Crystal growth, spectral, structural and optical studies of π-conjugated stilbazolium crystal: 4-Bromobenzaldehyde-4′-N′-methylstilbazolium tosylate

Article abstract of DOI:10.1016/j.saa.2014.01.029

Nonlinear optical (NLO) organic compound, 4-bromobenzaldehyde-4′- N′-methylstilbazolium tosylate was synthesized by reflux method. The formation of molecular complex was confirmed from ¹H NMR, FT-IR and FT-Raman spectral analyses. The single cr

Full text of DOI:10.1016/j.saa.2014.01.029

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Crystal growth, spectral, structural and optical studies of p-conjugated
stilbazolium crystal: 4-Bromobenzaldehyde-40-N0-methylstilbazolium
tosylate
M. Krishna Kumar a, S. Sudhahar a, G. Bhagavannarayana b, R. Mohan Kumar a,
⇑
a Department of Physics, Presidency College, Chennai 600 005, India
b Materials Characterization Division, National Physical Laboratory, New Delhi 110 012, India
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
The morphology of the BMST crystal
was depicted using WinXMorph
program.
ꢀ
FT-IR, FT-Raman and 1H NMR
assignments confirmed the functional
groups of BMST.
ꢀ
ꢀ
Bond length alternation value was
found to be 0.086 Å.
Band gap energy and refractive index
were found to be 2.96 eV and 2.41
respectively.
ꢀ
Kurtz powder test confirmed that
SHG intensity increases with increase
in particle size.
a r t i c l e i n f o
a b s t r a c t
Nonlinear optical (NLO) organic compound, 4-bromobenzaldehyde-40-N0-methylstilbazolium tosylate was
synthesized by reflux method. The formation of molecular complex was confirmed from 1H NMR, FT-IR and
FT-Raman spectral analyses. The single crystals were grown by slow evaporation solution growth method
and the crystal structure and atomic packing of grown crystal was identified. The morphology and growth
axis of grown crystal were determined. The crystal perfection was analyzed using high resolution X-ray
diffraction study on (001) plane. Thermal stability, decomposition stages and melting point of the grown
Article history:
Received 7 November 2013
Received in revised form 10 December 2013
Accepted 10 January 2014
Available online 27 January 2014
Keywords:
crystal were analyzed. The optical absorption coefficient (a) and energy band gap (Eg) of the crystal were
Organic compounds
Solution grown crystals
Infrared and Raman spectroscopy
Crystal morphology
Bond-length alternation
Photoluminescence
determined using UV–visible absorption studies. Second harmonic generation efficiency of the grown
crystal was examined by Kurtz powder method with different particle size using 1064 nm laser. Laser
induced damage threshold study was carried out for the grown crystal using Nd:YAG laser.
Ó 2014 Elsevier B.V. All rights reserved.
Introduction
efforts have mainly focused on organic systems [3]. The design of
new organic compounds with large nonlinear optical polarizabili-
Recent years, molecular-based second-order nonlinear optical
crystals have attracted much interest because of their potential
applications in emerging optoelectronic technologies [1,2]. These
ties is of crucial importance [4]. Much efforts have been made to
develop new materials with high second-order nonlinearities and
are expected to underpin applications in the areas such as elec-
tro-optic switching for telecommunications [5], optical informa-
tion processing [6], optical data storage [7,8], and the generation
of terahertz radiation [9]. Among the various classes of materials
⇑
Corresponding author. Tel.: +91 94446 00670; fax: +91 44 28510732.
E-mail address: mohan66@hotmail.com (R. Mohan Kumar).
http://dx.doi.org/10.1016/j.saa.2014.01.029
1386-1425/Ó 2014 Elsevier B.V. All rights reserved.

80
M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
investigated worldwide, ionic organic NLO crystals are of special
interest due to their advantageous in mechanical, chemical and
thermal properties [10]. Organic crystal 4-N,N-dimethylamino-
40,N0-methylstilbazolium tosylate (DAST) is one of the most widely
used NLO materials, showing large NLO susceptibility,
d11 = 1010 110 pm/V at 1318 nm, and large electro-optic coeffi-
cient, r11 = 50 5 pm/V at 1313 nm. It also has a lower dielectric
45
40
35
30
25
20
constant, e11 = 5.2 than standard NLO crystals [11,12].
To improve the second-order NLO response of ionic organic
crystals, a number of approaches have been demonstrated in re-
cent years. Also its high potential for the above applications
encourages the development of new DAST derivative crystals by
varying the counter anion and cation structure [3]. Furthermore,
Duan et al. shown that the electron-donating strength of the donor
unit in a series of stilbazolium salts increased (via CN, Br, Cl, H, CH3,
CHCl3, OCH3 or OH substitution) the quadratic NLO (b) value of
compound [13]. Mardar et al. reported that the second-order har-
monic generation efficiency of the title compound 4-bromobenzal-
dehyde-40-N0-methylstilbazolium tosylate (BMST) is five times
than Urea at 1064 nm radiation [14].
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Concentration (g/100 ml)
Fig. 2. Solubility and nucleation curves of BMST in methanol solvent.
The aim of the present work is to synthesis, crystal growth and
its different characterization for better understanding of properties
of the title compound. It crystallizes in P21/c space group of mono-
clinic system and also it possesses good second harmonic genera-
tion (SHG) property [15]. In the present report, the infrared,
Raman, 1H NMR spectra of the compound were recorded and ele-
mental analysis was carried out to analyze the formation of the
compound. The UV–visible transmission, Kurtz powder SHG and
photoluminescence properties have been studied to understand
the optical behavior. Furthermore, the structure, morphology, crys-
tal perfection and thermal properties of title compound have also
been studied.
equimolar quantity of reagents was used. The measured 4-picoline
(0.105 mol%) and methyl p-toluenesulfonate (0.105 mol%) were added
to 200 ml of toluene in the round bottom flask. It was refluxed by
applying heat, until it crystallizes as intermediate white salt, which
is insoluble in toluene. It was dissolved by adding 200 ml of N,N-
dimethylformamide (DMF) into the solution. After getting the clear
solution, 4-bromobenzaldehyde (0.105 mol%) was added slowly to
the solution. In addition, few drops of piperidine were added into
the solution and the solution color was changed into orange immedi-
ately. The solution was refluxed for 20 h using Dean–Stark trap to
remove water. Then, the solution was cooled to room temperature
and the orange colored BMST precipitate was collected from the round
bottom flask. Then, the salt was dried in the hot air oven at 100 °C for
an hour. The CHN elemental analysis was done to explore the compo-
sition of the synthesized compound. The composition of synthesized
compound with chemical formula, C21H20BrNO3S; was calculated as
C, 56.45%; H, 4.48%; N, 3.09%. These values are agreed very well with
the estimated values. The CHN analysis confirmed that, the synthe-
sized salt is 4-bromobenzaldehyde-40-N0-methylstilbazolium tosylate.
