Investigations on the physicochemical properties of the nonlinear optical crystal for blue green laser generation

Article abstract of DOI:10.1016/j.saa.2007.11.017

Bisglycinehydrogenchloride (BGHC) an intriguing material for frequency conversion has been grown by slow evaporation solution growth technique at room temperature. Their structural, optical and physicochemical properties were characterized by X-ray powder diffraction, FTIR, Raman and UV-vis spectra. The title crystal has found to crystallize into noncentrosymmetric orthorhombic P2₁2₁2₁ space group. The material has a wide transparency in the entire visible region. It is found that the cutoff wavelength lies in the UV region. The mechanical response of the crystal has been studied using Vickers microhardness technique. The Kurtz powder second harmonic generation test shows that the compound is a prospective crystalline material for second order nonlinear optical application.

Full text of DOI:10.1016/j.saa.2007.11.017

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Spectrochimica Acta Part A 71 (2008) 119–124
Investigations on the physicochemical properties of the nonlinear
optical crystal for blue green laser generation
V. Krishnakumar∗, S. Sivakumar, R. Nagalakshmi
Department of Physics, Periyar University, Salem 636011, India
Received 27 May 2007; received in revised form 10 November 2007; accepted 12 November 2007
Abstract
Bisglycinehydrogenchloride (BGHC) an intriguing material for frequency conversion has been grown by slow evaporation solution growth
technique at room temperature. Their structural, optical and physicochemical properties were characterized by X-ray powder diffraction, FTIR,
Raman and UV–vis spectra. The title crystal has found to crystallize into noncentrosymmetric orthorhombic P212121 space group. The material
has a wide transparency in the entire visible region. It is found that the cutoff wavelength lies in the UV region. The mechanical response of the
crystal has been studied using Vickers microhardness technique. The Kurtz powder second harmonic generation test shows that the compound is
a prospective crystalline material for second order nonlinear optical application.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Bisglycinehydrogenchloride; X-ray diffraction; FTIR; Optical transmission; Second harmonic generation
1. Introduction
outstandingmaterialstochallengetheexistingprospectivemate-
rials. These materials exhibit promising structural background
in view of their zwitterionic and protonated forms and structural
stabilization with hydrogen bonding. These factors accounting
for the delocalization and corresponding enhancement in sec-
ond order NLO activity. An amino acid exists as a dipolar ion
in which carboxyl group is present as carboxylate ion and the
amino group is present as ammonium ion. Thus, the presence
of zwitterion influences the physical and chemical properties
of amino acids [6]. Complexes of amino acid with inorganic
salts are promising materials for optical second harmonic gen-
eration as they tend to combine the advantages of the organic
amino acid with that of the inorganic acid/salt. Glycine and
its methylated analogues form complexes with mineral acids
exhibiting interesting physical properties like ferroelastic, fer-
roelectric or antiferroelectric behaviour often associated with
transition to commensurate or incommensurate phases. Accord-
ing to the structural studies made on the structural aspects of
glycine and its isomorphs [7–13], it has been found that there
exist two kinds of glycine groups, glycinium ions and zwitte-
rions. Such configuration of glycine ions is interconnected by
short N H. . .O hydrogen bonds, which is essentially respon-
sible for the ferroelectric and nonlinear optical behaviour of
the title crystal. In the present investigation, large and high
optical quality BGHC single crystals have been grown by
slow evaporation solution technique. Thus, the crystal growth
Developmentofnovelmolecularandcrystaldesigntechnique
for assembling the materials is a nifty concern. In particu-
lar, semi-organic system provides many structure and bonding
schemes for the molecular engineering of new materials. Semi-
organic nonlinear optical (NLO) crystals in recent years are
attracting a great deal of attention due to their high NLO coef-
ficient, high damage threshold and high mechanical strength
compared to organic NLO crystals. The crystal engineering of
this nonlinear optical material is based on the anchorage of
organic molecules exhibiting a large NLO efficiency on the
amino acid group, which ensure convenient, thermal, mechan-
ical and optical stabilities. Materials with large second order
optical nonlinearities, transparency at all wavelengths involved
and stable physicochemical performance are needed in order
to realize many applications [1,2]. In recent years, amino acid
family crystals are of interest due to their attracting nonlinear
optical properties [3–5]. Amino acid family crystals are gain-
ing importance as highly feasible second order NLO materials.
Considerable efforts have been made to combine amino acids
with interesting organic acid, inorganic materials to produce
∗
Corresponding author. Tel.: +91 427 2345766x246; fax: +91 427 2345565.
E-mail address: vkrishna kumar@yahoo.com (V. Krishnakumar).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.11.017

