Vol.1, No.3, 216-221 (2009)
Copyright © 2009 Openly accessible at http://www.scirp.org/journal/NS/
Natural Science
Studies on the growth aspects of organic L-alanine
maleate: a promising nonlinear optical crystal
D. Balasubramanian1,3*, R. Jayavel2, P. Murugakoothan3
1Department of Physics, Municipal School, Tiruvannamalai, India; dr.d.balu@gmail.com
2Crystal Growth Centre, Anna University, Chennai, India
3PG & Research Department of Physics, Pachaiyappa’s College, Chennai, India
Received 17 September 2009; revised 10 October 2009; accepted 12 October 2009.
A organic nonlinear optical material, L-alanine
maleate (LALM) was synthesized. Bulk Single
crystals of LALM have been grown by slow
cooling method with a solution pH of 5. The
solubility of L-alanine maleate has been deter-
mined for various temperatures. Large size sin-
gle crystal of 2.0 x 1.2 x 0.8 cm3 has been grown
with reasonable growth rate along the three
crystallographic directions by optimizing the
growth parameters. The structure of LALM crys-
tal was studied by single-crystal X-ray diffrac-
tion analysis. The presence of functional groups
was confirmed by Fourier transform infrared
spectroscopy. The LALM crystal was analysed
for its thermal and mechanical behaviours. The
grown crystals have also been subjected to
linear and non-linear optical property studies.
From these studies, it is inferred that the LALM
crystals exhibit better thermal and mechanical
stabilities with improved optical properties.
Thus satisfies the essential requirements for
optical device fabrication.
Keywords: Organic; LALM; Crystal Growth; Bulk
Single Crystals; Non-Linear Optical Crystal
Recently, there is considerable interest in the synthesis
of new nonlinear optical (NLO) material, both organic
and inorganic, with large second-order optical nonlin-
earities, as these materials have a significant impact on
laser technology, optical communication and optical
storage technology etc. Over the years many organic and
inorganic materials have been developed [1-4] to cover
the potential applications in ultra-violet, near-and far-
infrared wavelength regions. Amino acid nonlinear opti-
cal materials are often formed by weak van der Waals
and hydrogen bonds and hence possess high degree of
delocalization. The basic structure of organic NLO ma-
terials is based on the bond system. Due to the over-
lapping of orbital, the delocalization of electronic
charge distribution leads to a high mobility of electrons.
Functionalization of both ends of the bond system with
appropriate electron donor and acceptor groups can en-
hance the asymmetric electronic distribution in either or
both ground and excited states, this leads to an increased
optical nonlinearity. All these favourable properties
paved the way for the development of amino acid crys-
tals like L-arginine phosphate (LAP) [5], L-histidine
dihydrogen phosphate (LHP) [6], L-arginine tetrafluo-
roborate (L-AFB) [7], L-alanine tetrafluoroborate
(L-AlFB) [8], L-alanine [9], L-arginine acetate [10], and
L-alanine acetate [11]. LAP crystal was reported to have
promising NLO properties comparable to that of the
well-known inorganic crystal of KDP. L-alanine crystal
shows Type II phase matching, for doubling the Nd:YAG
fundamental, by propagating the pump beam nearly
normal to the {12 0} and {011} faces.
The growth of single crystals of L-alanine which is
the simplest acentric member of the amino acid family
has already been little investigated. In order to widen the
properties of L-alanine and to develop new crystals with
better NLO properties, L-alanine complexes with car-
boxylic acids have been tried. Maleic acid, a dicarbox-
ylic acid with relatively large pi-conjugation has at-
tracted much attention. Though the L-alanine maleate
crystal was already grown by slow evaporation method
[12], in the present study, bulk single crystals of
L-alanine maleate (LALM) single crystals have been
grown by the slow cooling method with optimized
growth conditions first time. The grown crystals were
subjected to various characterization studies. The im-
D. Balasubramanian et al. / Natural Science 1 (2009) 216-221
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/NS/
proved optical transmittance, NLO efficiency and the
mechanical stability of the grown crystals were realized.