The purity of the salt was improved by repeated recrystallization pro-
cess in methanol at least five times and was used for crystal growth.
Experimental procedure
Material synthesis
All the starting materials with high purity (Aldrich) were used to
synthesis 4-bromobenzaldehyde-40-N0-methylstilbazolium tosylate
compound by Knoevenagel condensation reaction [16] by using
Dean–Stark apparatus (Fig. 1). Knoevenagel reaction is generally car-
ried out in the presence of weak bases such as piperidine and ethylene-
diamine. This condensation is important for vinyl carbon–carbon
(C@C) double bond-forming reaction in water [17]. The round-bot-
tomed flask equipped with condensation setup was used for the syn-
thesis. The steps involved during chemical reaction are as follows: The
Crystal growth
The single crystals of 4-bromobenzaldehyde-40-N0-meth-
ylstilbazolium tosylate were grown by slow evaporation method
Fig. 1. Knoevenagel condensation reaction for BMST compound.

M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
81
experimental infrared and Raman spectra of the BMST compound.
It is observed that all the vibrational bands are asymmetric. It sug-
gested that, electron interaction between the strongest neutral do-
nor p-bromobenzaldehyde, methyl p-toluenesulfonate and the
strongest acceptor group of pyridine. The measured infrared and
Raman band positions and their assignments are given in Table 1.
Stilbazolium cation group vibrations
Vinyl group vibrations
The vibrations of ethylenic bridge (C@C) are highly sensitive to
degree of charge transfer between the donor and acceptor groups
and hence such stretching modes are of particular interest for spec-
troscopists [19,20]. Two characteristic vibrational bands at 1644
and 1585 cmꢂ1 occurred in IR spectrum are due to
p-conjugated
C@C ethylenic group. The downshift of the C@C stretching wave-
number reveals that the vinyl group is actively involved in the con-
jugation path. One is most localized on the aromatic donor (p-
bromobenzaldehyde) fragment and the other on the pyridine
acceptor (p-picoline) fragment. For each pair of related vibrations,
the pyridine component is occurred at lower wavenumber in the
p-conjugated molecules. This observation implies that the p-con-
jugation between the vinyl group and benzene ring mainly changes
the bond length alternation (BLA) of the vinyl group C@C and CAC
bonds. It is key parameter for second harmonic generation prop-
erty of centrosymmetric crystals. The characteristic vibrations of
vinyl aliphatic CAH stretching are expected between 3050 and
3000 cmꢂ1 for stilbazole derivatives [21]. The C7AH and C8AH
stretching modes are appeared at 3049 cmꢂ1 in infrared spectrum.
The in-plane and out-of plane deformation vibration bands of ali-
phatic CAH group are observed at 1195, 942 cmꢂ1 in infrared
and at 1183 cmꢂ1 in Raman spectra.
Phenyl and pyridine ring vibrations
Fig. 3. (a) Photograph of as-grown BMST single crystals. (b) Equilibrium morphol-
ogy of BMST single crystal.
The ring vibrational modes of cationic group of BMST were ana-
lyzed on the basis of stilbazolium ring, which consists of p-bromine
substituted benzene and pyridine moieties connected by ethylenic
bridge. The vibrational modes of pyridine component of BMST are
very similar to those of benzene and it derivatives. The ring
stretching vibrations are prominent in the infrared spectrum of
p-substituted benzene ring complexes. The substitution of ethyl-
enic bridge does not perturb much on CAC bond lengths
[18]. The solubility of BMST salt in methanol was determined
experimentally. The solubility and nucleation curves of BMST are
presented in Fig. 2. Based on the solubility data, the saturated
growth solution at 40 °C was prepared by continues stirring of
about 6 h. The pure solution was obtained by filtration and trans-
ferred to Teflon container to avoid sticking of the salt on the wall.
The beaker was sealed with perforated cover and kept at 40 °C
( 0.01 °C) using constant temperature bath. The solution was
allowed for slow evaporation. After the evaporation period of three
weeks, the spontaneous nucleation was occurred in the supersatu-
rated solution and the developed crystals were collected. The col-
lected as-grown BMST single crystals with size up to
9 ꢁ 3 ꢁ 3 mm3 are shown in Fig. 3a. Its equilibrium morphology
was obtained using WinXMorph program and it is shown in Fig. 3b.
100
90
80
70
60
50
40
30
20
Results and discussion
Spectral studies
Infrared and Raman spectral analyses
The infrared (IR) and Raman spectra were recorded using JASCO
FTIR 410 spectrometer (4000–450 cmꢂ1
) and Bruker RFS27
(4000–50 cmꢂ1) with 100 mW laser excitation respectively. KBr
pellet technique was used for infrared spectrum. These vibrational
methods can provide valuable information about the force field of
the molecules and can help to understand the chemical and phys-
ical properties in solid state. The vibrational spectral analysis was
performed on the basis of the characteristic vibrations of stilbazo-
lium cation and toluene sulfonate anion. Figs. 4 and 5 are the
10
1220
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 4. FT-IR spectrum of BMST compound.

82
M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
9
8
7
6
5
4
3
2
1
0
vibrations of CAH bonds in ortho-position and the C1ABr bond. The
band at the lower wavenumber in this region arises from the
mixed stretching vibrations of two CAH bonds (C3AH and C5AH)
in the ortho-position of the vinyl group (CAC). Similarly, the ring
CAH stretching vibrations is appeared at 3168 (pyridine – ring II)
and 3090 cmꢂ1 (methyl p-toluenesulfonate – ring III) vibrations
in the IR spectrum. The CAH in-plane bending character can be
expected in the region 1300–1000 cmꢂ1. The CAH in-plane defor-
mation ring characteristic vibrations are occurred at 1195 cmꢂ1
in IR spectrum and at 1208, 1183 cmꢂ1 in Raman spectrum. Fur-
thermore, such deformation changes the bond length between
the carbon atoms and the polarizability of such p-conjugated system.
The ring mode vibration is occurred at 1220 cmꢂ1 in the IR spectrum
as an intense band. The CAH out-of-plane bending vibrations of phe-
nyl ring are usually observed in the region 1000–675 cmꢂ1. The CAH
out-of-plane vibrations are observed as strong bands at 884 and
884 cmꢂ1 in IR and Raman spectrum respectively, and also a weak
band is appeared at 837 cmꢂ1 in IR spectrum. The existence of phenyl
ring modes at 713, 680 and 534 cmꢂ1 in IR spectrum, clearly indi-
cates that the BMST chromophore is non-polar [23].
