Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No. 8, pp 635-646, 2009 Printed in the USA. All rights reserved
Growth and Characterisation of Nonlinear Optical Single Crystals of
L-Alaninium Oxalate
K. J. Arun*and S. Jayalekshmi
Division for Research in Adv ance d Materials, Department of Phys ic s ,
Cochin University of Science and Technology, Kochi-682022, Kerala, India.
*Corresponding author:,
A potential organic nonlinear optical (NLO) single crystal based on amino acid, L alaninium
oxalate (LAO) is grown using the simple chiral amino acid L alanine and oxalic acid by slow
evaporation method. Grown crystals have an optimum dimension of 40 x15 x 8 mm3 with good
optical quality and are characterized by powder XRD, CHN analysis, FTIR spectroscopy, TGA-
DTA, DSC, UV/ VIS/ NIR absorption spectroscopy, photoconductivity and photoluminescence
studies. The third order nonlinear optical properties of the grown crystals are investigated
employing open aperture Z scan technique.
Key words: Nonlinear optical crystal, photoconductivity, photoluminescence, Z scan.
Over the past two decades much attention has been paid to the search of novel good quality NLO
materials that can generate large second order optical non linearities, significant for potential
applications including telecommunication, optical computing, and optical data storage and
processing[1 – 6]. In many of the organic NLO materials there is a solid framework of
conjugated π electrons along with weak Van der Waals and hydrogen bonds which are
responsible for most of their NLO properties. Organic NLO materials are superior to their
636 K. J. Arun and S. Jayalekshmi Vol.8, No.8
inorganic counterparts due to high conversion efficiency for second harmonic generation and
good transparency in the visible region, high resistance to optical damage and so on.
The α amino acid L alanine can be considered as the fundamental building block of more
complex amino acids which shows strong non linear behaviour and anomalous phonon coupling
and is a system exhibiting vibrational solitons [7]. An attempt is made to synthesize and grow an
organic NLO material L alaninium oxalate (LAO) using L alanine and oxalic acid. The top
seeded, aqueous solution grown crystals of LAO by slow evaporation are characterized by
powder XRD, TGA- DTA, DSC, FTIR,CHN analysis, UV/Vis/NIR absorption spectroscopy,
and photoconductivity and photoluminescence studies. Nonlinear absorption in the sample is
established by open aperture z scan technique.
L alaninium oxalate is synthesized from equimolar solution of L alanine and oxalic acid by
evaporation preventing decomposition. Synthesized samples are crystallized repeatedly to get a
pure, colorless, crystalline powder. Good quality seed crystals are prepared using this powder.
One or two seed crystals are then placed in its saturated solution kept in a bath at 30°C. Seed
crystals are grown to big crystals by slow evaporation to a size of about 40x 15 x 8 mm3 with
good transparency in three weeks time (Fig. 1). To the best of our knowledge, growth of big
LAO crystals of this size is not reported elsewhere [8].
Fig. 1. L-alaninium oxalate single crystal
Vol.8, No.8 Growth and Characterisation of Nonlinear Optical Single Crystals 637
Powdered XRD spectrum of the crystals is recorded using Rigaku X ray diffractometer with
CuKα radiation to identify the compound and structure. FTIR spectrum of the crystals is taken
with AVTAR 370 spectrometer having DTGS KBr detector with a resolution of 4 cm-1. TGA-
DTA studies are carried out with Perkin Elmer Diamond TGA / DTA equipment in nitrogen
atmosphere at a heating rate of 10°C / min for a range 28 - 600°C to assess the thermal stability.
The DSC spectrum of the sample is also recorded with a Mettler Toledo DSC822 instruments
with a heating rate of 10°C/min. CHN analysis is done with VarioEL III CHNS serial
number11035060 to confirm the molecular formula of the crystal. UV/Vis/NIR absorption
spectrum is recorded using a JASCO V 570 spectrophotometer in the range from 190nm to
2500nm. Negative photoconductivity is established using a KEITHLEY 236 source measuring
unit. Photo-luminescence spectrum is recorded with a Jobin Yvon Spectrofluorometer (Model
FL3-22). A 450 W xenon lamp is used as the source and PMT (Model R928P) as the detector.
