1(X) + B2(Y)
Lattice
Lattice
B1(X)
A1(Z) + A2
ν(Zn–Cl)
ν(Zn–Cl)
ν(Zn–Cl)
ν(Zn - S)
ν(Zn–S)
δ(C–N)
δ(C–N)
δ(C–N)
δ(C–N)
ν(C–N)
ν(C–N)
ν(C–N)
ρ(NH2)
ν(C–S)
ν(C–S)
ν(N–C–N)
δ(NH2)
δ(NH2)
ν(N–H)
ν(N–H)
ν(N–H)
ν(N–H)
ν(N–H)
ν(N–H)
ν(N–H)
1
A1(Z) + B1(X) + B2(Y)
A1(Z) + B1(X) + B2(Y)
2
2
A1(Z)
A1(Z) + B1(X) + B2(Y) + A2
A1(Z) + B2(Y) + A2
478
518
568
–
–
–
715
1103
1407
1446
1496
1612
–
3116
–
3288
–
5
–
–
–
–
A1(Z)
–
–
–
6
A1(Z)
B1(X)
A2
702
–
714.5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
13
–
707
715
1100
1407
–
–
7
712
1104
1419
1448
1502
1614
–
715
1103
1415
1450
–
714.5
–
–
–
–
–
–
–
A1(Z) + B1(X) + B2(Y) + A2
1
1
1
1
1
090
418
451
502
614
–
A1(Z) + A2
A1(Z) + A2
A1(Z)
–
–
A1(Z)
A1(Z)
A2
A1(Z)
A1(Z)
A1(Z)
A1(Z)
1612
–
1630
–
3
147
–
3146
–
3147
3236
3280
3292
3347
3371
3451
–
–
–
–
–
–
3
3
3
3
3
284
290
347
369
447
–
3291
3346
3369
–
–
3347
–
A1(Z) + B2(Y)
3327
3371
3445
–
A1(Z)
3452
–
A1(Z) + A2
ν, stretching; ρ, rocking; and δ, deformation.
Table 5
Effect of Group II B Metal complexation on wavenumbers (cm 1) of thiourea vibrations
−
TUa
Cu(tu)6Cl2a Cu(tu)6Br2a Ni(tu)6Cl2a Ni(tu)6Br2a Co(tu)6Cl2a Co(tu)6Br2a Cd(tu)2Cl2b Zn(tu)3SO4c Zn(tu)2Cl2d Assignments
3
3
3
3
1
1
1
380 3470
3460
3300
3253
3180
1604
1477
1422
1388
1089
3450
3367
3304
3279
1619
1482
1439
1398
1088
3400
3365
3279
3180
1619
1483
1440
1398
1089
3376
3400
3300
3200
1618
1543
1400
1420
1098
3422
3383
3288
3200
1617
1530
1420
1420
1099
3430
3310
3280
3184
1620
1498
1442
1406
3450
3380
–
3180
1634
–
3445
3371
3288
3116
1612
1495
1496
1407
1103
␯(N–H)
␯(N–H)
␯(N–H)
␯(N–H)
␦ (N–H)
␯(N–C–N)
␯C–S
279 3370
177 3260
090 3180
620 1608
477 1478
414 1422
–
1395
1408
1140
␯C–S
␳NH2
1
082 1095
1120
7
30
715
712
717
710
700
700
725
719
715
␯C–N
δ, deformation; ρ, rocking; and ν, stretching.
a
Ref. [8].
Ref. [7].
Ref. [6].
This work.
b
c
d

V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
505
Fig. 6. IR spectrum of BTZC.
wave number region at 3445, 3371, 3288 and 3116 cm 1
when compared to that of free ligand (TU) (Ref. Table 5)
are due to the formation of S → M bonds in Zn(tu)2Cl2
complex and is expected to increase the contribution of high
polar character of the structure of thiourea molecule, result-
ing in a stronger double bond character for the nitrogen to
carbon bond and a stronger single bond character for the
carbon to sulphur bond [9].
−
exhibits bands in the regions 1375–1570, 1260–1420 and
−
1
940–1140 cm , respectively [10]. These bands arises be-
cause of strong coupling between C–S and C–N vibrations.
In the polarized Raman spectra of BTZC, the C–S-stretching
vibrations in various scattering geometries such as a(bb)a,
c(aa) c, b(cc) b and c(ba) c are found at 1418, 1419, 1415
−
1
and 1407 cm , respectively. In the IR spectrum, the band
−
1
corresponding to this vibration is identified at 1407 cm
The IR band at 1103 cm
.
−
1
corresponds to NH2-rocking
vibration of BTZC, and in polarized Raman spectra, the
bands due to this vibrations are identified at 1090, 1104,
−
1
1
103 and 1100 cm for various scattering geometries. The
−
1
band located at 715 cm in the IR spectrum of the exper-
imental crystal is attributed to symmetric C–N-stretching
vibrations. In the polarized Raman spectra, the bands cor-
responding to this vibration are observed between 713 and
−
.3.2. 704–1620 cm 1 region
4
−
1
The bands observed between 1630–1620 cm in the in-
−
1
7
15 cm . The other types of vibrations arising due to the
vestigated Zn(tu)2Cl2 crystal corresponds to NH2-bending
vibrations (Ref. Table 4). The N–C–N stretching vibra-
various functional groups of BTZC are listed in Table 4.
−
1
tions of BTZC was found at 1496 cm in the observed
IR spectrum, and this could be due to the greater double
bond character of carbon to nitrogen on the formation of
Zn(tu)2Cl2 complex. The Raman counterpart for this vi-
4.4. Effect of Group II B metal complexation on thiourea
Infrared studies on metal coordination compounds of
thiourea have been made previously [8]. A comparison of
our results on BTZC with the infrared spectra of the free
−
1
bration (␯N–C–N) is identified at 1502 cm . Thiourea like
other thioamides, thiosemicarbazones and dithioamides