3200 2800 2400 2000 1600 1200
800
400
Wavenumber (cm-1)
Fig. 5. FT-Raman spectrum of BMST compound.
Halogen (CABr) group vibrations
Table 1
Infrared and Raman band positions (cmꢂ1) and their assignments of BMST.
In the Raman spectrum of BMST, the stretching vibration of bro-
tIR (cmꢂ1
)
tRaman (cmꢂ1
)
Assignments
mine (
t; CABr) contributes to the bands observed at 288 and
1073 cmꢂ1. Also the
t
(CABr) out-of-plane bending vibrations are
3168 m
3128 m
3090 w
3049 s
2965 w
2918 w
2861 w
1644 m
1624 vvs
1585 vs
1519 ssh
1488 m
1473 m
1404 m
1331 s
1220 s
1195 s
1122 s
1104 w
1073 m
1032 s
1010 s
979 w
942 w
884 s
–
Ring II
Ring I
t
(CAH)
3069 vs
t (CAH)
occurred at 308 cmꢂ1. These results show very good agreement
with the earlier report for p-bromo substituted aniline and phenol
[24]. It confirms the presence of bromine atom in the synthesized
complex.
–
–
–
–
–
–
–
–
Ring III t (CAH)
Vinyl CAH stretching
CH3 group of ring III asymmetric stretching
CH3 asymmetric stretching
CH3 group of ring III symmetric stretching
t
t
t
(C7@C8)/vinyl CAH rocking
(C7@C8)/ring (C@C)
(C@C)/ (CAC)
(C@C)
CH3 asymmetric deformation
t
Methyl group vibrations
t
1564 w, 1520 w
1478 vw
–
Ring t
Methyl groups are referred to as an electron donating substitu-
ent in the aromatic ring system [25]. Two kinds of methyl groups
are present in the BMST molecule, one type NACH3 attached with
pyridine ring and other type is CACH3 of tosylate anion. The CAH
stretching vibrations of the methyl group usually occurs in the re-
gion 3000–2840 cmꢂ1. It is observed that weak bands at 2965,
2918 cmꢂ1 in IR spectrum are referred to CAH stretching of methyl
group in pyridine ring and tosylate anion. The asymmetrical bend-
ing vibrations das (CH3) and symmetrical bending vibrations ds
(CH3) of methyl groups are occurred near 1450 and 1375 cmꢂ1
respectively. The asymmetrical bending vibrations are appeared
in the IR spectrum at 1473 cmꢂ1, while vibrations at 1331 cmꢂ1
in IR spectrum and 1349, 1332 cmꢂ1 in Raman spectrum are due
to symmetrical bendings of CH3 group. The rocking mode of the
CH3 groups generally lies between the region 1070–1010 cmꢂ1
and it is observed at 1073 cmꢂ1 in infrared and Raman spectra.
The twisting and torsion of methyl group vibrations are found at
563 cmꢂ1 in IR and 308, 288 cmꢂ1 in Raman spectra.
Ring
Ring
t
t
(C@C)
(C@C)
1404 w
1349 vw, 1332 w CH3 symmetric deformation
1286 w, 1208 w
1183 s
–
Ring d (CAH)
Vinyl CAH bending/ta (SO2)
Ring d (CAH)
Ring d (CAH)
CH3 rocking
Ring d (CAH)
Ring d (CAH)
Ring d (CAH)
Vinyl t (CAH)
–
1073 w
–
1010 vw
–
–
884 w
–
–
802 w
772 w
–
–
–
Ring d (CAH)/CAN stretch
Ring d (CAH)
837 s
821 s
Ring
Ring
t
t
(CAH)
(CAH)
807 w
772 w
713 w
680 vvs
563 s
Ring d (CCC)
Ring
Ring
t
t
(CCC)
(CCC)
CH3 twisting
CABr stretching
534 s
–
–
–
–
308 vw
288 vw
68 w
CH3 torsion/CABr stretching
Ring torsion/CABr bending
Tosylate vibrations
Sulfonate group and skeletal vibrations
The sulfonate functional groups contain the SO2 group and it
gives rise to strong IR bands between 1450–1150 cmꢂ1 for sym-
metric and asymmetric stretching vibrations. The sulfonate group
contains one SAO bond and two S@O bond stretches and it ap-
peared in the region 1430–1330 cmꢂ1 [26]. The sulfonate group
vibrations are observed at 1195 cmꢂ1 in IR and 1183 cmꢂ1 in
Raman spectra. The symmetric stretching vibrations of SO2 group
are identified at 1032 cmꢂ1 in IR and at 1073 cmꢂ1 in the Raman
spectra. The skeletal vibration bands rise from CAC and CAN
Abbreviations:
t – stretching vibration; d – deformation vibration; m – medium; w –
weak; s – strong; vs – very strong; vvs – very very strong.
(C4AC7 = 1.465 Å, C8AC9 = 1.470 Å) with benzene rings. The CAH
vibrations of p-bromo phenyl ring are observed at 3128, 3049
and 3069 cmꢂ1 in infrared and Raman spectra respectively and also
it agrees with previous reported assignments [22]. The symmetric
and asymmetric stretching vibrations in each pair of CAH bonds in
the bromobenzene (ring I) have very near frequency vibrations.
The band at the higher wavenumber can be assigned to two over-
lapping modes arising from the mixed C2AH and C6AH stretching
stretching are generally measured in the region 1150–850 cmꢂ1
The skeletal mode vibrations are observed at 884 cmꢂ1 both in
.

M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
83
the IR and Raman spectra and it is assigned to CAN stretching
mode.
methanol (MeOD) and is presented in Fig. 6a and b. It shows differ-
ent chemical shifts d through absorption peaks with respect to its
chemical environment in the molecule. The doublet chemical shift
‘d’ at 8.7 ppm is due to two (CAH) hydrogen atoms present in the
pyridine (ring II), and it is neighbor of (NAC) bond. The rest of
two hydrogen atoms in the pyridine ring raised the doublet chem-
ical shift ‘d’ at 8.14 ppm. Also the chemical shift of hydrogens in
methyl (NACH3) group attached with pyridine ring gives ‘d’ at
4.32 ppm. The two doublets chemical shifts observed ‘d’ at
1H NMR spectral studies
The 1H NMR spectrum provides information about the number
of different types of protons and also the nature of the immediate
environment of each of them. The BMST compound consists of dif-
ferent types of hydrogen attached with others in the molecular
structure. 1H NMR spectrum of BMST was recorded in deuterated
Fig. 6. (a) 1H NMR spectrum of BMST in MeOD. (b) 1H NMR spectrum of BMST in MeOD.