Double gratings are used for the excitation and emission spectrometers. Z scan technique is used
to reveal the nonlinear optical absorption behaviour of the crystal.
4.1 X-ray Diffraction Analysis
From the XRD data, it is found that L-alaninium oxalate belongs to the orthorhombic system
with a = 5.6302A°, b= 7.235 A° and c= 19.5973 A°, the space group being P212121 and has four
molecules in the unit cell with a volume of 803.146 A°. The close agreement with the observed,
calculated and reported d values [9] confirm the identity of the grown crystal. The recorded
powder XRD pattern is given in Figure 2. In the layered crystal structure of LAO (Fig. 3), the
amino group of the L-alaninium cation forms three N– H…O hydrogen bonds with the oxygen of
semi oxalate group and a symmetry-related alanine cation. There also exists two O– H…O
hydrogen bonds. The alaninium and semi-oxalate ions form alternate columns leading to a
layered arrangement parallel to the a.c. plane and each such layer is interconnected to the other
through N–H…O hydrogen bonds. Two short C…O contacts involving the carboxyl oxygen of
the alaninium ion [C1….O2 (1/2 + x, 3/2 – y, – z) = 2.931(3) Å and C2…O2 (– 1/2 + x, 3/2 – y, –
z) = 2.977(3) Å] are also observed in these layers. The slight difference observed in the bond
lengths of C5– O5 [= 1.219(2) Å] and C5–O6 [= 1.235(2) Å] in the carboxylate group of the
semi-oxalate ion may be attributed to the difference in strengths of the N–H…O hydrogen bonds
in which both O5 and O6 are involved.
638 K. J. Arun and S. Jayalekshmi Vol.8, No.8
Fig. 2. Powder XRD spectrum of L-alaninium oxalate crystal
Fig. 3. Layer structure of L-alaninium oxalate
Vol.8, No.8 Growth and Characterisation of Nonlinear Optical Single Crystals 639
4.2 Measurement of Density
The measurement of density is one of the important methods to study the purity of crystals. The
most sensitive method to find the density of the crystal, namely, the swim or sink method [10] is
used in the present study. In this analysis, theoretical density value is found using the formula,
density = (MZ)/(NV); where M is the molecular weight, Z is the number of molecules per unit
cell, N is the Avogadro number, and V is the volume of the unit cell. The density values are
experimental: 1.35 g/ cm3; and theoretical: 1.37 g/ cm3.
4.3 CHN Analysis
The chemical composition of the grown crystal determined using CHN analysis reveals that it
contains 33.18% carbon (33.51%), 4.96% hydrogen (5.02%), 7.79% nitrogen (7.82%) where
the figures in braket represent the theoretical composition. Thus the molecular formula of the
compound L alaninium oxalate is established as C5H9NO6 and it does not contain any water of
crystallisation in its structure. Figure 4 shows the molecular structure of LAO molecule.
Fig. 4. Molecular structure of L-alaninium oxalate.
4.4 Thermal Analysis
The good thermal stability of LAO crystals up to 196°C establishes that it has prospects in laser
applications where crystals should withstand high temperature. In the DTA curve (Fig. 5) there is
a large endothermic peak at196°C and a very small endothermic peak at around 98°C. The
major peak is the melting point of the crystal as the major weight loss occurs after that point and
the minor peak is assumed to be a weak solid to solid phase transition as there is no weight loss
at this temperature and there is no water of crystallization in the crystal.