2.1. Crystal Growth
High purity (99%) L-alanine and analar grade maleic
acid were taken in equimolar ratio and dissolved in
de-ionized water. The solution was slightly heated and
kept in undisturbed conditions. Three days later, trans-
parent seed crystals were obtained. The synthesized salt
was used to prepare the growth solution according to the
following reaction.
L-alanine + Maleic acid
 
L-alanine maleate (LALM)
The amount of L-alanine maleate dissolved in 10 ml
of water at 30 ºC was estimated from the saturated solu-
tion. The solubility was estimated for different tempera-
tures and the solubility of L-alanine maleate at 40 ºC is
estimated to be 33 g/100 ml. Crystals were grown from
aqueous solution prepared from the recrystallized salt of
LALM saturated at 40 ºC. In order to reproduce the su-
persaturation conditions, the solution was tested by
checking the dissolution of a probe crystal over a period
of one week. Then the solution was cooled down at a
rate of 0.1 C/day over a period of 25 days. Optical
quality single crystal elongated in c-axis was obtained.
2.2. Characterization
The structure of the crystals was examined by sin-
gle-crystal X-ray diffraction analysis and the lattice pa-
rameter values were determined. Powder X-ray diffrac-
tion analysis was also carried out using a Rich Seifert
diffractometer with CuK ( = 1.5418 Å) radiation to
verify the correctness of lattice parameter values. FTIR
spectrum was recorded by the KBr pellet technique us-
ing a Perkin–Elmer 783 spectrophotometer in order to
confirm the presence of functional groups in the crystal
lattice. Optical transmittance spectrum was recorded at
room temperature using Shimadzu 1601 (UV-VIS) spec-
trophotometer. Optical second-harmonic generation was
measured for the grown crystalline sample using Kurtz
and Perry technique. Thermo gravimetric (TG) and Dif-
ferential thermal analysis (DTA) for L-alanine maleate
dihydrate crystals were carried out by ZETZSCH–Ger-
atebau GmbH Thermal Analyzer. Etching studies were
carried out on the {011} face of L-alanine maleate crys-
tal using different etchants like water, methanol and
ethanol, in order to investigate the growth mechanism
and surface features. Microhardness studies were carried
out on the {011} face of the L-alanine maleate crystals.
As-grown crystal of L-alanine maleate (LALM) is
shown in Figure 1. The crystals possess well defined
morphology with reasonable growth rate along all the
three crystallographic directions. The molecular struc-
ture with the numbering scheme is shown in the Figure
2. The cationic alanine molecule exists with a positively
charged amino group and an uncharged carboxylic acid
group. The maleic acid molecule exists in the monoion-
ized state (i.e. as a semimaleate). The semimaleate ion is
essentially planar and the intramolecular hydrogen bond
between atoms O3 and O5 is found to be asymmetric, as
in the crystal structure of maleic acid [13]. The single
crystal X- ray diffraction studies confirm the orthorhom-
bic structure with space group P212121. The lattice pa-
rameter values of LALM were calculated as given in the
Table 1.
Figure 1. Bulk single crystal of LALM grown in opti-
mizedgrowth condition.
Table 1. The single crystal X-ray data for LALM single crystal.
Molecular formula C3H8O2N+. C4H3O4-
Molecular weight 205.17
System Orthorhompic
Space group P212121
Lattice parameter a = 5.5873 Å,
b = 7.3864 Å,
c = 23.688 Å
Volume (V) 977.6 Å3
Number of atom in unit cell (Z) 4
Density 1.394 Mgm-3
D. Balasubramanian et al. / Natural Science 1 (2009) 216-221
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The FTIR absorption spectrum of LALM is shown in
Figure 3. A broad, strong absorption in the 3300–2300
cm-1 range, including the absorptions at 3205 cm-1 cor-
responds to the stretching bonds of the NH3 + ion of the
amino acid. This region is due to superimposing of O–H
and NH3 + stretching bonds. Absorption in this region
was also characterized by multiple fine structures on the
lower wave number side of the bond and the weak ab-
sorptions due to COO- ions. The prominent absorption
band is relatively strong due to symmetric NH3+ bending
bond at 1569 cm-1. A strong band arising from C–COO -
stretching is observed at 1219 cm-1. Further strong car-
bonyl absorption at 1722 cm-1 confirms the COOH and
COO- groups of the compound. The C=O stretch of car-
boxylic acid was observed to produce its peak in the
same region of asymmetrical NH3+ bending vibration.