84
M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
7.62 ppm and 7.66 ppm are due to the hydrogen atoms of p-bro-
mobenzene (ring I) of the compound. The first doublet represents
the hydrogen atom which is neighbor of bromine (BrACACAH)
chemical environment and second is due to the rest of two hydro-
gens. The chemical shifts of two doublets of (CAH) at 7.23 ppm and
7.42 ppm are due to the ring hydrogens of methyl p-toluenesulfo-
nate (ring III). The singlet chemical shift ‘d’ at 2.36 is due to the
methyl (CH3) group hydrogens in ring III. All the observed chemical
shifts of 1H NMR spectra clearly confirmed that, all hydrogen
atoms with respective environment are present in molecular struc-
ture of the complex.
the space group of P21/c. The hall symbol of the system is ꢂP2ybc
and the unit cell contains four ion-pairs (Z = 4). The estimated cell
parameters of the grown crystal are given in Table 2. From the OR-
TEP diagram (Fig. 7) of the molecule, it is observed that the two
independent molecules, C14H13BrN and C7H7O3S are stabilized in
weak CAHꢃ ꢃ ꢃO interactions and forming asymmetric unit. The ring
I (p-bromobenzaldehyde) attached with ring II (p-picoline) via
strong vinyl C@C bond and its dihedral angle is 8.4(2)°. The torsion
angle between C4AC7 and C8AC9 is 178.1(3)°. The disorder in bro-
mine atom positions and disordered over two positions with site
occupancies of 0.72(3) and 0.28(3) was observed in ring I. The dis-
order in crystal structure contributes a sizable magnetic dipole and
an electric quadrupole moment to the bulk susceptibility [27–30],
which is responsible for second harmonic generation property. In
addition, bond length alternation (BLA) is found to be 0.086 Å,
which is the difference between average bond distances of single
and double bonds of carbon in the molecule [31,32]. The magni-
tude of BLA is a useful indicator of second-order NLO response
and compounds with BLA values approximately 0.05 Å are ex-
pected to exhibit the greatest NLO responses, while compounds
with BLA value zero are anticipated to exhibit no second-order
NLO response [33]. Some of the selected bond lengths of the crystal
are given in Table 3. The crystal structural analysis revealed that,
the structural key parameter BLA and observed disorder may be
the reason for observing second-order NLO property from BMST
compound.
X-ray diffraction studies
Single crystal X-ray diffraction study
The crystal structure of grown 4-bromobenzaldehyde-40-N0-
methylstilbazolium tosylate single crystal was determined using
Bruker Kappa APEX II diffractometer. Mo K radiation with wave-
a
length of k = 0.71073 nm from graphite monochromator was used
for recording the reflections. The collected data was solved using
WINGX programme and structure was refined by a direct method
using SHELXL-97. The refined crystal structure with ‘R’ factor
0.0428 revealed that, crystal belongs to monoclinic system with
Table 2
Crystal structure data of BMST.
It is useful to know the relation between morphological varia-
tions and the growth parameters which extends to applied fields
including metal solidification, quality control of optoelectronic
crystals and industrial crystallization. The morphology of BMST
crystal was predicted using WinXMorph program [34,35] and it
is depicted in Fig. 3b. The morphology of BMST revealed the well
Parameters
Crystal data
C21H20 BrNO3S
446.35 g/mol
Monoclinic
P21/c
Molecular formula
Molecular weight
Crystal system
Space group
Point group symmetry
Hall symbol
Unit cell dimensions
2/m
ꢂP2ybc
ꢀ
ꢀ ꢀ ꢀ ꢀ ꢀ
ꢀ
developed ten facets such as (100), ð100Þ; jð023Þ, ð123Þ, ð023Þ,
ꢀ
ꢀ
ꢀ
ꢀ ꢀ
(123), ð023Þ, ð123Þ, ð023Þ, ð123Þ. The morphology of the grown
ꢀ
crystal (Fig. 3b) shows that, (100) and ð100Þ facets have larger sur-
a = 9.0502(2) Å;
b = 6.4201(1) Å;
c = 33.9280(7) Å;
face area than other facets. The angle between the facets
ꢀ
ꢀ
ꢀ
ꢀ
ð100Þ—ð023Þ, ð100Þ—ð123Þ and ð123Þ—ð023Þ of grown crystal
found from morphology are 91.223°, 70.019° and 18.759° respec-
tively. Hence, the morphology of BMST crystal suggests that,
a
=
c
= 90°, b = 94.469(1)°
V = 1965.33 Å3
Fig. 7. Molecular ORTEP diagram of BMST crystal.

M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
85
Table 3
MoKα
70" 125"
Selected bond lengths and magnitude of BLA (Å) for BMST.
1
(+,−,−,+)
Bond
Distances (Å)
100
50
0
CABr1
CABr1*
C4AC7
C7AC8
C8AC9
C18AC21
BLA
1.878(4)
1.924(9)
1.466(4)
1.304(4)
1.468(4)
1.510(5)
0.086
68"
138"
145"
ꢀ
(100) and ð100Þ planes are the suitable facets for device
applications.
-400
-200
0
200
400
High resolution X-ray diffraction (HRXRD) study
The crystalline perfection of the BMST single crystal was char-
acterized by HRXRD using multicrystal X-ray diffractometer [36].
Glancing angle [arc s]
Fig. 8. HRXRD rocking curve of BMST single crystal on (100) plane.
A well-collimated and monochromated Mo K beam obtained
a
1
from the three monochromator Si crystals set in dispersive
(+,ꢂ,ꢂ) configuration was used as the exploring X-ray beam. The
specimen crystal of BMST was aligned in the (+,ꢂ,ꢂ,+) configura-
tion. Due to dispersive configuration, though the lattice constant
of the monochromator crystal and the specimen are different, the
unwanted dispersion broadening in the diffraction curve (DC) of
the specimen crystal is insignificant. The specimen can be rotated
about the vertical axis, which is perpendicular to the plane of dif-
fraction, with minimum angular interval of 0.4 arc sec. The rocking
or diffraction curves were recorded by changing the glancing angle
around the Bragg diffraction peak position hB starting from a suit-
able arbitrary glancing angle and ending at a glancing angle after
the peak, so that all the meaningful scattered intensities on both
sides of the peak include in the diffraction curve. The DC was re-
However, a quantitative analysis of such unavoidable defects is
of great importance, particularly in case of phase matching applica-
tions [41].