120
V. Krishnakumar et al. / Spectrochimica Acta Part A 71 (2008) 119–124
and characterization of the title compound is outlined in this
paper.
2. Experimental
2.1. Synthesis and crystal growth
BGHC was synthesized from glycine and hydrochloric acid
taken in the molar ratio of 2:1. The calculated amount of
the reactants were thoroughly dissolved in double distilled
water and stirred well for 2 h to yield a homogeneous mixture.
The mixture was evaporated to dryness by heating below an
optimum temperature of 60 ◦C. To prevent possible decompo-
sition, the colourless crystalline sample of the title compound
was obtained and recrystallized several times before starting
the growth process. The stoichiometric reaction is given as
follows
Fig. 1. Solubility curve of BGHC.
3. Spectral measurements
3.1. X-ray diffraction studies
2(NH2CH2COOH) + HCl
The structural properties of single crystals of BGHC have
been studied by X-ray powder diffraction technique. The X-ray
diffraction studies were carried out using SEIFERT diffractome-
→ (NH3+CH2COO−)(NH3+CH2COOH)Cl−
A
B
˚
terwithCuK␣1 (λ = 1.5406 A)radiation. Thepowderedsamples
were scanned over the range 10–70◦ at a rate of 1◦ per minute.
From the powdered X-ray data, the various planes of reflec-
tions were indexed using XRDA 3.1 program and the lattice
parameters were evaluated. The indexed X-ray diffraction pat-
tern is shown in Fig. 2. The XRD data prove that the crystal
is orthorhombic in structure with the space group P212121, a
well-known noncentrosymmetric space group thus satisfying
the requirements for second order NLO activity. The unit cell is
found to be tetra molecular. The lattice parameter values reveal
a very close agreement with reported values [15] thereby con-
firming the identity of the grown crystal. The crystallographic
data is given in Table 1. It has been observed that P212121 is one
among the popular space group and it allows maximal contri-
bution of the molecular nonlinearity of the macroscopic crystal
nonlinearity. It can be implied that A-phase occurs as a zwitte-
rion of the type NH3+CH2COO−, while B-phase has the normal
glycine configuration. This molecule receives an additional H+
2.2. Solubility
In order to determine the crystalline habit and large size single
crystal with good quality, the growth condition such as solvent,
temperature and pH are to be optimized. The solubility of the
material, which is available for growth and hence defines the size
of the crystal that can be grown. The selection is very definitive
for the growth of good quality crystal. The equilibrium solubility
and its temperature dependent are essential for solution growth.
The data from the solubility curve will suffice to start growing
fair quality crystal [14]. The solubility of the title compound
was assessed in different solvents apart from deionized water.
The title material was crystallized in very small size and found
to be defective in all the other solvents. Deionized water was
suitable for crystallizing large and transparent colourless single
crystal.
The solubility of BGHC was evaluated as a function of tem-
perature in the temperature range 30 ◦C. A thermostatically
controlled vessel was filled with the solution of the title com-
pound and with some undissolved BGHC and stirred for a day.
The next day a small amount of the supernatant solution was
pipetted out and the concentration of the solute was determined
gravimetrically. Fig. 1showsthesolubilitycurveofthetitlecom-
pound. We infer that the title compound exhibits good solubility
and a positive solubility temperature gradient in water.
Single crystals of the title compound have been grown
from saturated solution at a pH of 4.3 by slow evaporation
solution growth technique at 35 ◦C using constant tempera-
ture bath having controlled accuracy of 0.01 ◦C. The crystals
with perfect shape and transparency were formed by sponta-
neous nucleation in the supersaturated solution. The growth of
the title compound needed a week, single crystals with good
optical quality having rectangular shape having dimensions of
20 mm × 0.7 mm × 0.4 mm have been obtained.
Fig. 2. The indexed X-ray diffraction pattern of BGHC.