The CH3 bending modes were assigned to the peak at
1374 cm-1. O–H bending of the COOH group was ob-
served at 1333 cm-1. The peaks between 918 and 1106
cm-1 were assigned to asymmetrical coupled vibration of
maleic acid and alanine.
This analysis also indicates that the protonation of
carboxyl group in alanine takes place by maleic acid.
The absorptions of LALM have been compared with
those of the L-alanine [14] in Table 2. The shifts in the
positions of the characteristic peaks confirm the forma-
tion of the compound.
Figure 2. The molecular structure of LALM with atom-
numbering scheme and 50%probability displacement el-
Figure 3. FT-IR spectrum of LALM crystal.
Table 2. FT-IR spectral band assignments of LALM.
L-alanine LALM Assignments
NH3+ asymmetric stretching
C-H stretching
COO- stretching
NH3+ symmetric bending
C-H deformation in CH3
O-H plane deformation in COOH
=C-H deformation (over tone)
C-COO- stretching
C-O stretching, NH3 rocking
O–H out-of-plane deformation
CH2 rocking
O-C = O in plane deformation
COO- wagging
The transmission spectrum of LALM crystal was re-
corded in the range 200–1200 nm is shown in Figure 4.
A sample of thickness 2mm was used to record the
transmission spectrum. The crystal possesses 75 % tran-
smittance and the lower cutoff is found to be as low as
320 nm, allowing for frequency conversion down to
UV-region which account for the suitability of this mate-
rial for optoelectronics applications and the second and
third harmonic generation of Nd:YAG fundamental.
The powder second harmonic generation (SHG) test
was carried out for LALM using Kurtz and Perry tech-
nique. Powdered sample of LALM was tightly packed in
the micro capillary tubes of uniform diameter (1.5 mm)
and irradiated by an incident laser radiation 1064 nm of
pulse width 8 ns and pulse energy of 10–800 mJ from a
Q-switched quanta ray of Nd:YAG laser. KDP was used
for calibrating the SHG intensity. The second harmonic
nonlinearity of LALM was confirmed by the emission of
green radiation (532 nm) by the crystal. The powder
SHG efficiency of LALM was found to be 1.2 times that
of the standard KDP. This confirms that the LALM has
higher SHG efficiency than the relative efficiencies of
L-alanine (0.2) and L-alanine acetate (0.3) with respect
to KDP [15].
D. Balasubramanian et al. / Natural Science 1 (2009) 216-221
SciRes Copyright © 2009 Openly accessible at http://www.scirp.org/journal/NS/
Figure 4. Optical transmittance spectrum of LALM crystal.
Figure 5. (a) TG / DTG curves of LALM; (b) DTA/
TG curves of LALM.
Figure 6. Etch patterns obtained on (011) of LALM-
crystal for different etchants. (a) Water; (b) Methanol ;
(c) Ethanol.
Figure 7. Load (P) Vs Hardness number (Hv) of
LALM crystal.
400 600
8001000 1200
th (nm)
10 20 30 40 50
Hardness number HV (kg/mm2)
Load P (g )
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Simultaneous thermo-gravimetric analysis (TG) and
differential thermal analysis (DTA) were carried out for
the as-grown LALM crystals to study the thermal stabil-
ity. The characteristic curves are shown in Figure 5a and
5b. Finely powdered crystal was used for the TG/DTA
analysis in the temperature range of 26 to 500 C with a
heating rate of 5 C/min. The alumina (Al2O3) crucible
was used as a reference for the sample. A weight loss of
21% occur at 162 C in TGA corresponds to the decom-
position range of LALM. An endothermic peak observed
at 160 C in DTA is attributed to the utilization of ther-
mal energy to overcome the valence bonding between
the alaninium cation and the maleate anion, which hap-
pens during the initial stage of decomposition. As the
temperature is increased further, maleic acid decomposes
and becomes anhydride and results in the further release
of CO2 and CO molecules at 205 and 258 C, which is
evident from the DTG. The reactions of simplest amino
acids induced by heating include the condensation reac-
tions of carboxyl and amino groups leading to the for-
mation of peptide bonds. In the dehydration at the initial
stage, H2O molecule is not liberated immediately; in-
stead, it is absorbed by alumina, which acts as a catalyst,
and then is released along with another water molecule
obtained from the decomposition of alanine at 320C.