Optical characterization
UV–visible study
The optical properties of the grown crystal were studied by
recording UV–visible spectrum using SHIMADZU 1800 UV–visible
spectrophotometer. UV–visible transmittance spectrum was re-
corded in the range 200–800 nm (Fig. 9). It is very useful to identify
the optical absorption or transmission window and cut-off wave-
length of the crystal. Also it is useful to understand its electronic
states, when interacting with linear light. Absorption of ultraviolet
corded by the so called
x scan wherein, the detector was kept at
and visible light involves the promotion of the electrons in
r- and
the same angular position 2hB with wide opening for its slit. This
arrangement is very appropriate to record the short range order
scattering caused by the defects or by the scattering from local
Bragg diffractions from agglomerated point defects or due to low
angle and very low angle structural grain boundaries [37,38]. Be-
fore recording the diffraction curve, the non-crystallized solute
atoms remained on the surface of the crystal and the possible lay-
ers which may sometimes form on the surfaces of grown crystals
by solution methods can be removed [39]. Also to ensure the sur-
face planarity, the specimen was first lapped and chemically
etched in a non-preferential etchant.
Fig. 8 shows the recorded HRXRD curve on (100) diffraction
plane for a typical BMST single crystal specimen. On close observa-
tion, one can realize that the curve is not a single peak. On decon-
volution of the diffraction curve, it is clear that the curve contains
two additional peaks. The solid line of convoluted curve is well fit-
ted with the experimental points represented by the filled circles,
which are 70 and 125 arc sec. away from the main peak on both
the sides. These two additional peaks correspond to two internal
structural low angle boundaries with tilt angle >1 arc min. but <1
degree boundaries [40]. The tilt angle may be defined as the mis-
orientation angles between the two crystalline regions on both
sides of the structural grain boundary are 70 and 125 arc sec. from
their adjoining regions. The full width at half maximum (FWHM) of
the main peak and the two very low angle boundaries are respec-
tively 68 and 138 and 145 arc sec. The relatively low values of
FWHM of the grains in comparison with that of the real life crystals
are depicts that the crystalline perfection is fairly good. It may be
mentioned here that such low angle boundaries could be detected
in the diffraction curve only because of the high-resolution of the
diffractometer used in the present investigations. The presence of
such defects may not influence much on the NLO properties.
p-orbitals from the ground state to excited energy states [42]. The
BMST crystal was transparent from 419 nm to 900 nm and below
419 nm wavelength, the crystal was opaque. From the spectrum,
it is evident that the BMST crystal has optical cut-off at 419 nm
and found no absorption in the most visible region, which is
needed quality of a crystal for optical applications.
Optical constants estimation
The optical constants of the materials are important parameters
for the fabrication of optical devices. The estimation of energy band
gap (Eg) and refractive index (n1) of a specific material are the
essential criteria for linear and nonlinear optical applications. The
energy band gap of the material is very closely related to the atom-
ic structure and electronic band structure of the materials [43]. The
energy band gap (Eg) of the material is very closely related to the
materials with atomic and electronic band structures. The band
gap energy was calculated using the relation,
Eg ¼ hc=k
ð1Þ
where h is the Planck constant, c is the velocity of light and k is the
cut-off wavelength of the crystal. Hence, the band-gap energy of the
grown crystal was estimated to be 2.96 eV.
Typically, main optical parameters of the grown crystal are
absorption coefficient (
absorption coefficient (
a
) and refractive index (n). The optical
a
) of BMST crystal at different wavelengths
was calculated using Eq. (1) [44].
ꢀ ꢁ
1
2:3026 log
T
a
¼
ð2Þ
t
where T is the transmittance and t is the thickness of the sample
used for this study. A typical relationship between absorption

86
M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
ꢂðR þ 1Þ ꢄ ꢂ3R2 þ 10R ꢂ 3
419 nm
n ¼
ð4Þ
100
80
60
40
20
0
+
2ðR ꢂ 1Þ
The linear refractive index (n) was calculated in terms of reflectance
using Eq. (4) and the calculated linear refractive index of the BMST
crystal is found to be n = 2.41. The optical studies revealed that,
BMST crystal possesses good optical behavior for using it in device
applications.
Kurtz powder SHG test
The second harmonic generation property of the BMST crystal
was examined by using Kurtz powder SHG test [45]. The funda-
mental beam obtained from 1064 nm Q-switched Nd:YAG laser
radiation of 2.8 mJ/pulse with 10 Hz repetition rate was used. In
the present study, the crystalline powder was sieved into different
+
365 nm
particle size ranges such as 100, 120, 140, 160 lm. The Urea sam-
ples were used as reference for the SHG test. The ground BMST and
Urea samples were loaded into uniform bore glass capillaries,
200
300
400
500
600
700
800
Wavelength (nm)
which is having an inner diameter of about 600 lm. The sample
Fig. 9. UV–visible transmittance spectrum of BMST crystal.
was subjected to 1064 nm monochromatic radiation and green sig-
nal (532 nm) was observed from the sample. It confirms the capac-
ity of frequency conversion (SHG) property of the BMST material.
The intensity of the green signal was detected using monochroma-
tor, PMT, and oscilloscope. The output SHG signal voltages of the
different sized powdered samples were observed to be 32, 40, 44
coefficient and the photon energy observed in the crystal is pre-
sented in Fig. 10. From the spectrum, all the possible absorption
and electronic processes were studied. It is due to the possible elec-
tronic transitions at the molecular levels. Free carrier absorption is
due to the presence of free electrons and holes and that decreases
with increase in photon energy. There was no absorption band
due to presence of impurities in the compound. The lattice absorp-
tion is due to the radiation absorbed by vibrations of the crystal ions
(Fig. 10). The fundamental absorption of photons is observed in the
region of 5.09 eV and due to band-to-band excitation, in which an
electron can move from the valence to the conduction band. It is ob-
served from the spectrum that, it has large absorption coefficient
and it is occurred, when the photon energy reaches the bandgap en-
ergy. The maximum absorption is noticed at the lower energy,
and 47 mV for 100, 120, 140 and 160 lm respectively and the ref-
erence Urea showed 10 mV. It is observed that the SHG intensities
increase with increase in particle size (Fig. 11). The relative SHG
efficiency of BMST is about 4.7 times that of Urea at about
160 lm particle size and also it agrees with the reported data.
The counter-anion and cation substituted DAST derivative crystals
exhibited strong second-harmonic signals by powder SHG test. The
powder SHG efficiencies of few counter-cation substituted stil-
bazolium derivative crystals such as MBST [46], HMSB [47] and
CMST [48] are compared and given in Table 4.
which is below 419 nm and
this region, when the applied energy reached the higher energy va-
p–
p* energy transition takes place in
Photoluminescence spectra
Luminescent materials are substances which convert incident
energy input into the emission of electromagnetic waves in the
ultraviolet, visible or infrared regions and above that due to
black-body emission. Photoluminescence (PL), where the lumines-
cence is stimulated by UV or visible light, is a widely used tech-
nique to identify the impurities and finds applications in lighting
technologies [49]. It is the mechanism through which the excited
specimen relaxes to the equilibrium state. Luminescence processes
lue of about 2.96 eV in terms of light.
