Journal of Minerals & Materials Characterization & Engineering, Vol. 10, No.10, pp.913-921, 2011 Printed in the USA. All rights reserved
Influence of Picric Acid on the SHG Efficiency of L-Prolinium Tartrate
S. Natarajan
, K. Moovendaran
, J. Kalyana Sundar
, G. Bhagavannarayana
S.A. Martin Britto Dhas
School of Physics, Madurai Kamaraj University, Madurai-625 021, India
Material Characterization Division, National Physical Laboratory, New Delhi 110012, India
Corresponding author:
Single crystals of the NLO material L-Prolinium tartrate (LPT) have been grown in the
presence and absence of picric acid, by slow evaporation technique. Good quality crystals
were harvested within 30 days. The grown crystals were characterized by single crystal X-ray
diffraction. The quality of the crystals grown in the presence and absence of picric acid was
examined by high resolution X-ray diffractometry (HRXRD). Fourier transform infrared
spectroscopy (FTIR) and Fourier transform Raman spectroscopy were used to confirm the
presence of the functional groups of L-Prolinium tartrate and the absence of picric acid in
the crystals. The thermal stability and the trend of decomposition of the grown crystals were
analyzed by thermo gravimetric analysis (TGA). The influence of picric acid on the second
harmonic generation efficiency was studied by Kurtz and Perry method. It was observed that
the SHG efficiency increased by about 2.3 times compared to that of normal LPT crystals.
The results are discussed in detail.
Keywords: organic compounds; crystal growth; X-ray diffraction; thermo-gravimetric
In recent years, several efforts have been made to develop new inorganic, organic and semi-
organic nonlinear optical (NLO) crystals [1-5]. The science and technology of crystal growth
has advanced rapidly for the development of new NLO materials for several applications in
telecommunication, optical data storage, optical information processing, optical switching,
frequency conversion and electro-optical modulation [6]. Second harmonic generation (SHG)
or frequency doubling is the most readily understood and widely used property of the NLO
914 S. Natarajan, et al Vol.10, No.10
phenomenon, which allows for example the production of high intensity green laser pointers
[7]. One way to improve the above properties is growing the crystal in different chemical
environments. The presence of additives or impurities during the crystallization is one of the
most important factors that affect the morphology and the physical properties of the crystal.
Podder [8] reported that the presence of KCl in the growth medium suppresses the metal-ion
impurities and increases the growth rate. The increase in the quality of potassium dihydrogen
phosphate (KDP) crystals in the presence of KCl is due to the complexation of trace metal-
ion impurities in solution by Cl
ions. Hence, it was planned to investigate the influence of
dopant in L-Prolinium tartrate (LPT) crystal, a SHG material. The structure of LPT was
elucidated by Subha Nandhini et al., in 2001 [9] and the nonlinear optical and other
characterizations were carried out in 2006 [10] from our laboratory. The second harmonic
generation (SHG) efficiency of LPT crystal was found to be 1.2 times higher than that of
KDP. In this paper, the crystalline perfection and SHG efficiency of LPT crystals in the
presence and absence of picric acid are reported. The grown crystals were characterized using
single crystal X-ray diffraction. The crystalline perfection was tested by high resolution X-
ray diffractometer. FTIR and FT-Raman studies were carried out to confirm the absence of
picric acid inside the lattice. TGA/DTA studies and SHG studies were also carried out and
the results are discussed.
2.1 Crystal Growth
Single crystals of LPT [(C
] were grown by mixing L-Proline and
L-Tartaric acid in 1:1 stoichiometric ratio in aqueous solution, using slow evaporation
method. Optically clear and well-shaped crystals were obtained in a period of 15-20 days.
These crystals were powdered and recrystallized in the presence of picric acid (about 1
mole %). Good quality crystals (referred hereafter as PLPT,) were harvested in a period of
25-30 days.
2.2 Characterization
The grown crystals were subjected to single crystal X-ray diffraction using Nonius CAD-
4/MACH 3 diffractometer, with MoKα radiation (λ=0.71073 Å). The cell parameters were
obtained from the least-squares refinement of the setting angles of 25 reflections. The FTIR
spectra of the samples were recorded in the frequency region of 400 – 4000 cm
using a
Bruker Alpha-p FTIR Spectrometer, at a resolution of 4 cm
. Raman spectral measurements
were made with a model RFS 100/S Bruker. Nd: YAG laser operated at 1064 nm with a
power output of 200 Mw used as source. The spectrum was recorded over the range 4000–
500 cm
. The sample was finely powdered and pressed into a small depression on a metal
disc and mounted on the sample compartment. This instrument has a resolution of 4 cm
The second harmonic generation (SHG) conversion efficiency was tested using a modified
setup of Kurtz and Perry. The fundamental beam of 1064 nm from a Q switched Nd:YAG
laser was used to test the SHG efficiency of LPT and PLPT crystals. The laser beam used had
Vol.10, No.10 Influence of Picric Acid on the SHG Efficiency 915
the pulse energy 3.5 mJ/pulse and pulse width 8 ns and repetition rate 10 Hz. A photo
multiplier tube (Hamahatsu R2059) was used as detector and 90 degree geometry was
employed. The input laser beam was directed on the microcrystalline powdered sample
packed in a capillary tube of diameter 0.154 mm. The assembly of an oscilloscope and
photomultiplier detector was employed to measure the emitted light by the sample. The SHG
signal generated in the sample was confirmed from the emission of green radiation from the
sample. The optical signal generated from the sample was converted into an electrical signal
and measured on the oscilloscope. Simultaneous thermogravimetric analysis (TGA) and
differential thermal analysis (DTA) were carried out for the crystals, using a SDT Q600 V8.3
build 101 thermal analyzer. A powder sample was used for the analysis in the temperature
range of 26°C to 800°C with a heating rate of 20°C/min. The crucible used was of alumina
), which also served as a reference for the sample.