V. Krishnakumar et al. / Spectrochimica Acta Part A 71 (2008) 119–124
121
Table 1
Crystallographic data of BGHC
Identification code
BGHC
Empirical formula
Formula weight
Crystal structure
Space group
C4H11N2O4Cl
186
Orthorhombic
P212121
Cell parameters
´
˚
a (A)
5.2990
8.1181
´
˚
b (A)
´
˚
c (A)
18.0622
90
90
α (◦)
β (◦)
γ (◦)
90
´
3
˚
Volume (A)
Z
777.00923
4
D24
20 mm × 0.7 mm × 0.4 mm
Point group symmetry
Crystal size
Fig. 4. FT Raman spectrum of BGHC.
ion from HCl and these H+ acts as ionic connection between
molecules A and a pair of Cl− ions. The carboxyl group is a
proton donor and the amino group is a proton acceptor. In the
glycine molecule carboxylic acid group donates its proton to
the amino group to form molecule A i.e. NH3+CH2COO−. The
Cl chains are bonded on both sides to two molecules by means
of hydrogen and Vanderwaals bonds. Thus, the hydrogen bond-
ing and Vanderwaals forces existing in the intermolecular and
intramolecular structure of the molecule. The dipole moment of
the title crystal is large, which influences the essential nonlinear
optical properties such as second harmonic generation (SHG)
efficiency etc.
Fourier transform Infrared Spectrophotomer, model SPEC-
TRUM RX1, using KBr pellets containing a fine BGHC
powder obtained from the grown single crystals, equipped
with a LiTaO3 detector, a KBr beam splitter, He–Ne
Laser source and boxcar apodization used for 250 averaged
interferogrammes collected for both the sample and the back-
ground.
3.3. FT Raman measurements
The FT Raman spectrum was recorded on a BRUKER
IFS 66V model interferometer equipped with an FRA-106 FT
Raman accessory. The observed spectrum given in Fig. 4 was
recorded in the 3500–100 cm−1 stokes region using the 1064 nm
line of a Nd:YAG laser for excitation operating at 200 mW
power. The reported wave numbers are believed to be accurate
3.2. FTIR measurements
The room temperature mid Fourier transform infrared spec-
trum of BGHC shown in Fig. 3 was recorded in the region
400–4000 cm−1 at a resolution of 4 cm−1 using PerkinElmer
within 1 cm−1
.
Fig. 3. Experimental FTIR spectrum of BGHC.

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V. Krishnakumar et al. / Spectrochimica Acta Part A 71 (2008) 119–124
4. Results and discussions
of amino acid cation and the amino group and also between the
amino group and the chlorine atom. In several cases, specific
intermolecular hydrogen bonding networks have been used as
a steering force for structure accentricity in crystal engineering
particularly of Etter’s [16,17] and Gunter’s group [18–20]. The
hydrogen-bonding network seems to tailor the molecular dipoles
of the ionic species in similar direction.
4.1. Vibrational spectral analysis
Vibrational spectroscopy is an important tool in understand-
ing the chemical bonding and provides useful information in
studying the microscopic mechanism of the NLO properties of
new materials. From the spectral analysis it is confirmed that the
amino groups are protonated and counter balance the negative
charge of the chlorine ion functionality. Thus, the molecular
structure of the synthesized compound was confirmed by the
spectral analysis. The absence of C O and NH2 group frequen-
A broad strong absorption in the 3300–2300 cm−1 corre-
+
spond to the NH3 ion of the amino acid. Generally N H and
O H bonds participate in hydrogen bonding. Hence this region
resultsfromsuperimposedOHofCOOH, HClandNH3+ stretch-
ing bonds, hence their stretching vibrations should be shifted
to low wavenumber side and also they are asymmetric. The
ammonium group NH3+ exists with pyramidal symmetry in free
state. Its normal mode of vibration are ν1(A1), ν2(A1), ν3(E) and
ν4(E). All the modes of vibrations are both IR and Raman active
[21,22]. The charged amino group has stretching wavenumber
3300–2500 cm−1 [23]. The medium intensity band at 3434 and
3343 cm−1 appearing in IR and a very weak intensity band at
cies and the presence of COO− and NH3 frequencies indicate
+
that the molecule exists in zwitterionic form and the molecules
are held by their dimensional network of hydrogen bonds. The
internal vibrations of glycinium cation may be classified into
those arise from functional groups NH3+, CH, COOH. However,
their vibrations may be strongly coupled between themselves.
The observed wavenumbers, relative intensities obtained from
the recorded spectra and the assignments proposed for the title
nonlinear optical crystal is given in Table 2.
+
Raman at 3476 cm−1 are due to NH3 asymmetric structural
+
mode. The symmetric and asymmetric bending NH3 mode
ν4(E) are identified as medium intensity at 3117 cm−1 and a
strong mode at 1497 cm−1 in the infrared and Raman spectra of
4.2. Vibrations of NH3+ group
+
the title crystal. The NH3 deformation appears at 1607 cm−1
which is attributed to the extensive hydrogen bonding of the
crystal.
The broad band observed in high wavenumber region indi-
cates the presence of hydrogen bonding in the title crystal. The
hydrogen bonds connect a cation pair to the adjacent anion of
another in pair. Thus, it is formed between carboxylate group
4.3. CH2 vibrations
The stretching vibrations of CH2 group are best resolved in
the Raman spectrum and the internal vibrations of CH2 group
are well separated from other parts of the glycine molecule.
The carbon hydrogen stretching vibration give rise to hydrogen
bonds in the region 3100–3000 cm−1 in all the amino acid com-
pounds. In IR spectrum the bands due to this group are observed
in 3042 and 2611 cm−1, which formally are assigned to νasCH2
and νsCH2, respectively. The wavenumbers of methylene group
are well compared with those reported for amino acids [24].
Table 2
Observed IR and Raman wavenumbers (in cm−1) of BGHC and its vibrational
assignments
Wavenumber Raman Wavenumber
FTIR
Assignments
3476w
–
3104w
2966ms
–
3434m
3343m
3117ms
2611w
2168w
1720ms
–
1607ms
1497m
1407ms
1330m
νas(NH3)+
νas(NH3)+
νs(NH3)+, ϕ (C H) stretching
νasCH2
νsCH2
ν(C O)
4.4. Carboxyl vibrations
1680m
–
1440ms
–
δas(NH3)+
δas(NH3)+
The deprotonated carboxylic group (COO−) has two impor-
ν(COO−), δCH2
tant characteristic absorption bands around 1600–1420 cm−1
,
ρCH2, ωCH2, νC
ωCH2, νC C ν(COO)−, δin plane
O
1338ms
which are asymmetric and symmetric stretching modes, respec-
tively. In the title crystal, the bands around 1607, 1497,
1407 cm−1 in the infrared spectrum suggest the presence of
COO− group. They also attributed to the Fermi resonance with
the CH2 rocking mode. The in plane and out of plane C OH
occurs at 1330 cm−1 as a strong band in the Raman.
C
O(H), δin plane(C H)
1266ms
1138w
1050ms
986w
–
894s
686ms
–
502ms
466ms
356w
332ms
–
τCH2, δin plane(C H), ρ(NH3)+
δin plane(C H), ρNH3+, ν(C C)
ν(C C)
1123m
1036m
–
+
δout of plane(C H), ρNH3
926w
890ms
686ms
607ms
503s
–
C (O)H out of plane
ν(C C)
(COO)− scissoring
The protonated carboxyl group R COOH is characterized
by three main IR bands O H stretching, C O stretching at
1700–1780 cm−1 and O H bending at 1200–1300 cm−1. The
presence of protonated free carboxylic acid group in the ionic
δin plane(C
τ(NH3)+
ρ(COO)−
δ(C N)
C C), δ(C N)
part of the glycinium ion is observed at 1720 cm−1
.
–
–
Lattice modes
The presence of glycinium ion is reflected in the highly mixed
bands of the C O stretch, C O H in plane and out of plane
deformation. The wagging, twisting and rocking of CH2 have
ν: stretching; ρ: rocking; δ: deformation; ω: torsion; ϕ: ring: w: weak; m:
medium; ms: medium strong; s: strong.