The refractive index of the optical material can be calculated
using transmission spectrum of the crystal and the reflectance of
the crystal in terms of absorption coefficient was calculated using
Eq. (3),
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1 ꢂ expðꢂ
a
tÞ þ expð tÞ
a
R ¼ 1 ꢄ
ð3Þ
1 ꢂ expðꢂ tÞ
a
48
46
44
42
40
38
36
34
32
30
Lattice absorption
2
1
Fundamental absorption
0
Eg
-1
-2
Transmission Window
+
Absorption edge
5 6
2.97 eV
Free carrier
100
110
120
130
140
150
160
2
3
4
Energy bandgap (eV)
Particle size (μm)
Fig. 10. Plot of absorption coefficient (
a
) vs. photon energy of BMST crystal.
Fig. 11. Kurtz powder SHG signal voltage vs. particle size (lm).

M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
87
Table 4
formation, etc., when an optical material exposed to laser radia-
tion. LID threshold refers to the fluence, which causes such dam-
ages. LID threshold of the optical materials are getting an
important role due to high optical intensities involved in nonlinear
process and it must withstand to high power intensities [55]. For
LID experiment, the Q-switched 1064 nm Nd:YAG pulsed laser
was used. The laser beam profile with repetition rate of 10 Hz
and pulse width of 6 ns was used. The (100) plane of the crystal
was polished and subjected to the LID threshold measurement.
The polishing technique of the material produces minimum sub-
surface damage and it can be raised the surface damage threshold.
The rear surface damages occurred due to Fresnel reflection loss at
the front surface result in less energy [56]. The multishot LID mea-
surement was made on the crystal with 1 mm beam spot. The out-
put intensity of the laser was controlled with variable attenuator
and delivered on the surface of the sample located at the near
focusing of the converging lens. The energy density (E) of the laser
beam was noted for which the crystal gets damaged. The surface
LID threshold of BMST crystal was calculated using,
Comparison of powder SHG efficiency of BMST with some DAST based organic
crystals.
Crystal
SHG efficiency
References
DAST
MBST
HMSB
CMST
BMST
1000
17
11
10.2
4.7
[14]
[46]
[47]
[48]
Present work
is being excited using a beam of light that leads to the creation of
an electron–hole (e–h) pair and it may recombine across the gap
and emit a photon with energy equal to band gap [50]. The e–h
recombination process can be occurred because, excited electron
and hole cannot remain in their initial excited states for very long,
instead they can relax very rapidly (ꢅ10–13 ns) to the lowest en-
ergy states within their respective bands by emitting phonons.
Fig. 12 shows the room temperature PL spectrum of BMST crystal,
which is excited at 360 nm. The donor–acceptor type and ionic
property of the compound may give rise to luminescence. The
broad photoluminescence emission (PLE) peak seen in the spectra
at 432 nm is due to interaction between the electronic system of
the luminescent center and the vibrations of ions which have it.
Such a broad PLE emission was caused by diverse electronic tran-
sitions occurring in different energy levels due to deep or shallow
holes within the band gap [51]. PL spectra consists two main peaks
and one is near band edge transition usually lying at the ultraviolet
region at 432 nm and other is in red region at 646 nm, which is re-
lated to deep level defect transitions. The tail band structure PL
emission peak at 646 nm is due to the defects of the crystal [52].
The deep holes are origin states for the green, yellow, orange and
red PL emissions, while the shallow holes are responsible for the
violet and blue emissions [53]. In the present study, broad PL emis-
sion at 432 nm violet emission is an indicative of the charge trans-
fer process as well as the trapping of electrons and it is due to the
contribution of the shallow holes than the deep holes [54]. In addi-
tion, the weak PL emission peak at 646 nm is due to fewer contri-
butions from deep holes in the energy levels. Thus, PLE spectrum
revealed that, BMST crystal exhibits blue shift emission, which is
most useful for luminescent applications.
PðdÞ ¼ E=
s
A
ð5Þ
where E is the intensity of the irradiant laser beam (mJ),
s
is the
pulse width (ns) and A is the area of the circular spot size (cm2).
The calculated LID threshold value of BMST crystal is found to be
1.50 GW/cm2. Further, the LID threshold values are compared with
familiar NLO crystals and are given in Table 5 [57,58]. It is clear that,
BMST possesses lesser LID than DAST crystal and has higher value
than KDP crystal, which is most widely used in NLO applications.
Thermal analyses
Thermogravimetric analysis measures the change in mass of a
sample on heating and useful to study the crystallization. Thermo-
gravimetry (TG), differential thermal analysis (DTA) and differen-
tial scanning calorimetry (DSC) are quite useful, since they
provide reliable information on the physico-chemical parameters,
characterizing the processes of transformation of solids or partici-
pation of solids in processes of isothermal or non-isothermal heat-
ing [59,60]. The SDT Q600 V8 instrument was used to record
TG-DTA thermogram of the compound in nitrogen atmosphere at
the heating rate of 20 °C/min (Fig. 13). The TGA curve revealed that,
three distinct weight losses stages occurred on sample: the first
one at about 294.93 °C connected with ꢅ2% mass loss was accom-
panied by endothermic effect and was attributed to the removal of
moisture absorbed by the compound. The second one begun at
about 294.93 °C after melting and it extended up to 474.5 °C. It is
connected with ꢅ71% mass loss which is indicative for the decom-
position of pyridine moiety, vaporization and burning of volatile
gaseous such as SO2, CO2, and NO2 produced from the thermal deg-
radation of the compound. Similarly in the DTA- curve, one main
endothermic peak was observed at about 288.07 °C and it is con-
nected with melting of the compound. After the melting tempera-
ture, compound led to decomposition on further heating. The third
decomposition step at temperature above 474.5 °C was observed in
Laser induced damage threshold study
Laser-induced damage (LID) in optical materials refers to
permanent damage caused by melting, ablation, cracking, plasma
7.4
432 nm
+
7.2
7.0
6.8
6.6
6.4
6.2
6.0
646 nm
+
Table 5
Comparison of laser damage threshold value of BMST crystal with some well-known
NLO crystals.
Crystal
Laser damage threshold (GW/cm2)
References
KDP
0.2
[57]
Urea
1.5
[57]
Beta-barium borate
L-Arginine phosphate
DAST
5.0
10.0
2.8
[57]
[57]
[58]
350
400
450
500
550
600
650
700
750
Wavelength (nm)
BMST
1.24
Present work
Fig. 12. Photoluminescence emission spectrum of BMST crystal excited at 360 nm.