5
06
V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
Fig. 7. Optical transmission spectrum of BTZC.
ligand and its complexes of Group II B metals are given
in Table 5, together with their suggested assignments. The
infrared bands found at 1620, 1608, 1604, 1619, 1618 and
mission in the entire visible range. The less absorbance be-
haviour in the entire visible region also confirms the colour-
less nature of the crystal. A notable feature in the spec-
trum is reduction in absorption at Nd:YAG laser fundamen-
tal (1064 nm). This observation may be attributed to that
the number of N–H bonds, which causes absorption around
1050 nm by vibrational overtones is lesser in the experimen-
tal crystal. This reduction in absorption around 1064 nm has
a significant contribution towards an improvement in the re-
sistance to laser-induced damage. In the UV-Vis spectrum,
the lower cut-off of BTZC is found near 300 nm, which is an
advantage in semiorganic non-linear optical materials over
their organic counterparts. The extended transparency win-
dow will enable higher harmonic generation and wavelength
extension by cascaded frequency conversion processes.
−1
1
617 cm for free ligand (TU), and its transition metal
complexes with chloride and bromide salts of Cu(II), Ni(II)
and Co(II) ions have been assigned to N–H-bending vibra-
tions by El-Bahy et al., [8] In Cd(tu)2Cl2 and Zn(tu)3SO4,
the bands for this vibrational mode have been identified
−
1
at 1620 and 1634 cm by Bhat and coworkers [6,7]. In
the present study of BTZC, on coordination of ZnCl2 with
−
1
thiourea, this band was shifted marginally to 1612 cm
.
Similarly, in N–C–N stretching vibrations also, the title
non-linear optical crystal exhibited a shift in the observed
IR spectrum when compared to the various thiourea metal
complexes (Table 5). The wave number increase observed
−1
(
18 cm ) for νN–C–N vibrations for BTZC when com-
pared to that of free ligand (TU) could be due to the greater
double bond character of the carbon to nitrogen bond on
complex formation.
4.6. Second harmonic generation
The second harmonic generation of BTZC single
crystal was studied using Nd:YAG laser (model contin-
uum YG501C, λ = 1063 nm). The crystal was set in
phase-matching conditions, and second harmonic signal
(SHS) with λ = 531 nm was detected using an optical
cable attached to a fluorescence spectroscope (Prince-
ton Instrument Inc., Spectroscopy, Instruments GmbH,
model–DIDA–512 G/R). The wavelength was scanned con-
tinuously with an aid of a monochromator in the spec-
4
.5. Optical transmission spectrum
The transmission spectrum of BTZC was recorded using
a 2-mm-thick (1 0 0) plate on Varian Cary 5E UV-Vis-NIR
spectrophotometer in the range 200–2000 nm with high res-
olution, and it is shown in Fig. 7. The significant absorption
was found around 1500 nm. The crystal has a good trans-

V. Krishnakumar, R. Nagalakshmi / Spectrochimica Acta Part A 61 (2005) 499–507
507
property of second harmonic generation of the experimen-
tal crystal was also verified. Studies on the possibilities of
enhancement of SHG efficiency in BTZC by irradiation of
heavy ions are in progress.
Acknowledgements
The authors are thankful to RSIC, IIT Madras, Chennai,
Dr. László Kovács and Dr. Katalin Polgar, Research Institute
for solid state physics and optics, Hungarian Acadamy of
Sciences for the spectral facilities. The financial assistance
of Defence Research Development Organisation, Ministry of
Defence, Government of India is gratefully acknowledged.
One of the authors (V.K.) is thankful to Indian National Sci-
ence Academy (INSA), New Delhi, for the award of visiting
fellowship to Hungarian Academy of Sciences, Budapest.
Fig. 8. Second harmonic generation.
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observed SHS was shown in Fig. 8.
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