640 K. J. Arun and S. Jayalekshmi Vol.8, No.8
Fig. 5 TGA/DTA thermogram of L-alaninium oxalate
The differential scanning calorimetry (DSC) spectrum (Fig. 6) shows a minor endothermic peak
at 980C and a major peak at 1970C, supporting the observation found in TGA/DTA
measurement. In organic crystals phase transitions between polymorphs are very common and
the mechanisms remain unknown despite of extensive investigations. No such detailed phase
transition studies are reported in the literature for LAO, even though results are available for
other organic crystals [11, 12]. Hashizume et al [13] have observed the phase transition of L-
ethyl-3-urea by means of detailed temperature resolved single crystal diffraction method. Crystal
structures before and after the phase transition are isostructural where one-dimensional hydrogen
bonding structure is formed and stacked to form a molecular layer. Their work shows a solid to
solid phase transition at 90°C. Since these crystals are grown by slow evaporation method, they
have taken care in omitting the occluded water, if any, during the growth by annealing the
samples fairly at higher temperatures. In the present study LAO shows a phase transition (solid
to solid) at 371 K (98°C), when the temperature is slowly increased from room temperature,
which is of the same order as for L-ethyl-3-urea. The comparison with L-ethyl-3-urea is made
here because both of them have layered structures. The layer structure of the present LAO,
Vol.8, No.8 Growth and Characterisation of Nonlinear Optical Single Crystals 641
viewed from the b axis is shown in Figure 3. Even though the geometry of the layers is retained,
the relative positions of the layers in LAO with their neighbors change gradually with
temperature. The change is accelerated at the temperature representing the start of the endotherm
in the TGA and DTA curves. The structural variation thus creates a void space in between the
layers and as this grows, naturally the crystal will be unstable and so the carboxyl group of the
molecules turns into a disordered structure with abrupt conformational changes to fill up the void
space. This transition, being isostructural, may be visualized as the transition of two elementary
processes—supramolecular and molecular.
Fig. 6. Differential scanning calorimetry curve of L-alaninium oxalate
4.5 Fourier Transform Infra red Spectroscopic Studies
The characteristic vibrations of the carboxylate ions and the zwitter ionic group NH3+ of LAO are
depicted in the spectrum [Fig. 7]. Observed frequencies are compared with those of similar
functional groups including carboxylate ions. During the formation of the salt; NH2 group in the
free acid is converted into NH3+ ions. In the spectrum, the peak at 3243 cm-1 corresponds to OH
stretching of COOH group of amino acid; the one at 2900 cm-1 to NH3+ symmetric stretching in
plane and the peak at 2509 cm-1 to NH3+ symmetric stretch out of plane vibrations. The C-C
overtone vibration is observed at 1915 cm-1, and C=C stretching at 1716 cm-1. The vibration peak
at1585 cm-1 is due to NH3+ asymmetric bending and that at 707 cm-1 corresponds to C=O
bending [14, 15]. The FTIR vibrational spectrum establishes the presence of NH3+ group in the
crystal confirming the protonation of amino acid group leading to the formation of LAO
molecule [16].
642 K. J. Arun and S. Jayalekshmi Vol.8, No.8
Fig. 7. FTIR spectrum of L-alaninium oxalate
4.6 UV/ Vis/ NIR Absorption Spectroscopy
UV/Vis/NIR absorption spectrum [Fig. 8] shows that LAO crystal has a wide transparency
window without any absorption in the UV, visible and near IR regions, ranging from 318 nm to
1524 nm suggesting its suitability for second and third harmonic generations of the 1064 nm
radiation and other applications in the blue – violet region.
Fig. 8. UV/Vis/NIR absorption spectrum of L-alaninium oxalate
Vol.8, No.8 Growth and Characterisation of Nonlinear Optical Single Crystals 643
4.7 Photoconductivit y S t ud i es
Field dependence of dark and photo currents of LAO crystals is shown in Figure 9. The
photocurrent is found to be less than the dark current at every applied electric field. This
phenomenon is known as negative photoconductivity which in this case may be due to the
reduction in the number of charge carriers or their lifetime in the presence of radiation. Decrease
in lifetime with illumination, could be due to the trapping process and increase in carrier velocity
according to the relation:
τ = (vsN)-1
Where v is the thermal velocity of the carriers, s is the capture cross section of the recombination
centers and N is the carrier concentration. As intense light falls on the sample, the lifetime
decreases [17]. In Stockmann model, a two level scheme is proposed to explain negative
photoconductivity [18]. The upper energy level is situated between the Fermi level and the
conduction band, whereas the other one is located in the neighborhood of the valence band. The
lower level has high capture cross section for electrons from the conduction band and holes from
the valence band. As a result, as soon as the sample is kept under exposure to light, the
recombination of electrons and holes takes place, resulting in decrease in the number of mobile
charge carriers, giving rise to negative photoconductivity.