88
M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
3
spectral studies. The single crystal X-ray diffraction study reveals
294.93oC; 97.81 %
that BMST crystallizes in the space group of P21/c with monoclinic
system. The equilibrium morphology of grown crystal was deter-
mined using WinXMorph program. The crystal perfection was
studied by using HRXRD analysis. The optical transmission
spectrum confirms that, crystal possess transmission window in
419–900 nm region and band gap energy (Eg) and linear refractive
index were found to be 2.96 eV and 2.41 respectively. The SHG
property was analyzed for different particle sizes by Kurtz method
and found that SHG intensity increases with increase in particle
size. Photoluminescence spectral studies suggest that strong
charge transfer occurs in the molecule. Laser damage threshold
study confirms that, BMST possess greater LID threshold than
familiar NLO crystals such as KDP crystal. The decomposition
stages were analyzed from thermal analyzes and found that the
melting point is 289.4 °C; hence, it suggests the application limit.
From the characterization results, it is observed that the BMST
crystal is potential material for optical applications.
100
80
60
40
20
0
+
+
1.505 mg
2
+
288.07 o
C
1
0
474.5 oC; 26.85%
-1
-2
-3
+
+
731.07 oC; 2.39%
0
100
200
300
400
500
600
700
800
Temperature (ºC)
Fig. 13. TG-DTA thermogram of BMST compound in N2 atmosphere.
Acknowledgement
One of the authors (M.K.) would like to acknowledge the CSIR,
New Delhi for providing financial support (Project No. 03(1200)/
11/EMR-II dated: 18.04.2011).
0
-5
180 mJ/mg
References
-10
-15
-20
-25
-30
[1] G.G.A. Balavoine, J.C. Daran, G. Iftime, P.G. Lacroix, E. Manoury, J.A. Delaire, I.M.
Fanton, K. Nakatani, S.D. Bella, Organometallics 18 (1999) 21–29.
[2] H.O. Mercy, L.F. Warren, M.S. Webb, C.A. Ebbers, S.P. Velsko, G.C. Kennedy, G.C.
Catella, Appl. Opt. 31 (1992) 5051–5060.
[3] Z. Yang, M. Jazbinsek, B. Ruiz, S. Aravazhi, V. Gramlich, P. Gunter, Chem. Mater.
19 (2007) 3512–3518.
[4] M. Todorova, R. Bakalska, T. Kolev, Spectrochim. Acta, Part A: Mol. Biomol.
Spectrosc. 108 (2013) 211–222.
[5] Y. Shi, C. Zhang, H. Zhang, J.H. Bechtel, L.R. Dalton, B. Robinson, W. Steier,
Science 288 (2000) 119–122.
[6] A.F. Benner, M. Ignatowski, J.A. Kash, D.M. Kuchta, M.B. Ritter, IBM J. Res. Dev.
49 (2005) 755–775.
289.4 o
C
[7] L. Li, H. Cui, Z. Yang, X. Tao, X. Lin, N. Ye, H. Yang, CrystEngComm 14 (2012)
1031–1037.
[8] S.R. Marder, B. Kippelen, A.K. Jen, N. Peyghambarian, Nature 388 (1997) 845–
851.
0
50
100
150
200
250
300
350
Temperature (ºC)
[9] A. Schneider, M. Neis, M. Stillhart, B. Ruiz, R. Khan, P. Gunter, J. Opt. Soc. Am. B
23 (2006) 1822–1835.
Fig. 14. DSC curve of BMST crystal.
[10] S. Kwon, M. Jazbinsek, O. Kwon, P. Gunter, Cryst. Growth Des. 10 (2010) 1552–
1558.
[11] B. Cai, T. Hattori, H.H. Deng, K. Komatsu, C. Zawadzki, N. Keil, Jpn. J. Appl. Phys.
40 (2001) 964–966.
[12] T. Vijayakumar, I.H. Joe, C.P. Reghunadhan Nair, M. Jazbinsek, V.S. Jayakumar, J.
Raman Spectrosc. 40 (2009) 52–63.
[13] X.M. Duan, S. Okada, H. Nakanishi, A. Watanabe, M. Matsuda, K. Clays, A.
Persoons, H. Matsuda, Proc. SPIE 2143 (1994) 41.
[14] S.R. Marder, J.W. Perry, C.P. Yakymyshyd, Chem. Mater. 6 (1994) 1137–1147.
[15] M. Krishna Kumar, S. Sudhahar, B.M. Sornamurthy, G. Chakkravarthi, R. Mohan
Kumar, Acta Crystallogr. E69 (2013) o694.
[16] S. Fioravanti, L. Pellacani, P.A. Tardella, M.C. Vergari, Org. Lett. 10 (2008) 1449–
1451.
[17] K. Ebitani, K. Motokura, K. Mori, T. Mizugaki, K. Kaneda, J. Org. Chem. 71 (2006)
5440–5447.
[18] N. Zaitseva, L. Carman, A. Glenn, J. Newby, M. Faust, S. Hamel, N. Cherepy, S.
Payne, J. Cryst. Growth 314 (2011) 163–170.
the course of TG-curve, which is extended up to 731.07 °C. It may
be due to the change of the structure of the material and it is con-
nected with ꢅ26.8% mass loss which is indicative of break of vinyl
ligand (C@C). The DSC analysis was carried out (Fig. 14) for the
sample using EXSTAR SII DSC 6220 instrument. A sharp endother-
mic peak observed at about 289.4 °C is due to endothermic absorp-
tion of about 180 mJ/mg energy. It indicates that the sharp melting
point nature of the crystal and also the indication of absence of
impurities. TG-DTA and DSC analyses confirm that, BMST crystal
melts at about 289.4 °C and then it starts to degradation. Thus, it
is capable to function at temperature up to 289.4 °C in optical
applications.
[19] Y. Li, Z.R. Li, D. Wu, R.Y. Li, X.Y. Hao, C.C. Sun, J. Phys. Chem. B 108 (2004) 3145–
3148.
[20] M.D. Zoppo, M. Tommasini, C. Castiglioni, G. Zerbi, Chem. Phys. Lett. 287
(1998) 100–108.
Conclusion
[21] R.M. Silverstein, F.X. Webster, Spectroscopic Identification of Organic
Compounds, sixth ed., John Wiley & Sons, New York, 2003.
[22] P.M. Wojciechowski, D. Michalska, Spectrochim. Acta, Part A: Mol. Biomol.
Spectrosc. 68 (2007) 948–955.
[23] C.H. Choi, M. Kertesz, J. Phys. Chem. A 101 (1997) 3823–3831.