V. Krishnakumar et al. / Spectrochimica Acta Part A 71 (2008) 119–124
123
also been observed. The other skeletal modes of vibrations such
as C N, C C, O C O have been identified and assigned. The
external modes that are due to the translatory and rotatory modes
have been appeared in Raman below 200 cm−1. The C H in
plane bending is expected in the region 1300–1000 cm−1. The
out of plane C H bending are appeared as a strong band at
890 cm−1. The medium band at 1330 cm−1 in IR and the Raman
counterpart at 1338 cm−1 is assigned to C O stretching which
may be coupled with C H bending mode. So they cannot be
separated and identified in an unambiguous manner. The conju-
gation and the influence of hydrogen bonding type network in
the crystal results in the lowered C O stretching wave number.
The simultaneous IR and Raman activation of C O stretching
modes clearly point to the charge transfer into action between
carboxyl through C O group with lone pair of chlorine. The
intramolecular charge transfer from the carboxyl groups to the
chlorine group makes the molecule highly polarized and into a
molecular charge transfer into action may be responsible for the
NLO properties of the title compound.
tube (Hamamatsu RC 109, a visible PMT) and displayed on a
storage oscilloscope (TDS 3052 B 500 MHz phosphor digital
oscilloscope). KDP crystal was powdered to the identical size
and was used as reference materials in the SHG measurement.
The non-zero measured powder SHG signal is reliable with the
SHG activity, which can be correlated with noncentrosymmet-
ric crystal structure. The relative efficiency of BGHC with that
of KDP has been measured. It is found that the efficiency of
the title crystal (142 mV), which is 15 times greater than that of
KDP (9 mV). The result of SHG is also supporting for further
studies.
7. Microhardness measurements
Hardness of the material is a measure of resistance, it offers
to deformation [26]. The transparent crystals free from cracks
were selected for microhardness measurements. The indenta-
tions were made on the grown surface with the load ranging from
25 to 300 g using Vickers microhardness tester. Leitz–Wetzlar
fitted with a Vickers diamond pyramidal indenter and attached
to an incident light microscope. The indentation time was kept
as 5s for all the loads. The Vickers hardness number Hv was
calculated from the following equation
5. Optical studies
5.1. Transparency range
The optical transmittance plays an important role in identify-
ing the potential of the NLO material. Because the material can
be of utility if it has wide transparency window with reduced
absorption around the fundamental and second harmonic wave-
length. The UV–vis transmittance has been performed using
PerkinElmer Lambda 35 UV visible spectrophotometer in the
region 200–1100 nm. The lower cutoff wavelength lies around
200 nm which is due to the ␲–␲* transition in this compound.
The reduction in absorption around Nd:YAG laser fundamen-
tal wavelength contributes the resistance of the material to laser
damage threshold. The step decrease in transmittance around
200 nm may be assigned to electronic excitation in COO− group.
As there is no change in transmittance in the entire visible range
up to 200 nm the materials can find application as window in
spectral instruments in that region.
P
d2
Hv = 1.8544 ×
kg/mm2
where P is the applied load in kg and d is the diagonal length of
the indentation impression in micrometer and 1.8554 is constant
of a geometrical factor for the diamond pyramid. A plot obtained
between ln(P) against ln(d) gives a straight line which is derived
from the Meyers law, the relation connecting the applied load is
given by P = adn. Here n is the Meyer’s index or the hardening
coefficient that has been calculated from the slope of the straight
line. The value of n is 2.04. According to Onitsch, n is greater
than 2 when hardness decreases with the intensity of load. It
satisfies the prediction of Onitsch [27]. The plot of hardness
(Hv) versus load for the title crystal is shown in Fig. 5.
6. Nonlinear optical studies
Kurtz Perry technique remains an extremely valuable tool for
initial screening of NLO materials [25]. A quantitative measure-
ment of the conversion efficiency of BGHC was determined by
the modified version of powder technique developed by Kurtz
and Perry. The crystal was ground into powder and it was packed
densely between two transparent glass slides. An Nd:YAG(DCR
11) laser was used as a light source. A fundamental laser beam
of 1064 nm wavelength, 8 ns pulse in depth with 10 Hz pulse
rate was made to fall normally on the sample cell. The power
of the incident beam was measured using a power meter and it
is 5.7 mJ/pulse. The transmitted fundamental wave was passed
over a monochromator (Czerny turney monochromator), which
separates 532 nm (SHG signal) from 1064 nm and absorbed by a
CuSO4 solution. Suitable filters were used to remove the resid-
ual light. The green light was detected by a photo multiplier
Fig. 5. Plot of Hv vs. Load of BGHC.