Because of this, an endothermic effect is noted in DTA.
CO and CH4 molecules are liberated at around 450C.
The NLO efficiency of the grown crystals mainly de-
pends on their optical quality, because the segregated
impurities and dislocations occurring during the growth
results in the distortion of the optical beam to be proc-
essed. In the present investigation, grown crystal of
LALM was subjected to chemical etching to study the
microstructural imperfection or crystal defects in the
grown crystal. A thin plates of 3 mm thickness parallel to
{0 1 1} face were cut from the as grown crystal of
LALM with the help of a wet thread. Polishing of the
surfaces was carried out using soft felt-cloth wetted with
ethanol and tertiary butanol mixture (3 : 1). Polished
plates of 2 mm in thickness, free from visible inclusions
or cracks were selected for etch pit study. Etchants em-
ployed to reveal dislocations are taken in homologous
series of alcohols i.e. water, methanol and ethanol.
Etching of the surfaces was carried out by dipping the
plates in etchants for few seconds to few minutes at
room temperature and then wiping them with dry filter
paper. Etch patterns were observed and photographed
under an optical (Carl-Zesis Jenavert) microscope in the
reflected light. Elongated circular etch pits were ob-
served when LALM single crystal was etched with water
for five seconds as shown in Figure 6a. There was no
change in etch pit dimension and density with varying
etching time (10–20 s). Trapezoid etch pits were ob-
served as shown in Figure 6b when LALM single crys-
tal was etched with methanol for five seconds. Rectan-
gular etch pits were observed when the crystal was
etched with ethanol for fifteen seconds as shown in Fig-
ure 6c. From the results of etching behavior of different
etchants on LALM crystals, it is inferred that all the or-
ganic solvents used in this experiment have successfully
revealed the presence of dislocation in the crystal. The
observed etch pits, due to layer growth, confirmed the
two dimensional (2D) nucleation mechanism with less
dislocations. Fast dissolving etchant like water produces
better contrasting dislocation etch pits of all surfaces and
hence it is intensive to surfaces orientation.
Hardness value on the (011) of LALM crystal was es-
timated for different loads. The relation between hard-
ness number (Hv) vs load (P) for LALM is shown in
Figure 7. At lower loads, hardness is relatively lower
and it increases for higher loads and remains constant up
to 40g. Above 40g, a significant cracking occurred due
to the release of internal stress generated locally by in-
dentation. It has been observed that the hardness values
of LALM are comparable with pure L-alanine.
A new organic optical material for second order NLO
applications, L-alanine maleate (LALM) was synthe-
sized. Single crystals were grown and characterized by
X-ray diffraction (single crystal XRD) to confirm the
formation of the crystalline phase. FT-IR spectroscopic
analysis confirms the presence of all the functional
groups in the crystal lattice. Etching studies were carried
out for LALM crystal using various etchants. Mechani-
cal behavior of grown crystal was studied on (011) using
micro hardness measurement and the hardness values are
found to be comparable with pure L-alanine. TG-DTA
studies reveal that the material starts decomposing at
162.2 oC. The UV-Vis spectrum establishes the good
transmittance window and the lower cutoff are found to
be as low as 320 nm, allowing for frequency conversion
down to UV-region. From the Kurtz-Perry powder tech-
nique, the second harmonic generation efficiency of the
grown LALM crystal was found to be 1.2 times that of
KDP crystal.
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