[24] W. Zierkiewicz, D. Michalska, T.Z. Huyskens, J. Phys. Chem. A 104 (2000)
11685–116692.
[25] M. Gussoni, C. Castiglioni, J. Mol. Struct. 521 (2000) 1–18.
[26] A. Subashini, S. Leela, K. Ramamurthi, A. Arakcheeva, H.S. Evans, V. Petricek, G.
Chapuis, P. Pattison, P. Reji, CrystEngComm 15 (2013) 2474–2481.
[27] B. Koopmans, A.M. Janner, H.T. Jonkman, G.A. Sawatzky, E. van der Woude,
Phys. Rev. Lett. 71 (1993) 3569–3572.
A novel DAST derivative crystal, 4-bromobenzaldehyde-40-N0-
methylstilbazolium tosylate salt was synthesized by condensation
method and the elemental analysis confirmed its composition. The
solubility and metastable zone width of the synthesized salt in
methanol solvent was determined and single crystals of BMST with
dimension of 9 ꢁ 3 ꢁ 3 mm3 were grown by solvent evaporation
method. IR and Raman frequencies explored all the functional
groups present in the compound. All the chemical shifts of func-
tional groups of compound were confirmed by using 1H NMR

M. Krishna Kumar et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 125 (2014) 79–89
[28] B. Koopmans, A. Anema, H.T. Jonkman, G.A. Sawatzky, F. van der Woude, Phys. [45] S.K. Kurtz, T.T. Perry, J. Appl. Phys. 39 (1968) 3798–3812.
89
Rev. B 48 (1993) 2759–2764.
[29] G.J. Ashwell, G. Jefferies, D.G. Hamilton, D.E. Lynch, M.P.S. Roberts, G.S. Bahra,
C.R. Brown, Nature 375 (1995) 385–388.
[30] G. Lupke, D.J. Bottomley, H.M. Van Drile, J. Opt. Soc. Am. B 11 (1994) 33–43.
[31] C.B. Gorman, S.R. Mardar, Proc. Natl. Acad. Sci. USA 90 (1993) 11297–11301.
[32] N.A. Murugan, J. Kongsted, Z. Rinkevicius, H. Agren, Proc. Natl. Acad. Sci. USA
107 (2010) 16453–16458.
[46] C.K.L. Perumal, A. Arulchakkaravarthi, N.P. Rajesh, P.S. Raghavan, Y.C. Huang,
M. Ichimura, P. Ramasamy, J. Cryst. Growth 240 (2002) 212–217.
[47] A.S. Gill, S. Kalainathan, G. Bhagavannarayana, Mater. Lett. 64 (2010) 1989–
1991.
[48] A.S. Gill, S. Kalainathan, Arch. Phys. Res. 1 (4) (2010) 41–47.
[49] S. Kasap, P. Capper, Springer Handbook of Electronic and Photonic Materials,
Springer Science Inc., 2006. pp. 983–996.
[33] A. Teshome, M.D.H. Bhuiyan, G.J. Gainsford, M. Ashraf, I. Asselberghs, G.V.M.
Williams, A.J. Kay, K. Clays, Opt. Mater. 33 (2011) 336–345.
[34] W. Kaminsky, J. Appl. Crystallogr. 38 (2005) 566–567.
[35] W. Kaminsky, J. Appl. Crystallogr. 40 (2007) 382–385.
[36] K. Lal, G. Bhagavannarayana, J. Appl. Crystallogr. 22 (1989) 209–215.
[37] G. Bhagavannarayana, S.K. Kushwaha, J. Appl. Crystallogr. 43 (2010) 154–162.
[38] K. Senthilkumar, S. Moorty Babu, G. Bhagavannarayana, J. Appl. Crystallogr. 44
(2011) 313–318.
[39] G. Bhagavannarayana, S. Parthiban, S. Meenakshisundaram, J. Appl.
Crystallogr. 39 (2006) 784–790.
[40] G. Bhagavannarayana, R.V. Ananthamurthy, G.C. Budakoti, B. Kumar, K.S.
Bartwal, J. Appl. Crystallogr. 38 (2005) 768–771.
[50] M. Bass, Hand Book of Optics, vol. IV, Tata McGraw-Hill Companies, Inc., 2010.
pp. 5.70–5.83.
[51] Y. Liu, M. Li, Q. Fang, Q. Lv, M. Wu, S. Cao, Chinese J. Phys. 48 (2010) 523–531.
[52] V.M. Longo, L.S. Cavalcante, R. Erlo, V.R. Mastelaro, A.T.D. Figueiredo, J.R.
Sambrano, S. De Lazaro, A.Z. Freitas, L. Gomes, N.D. Vieira, J.A. Varela, E. Longo,
Acta Mater. 56 (2008) 2191–2202.
[53] L.S. Cavalcante, J.C. Sczancoski, F.S. De Vicente, M.T. Frabbro, M. Siu Li, J.A.
Varela, E. Longo, J. Sol–Gel. Sci. Technol. 49 (2009) 35–46.
[54] J.C. Sczancoski, L.S. Cavalcante, T. Badapanda, S.K. Rout, S. Panigrahi, V.R.
Mastelaro, J.A. Varela, M. Siu Li, E. Longo, Solid State Sci. 12 (2010) 1160–1167.
[55] G.C. Bhar, A.K. Chaudhary, P. Kumbhakar, Appl. Surf. Sci. 161 (2000) 155–162.
[56] M. Bass, Handbook of Optics, vol. IV, The McGraw-Hill companies, Inc., 2010.
pp. 19.1–19.4.
[41] G. Bhagavannarayana, B. Riscob, M. Shakir, Mater. Chem. Phys. 126 (2011) 20–
23.
[57] H.L. Bhat, Bull. Mater. Sci. 17 (1994) 1233–1249.
[42] S. Kasap, P. Capper, Springer Handbook of Electronic and Photonic Materials,
Springer Science Inc., 2006. pp. 47–69.
[58] Y. Takahashi, S. Onduka, S. Brahadeeswaran, M. Yoshimura, Y. Mori, T. Sasaki,
Opt. Mater. 30 (2007) 116–118.
[43] R.N. Rai, S.R. Mudunuri, R.S.B. Reddi, V.S.A.K. Satuluri, S. Ganeshmoorthy, P.K.
Gupta, J. Cryst. Growth 321 (2011) 72–77.
[59] L. Vlaev, N. Nedelchev, K. Gyurova, M. Zagorcheva, J. Anal. Appl. Pyrol. 81
(2008) 253–262.
[44] R. Robert, C. Justin Raj, S. Krishnan, S. Jerome Das, Physica B 405 (2010) 20–24.
[60] V. Georgieva, D. Zvezdova, L. Vlaev, Chem. Cent. J. 6 (2012) 81–90.