124
V. Krishnakumar et al. / Spectrochimica Acta Part A 71 (2008) 119–124
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8. Conclusions
279.
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[6] V. Krishnakumar, R. Nagalakshmi, Spectrochim. Acta Part A 64 (2006)
736.
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Bisglycinehydrogenchloride was synthesized by the reaction
of glycine with hydrochloric acid in water. The solubility curve
shows that it has reasonable solubility in water. A single crystal
of BGHC was grown from the aqueous solution by slow evapora-
tion method. FTIR and Raman confirmed the various molecular
group vibrations and protonation of amino acids in the grown
crystals. As there is no change in transmittance in the entire vis-
ible range up to 200 nm, these materials can find application as
window in spectral instruments in that region. The output SHG
relative efficiency was found to be 15 times greater than that of
KDP. The vibrational spectral analysis also justifies the presence
of carboxylic group and the title molecule is a positive ion with
the charge formally residing on the NH3+. Hence the BGHC
have nonlinear polarizability and exhibit SHG property. Vick-
ers hardness was carried out and found that the present material
obeys the Indentation Size Effect. Based on these facts it could
be concluded that the present material has good propensity for
nonlinear optical activity.
[19] M.S. Wong, F. Pan, V. Gramlich, C. Bosshard, P. Gunter, Adv. Mater. 9
(1997) 554.
[20] M.S. Wong, F. Pan, M. Bosch, R. Spreiter, C. Bosshard, P. Gunter, V.
Gramlich, J. Opt. Soc. Am. B 15 (1998) 426.
Acknowledgement
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Spectroscopy, Academic Press, New York, 1990.
[22] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Wiley, New York, 1986.
The authors are thankful to University Grants Commission
(UGC), New Delhi for the financial assistance under a major
research project.
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