Fig. 9. Photoconductivity curve of L-alaninium oxalate crystal
644 K. J. Arun and S. Jayalekshmi Vol.8, No.8
4.8 Photoluminescence Studies
Photoluminescence (PL) spectrum of LAO crystal recorded is shown in Figure 10. Excitation
wavelength used is 300 nm. Spectrum shows a broad peak centered at 440 nm with intensity
comparable to that of conducting polymers and polymer composites. Intensity is slowly reduced
in the higher wavelength region. The reason for enhanced PL emission in the case of LAO would
be the presence of electron donating group NH and electron-withdrawing group COOH that can
enhance the mobility of π electrons. The maximum intensity peak at 440nm is due to the
protonation of amino group to the carboxyl group. The lowering of photoluminescence intensity
at higher wavelength region may be attributed to a relatively low barrier for rotation of the
carboxyl group around the central C-C bond [19].
Fig. 10. Photoluminescence spectrum of L-alaninium oxalate
4.9 Open Aperture Z Scan Measurements
The result of the open aperture Z scan measurement of the crystal is shown in Figure 11. The
open aperture (OA) curve demonstrates a nonlinear absorption and the characteristic pattern of
the curve shows that the nonlinear absorption is reverse saturation absorption (RSA). For 532nm
resonant absorption, both excited state absorption and two-photon absorption (TPA) can be
responsible for the observed NLO effects. The RSA coefficient β (m/W) can be obtained from a
best fitting performed on the experimental data of the OA measurement with the equations (1)
and (2)[20] where α and β are the linear and effective third order NLO absorption coefficients,
respectively, τ is the time, I(z) is the irradiance and L is the optical path length.
Vol.8, No.8 Growth and Characterisation of Nonlinear Optical Single Crystals 645
T(Z)= (1)
Q(Z)= βI(Z) (2)
The value of the non linear absorption coefficient β is calculated to be equal to be 2.9×10-10
m/W. The reason for the large value of RSA coefficient may be due to the large number of
delocalized π electrons resulting from the protonation of amino group which is clear from the
structure of the crystal (Fig.3). This delocalization gradually enhances the hyperpolarizability
and the nonlinear susceptibility, and leads to large third-order NLO properties [21].
Fig. 11. Open aperture Z scan spectrum of L-alaninium oxalate crystal
The bulk size single crystals of L-alanininium oxalate are successfully grown by the slow
evaporation solution growth method at room temperature. Exposure of the crystal surfaces to
humid and dry atmospheres indicates that LAO is stable and non hygroscopic. The grown
crystals have been subjected to various characterization studies. Crystal structure has been
646 K. J. Arun and S. Jayalekshmi Vol.8, No.8
confirmed by powder XRD .CHN analysis reveals that the crystal doesn’t contain any water of
crystallization in its structure and suggests the molecular formula to be C5H9NO6. TGA/DTA
and DSC studies establishes the good thermal stability of the crystal upto its melting
temperature (1970C), suggesting it to be a potential material for laser applications where crystal
should withstand high temperatures. Since there is no decomposition observed up to 200°C,
crystallization can be done by melt method too.
FTIR studies confirm the various functional groups and their vibrational interactions. The
UV/Vis/NIR absorption studies highlight the excellent transparency of the material in the range
318 nm to 1524nm. It is an important requirement for materials for applications in second and
third harmonic generations. Open aperture Z scan studies establish that the material can be
effectively used for optical limiting applications.
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