3.1 Single Crystal X-ray Diffraction
The unit cell dimensions obtained are listed in Table 1. It is seen that the crystal belongs to
the monoclinic system with the space group P2
. The cell dimensions agree well with the
reported values [9]. The density of pure and PLPT crystals was found to be 1.55(1) g/cm
it is in good agreement with the literature values (1.55 g/cm
). No appreciable changes in the
values of the cell parameters indicate that the inclusion of picric acid in the lattice of LPT is
not to an appreciable extent.
Table 1: XRD data for LPT and PLPT
value LPT PLPT
a (Å) 5.007(1) 5.009(2)
b (Å) 17.673(3)
c (Å) 6.523(1) 6.514(2) 6.523(3)
β (°) 100.40(2)
V (Å
) 567.8(2) 567.1(7) 567.6(4)
3.2 Multi-Crystal X-ray Diffraction
The crystalline perfection of the grown single crystals was characterized by HRXRD by
employing a multicrystal X-ray diffractometer developed at NPL [11]. The well-collimated
and monochromated MoKα
beam obtained from the three monochromator Si crystals set in
dispersive (+,-,-) configuration has been used as the exploring X-ray beam. The specimen
crystal is aligned in the (+,-,-,+) configuration. Due to dispersive configuration, though the
lattice constant of the monochromator crystal(s) and the specimen are different, the unwanted
916 S. Natarajan, et al Vol.10, No.10
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
diffraction, with minimum angular interval of 0.4 arc sec. The rocking or diffraction curves
were recorded by changing the glancing angle (angle between the incident X-ray beam and
the surface of the specimen) around the Bragg diffraction peak position θ
(taken as zero for
the sake of convenience) starting from a suitable 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 recorded by the so-called ω scan
wherein the detector was kept at the same angular position 2θ
with wide opening for its slit.
Before recording the diffraction curve to remove the non-crystallized solute atoms which
remained on the surface of the crystal and also to ensure the surface planarity, the specimen
was first lapped and chemically etched in a non preferential etchant of water and acetone
mixture in 1:2 volume ratio.
Fig. 1(a) and (b) shows the high-resolution diffraction curves (DCs) recorded respectively for
LPT and PLPT single crystal specimens grown by slow evaporation solution growth
technique (SEST) using (040) diffracting planes in symmetrical Bragg geometry by
employing the multicrystal X-ray diffractometer with MoKα
radiation. Form the figure 1(a),
it is clear that the DC of the undoped specimen is not having a single peak but contains four
well resolved peaks. These additional peaks depict internal structural low angle (tilt angle >
1 arc min but less than a deg.) boundaries [12] whose tilt angles (the mis-orientation angle
between the two crystalline regions on both sides of the structural grain boundary) are 302,
168 and 326 arc s from their adjoining regions. The full width at half maximum (FWHM) of
these peaks are respectively 105, 150, 60 and 184 arc s.
As seen in curve (b) the doped specimen contains only one clear low angle boundary with tilt
angle 356 arc s. It is worth to observe the prolonged scattered intensity at higher glancing
angles from the peak position of the low angle boundary. This type of scattered intensity at
higher diffraction angles shows that the crystal contains interstitial type of point defects and
their clusters probably due to dopants (picric acid) which may be explained in the following
way. Due to interstitial defects (self interstitials or impurities or dopants at interstitial sites),
the lattice around these defects undergo compressive stress [13] and the lattice parameter d
(interplanar spacing) decreases and leads to give more scattered (also known as diffuse X-ray
scattering) intensity at slightly higher Bragg angles (θ
) as d and sin θ
are inversely
proportional to each other in the Bragg equation (2d sin θ
= nλ; n and λ being the order of
reflection and wavelength, respectively which are fixed). The inset in curve (b) shows the
schematic to illustrate how the lattice around the defect core undergoes compressive stress.
The converse explanation is true in the case of vacancy defects which cause tensile stress in
the lattice around the defect core leading to increase of lattice spacing which in turn results in
more scattered intensity at the lower Bragg angles. It is worth to mention here that the
observed scattering due to point defects is of short range order as the strain due to such
minute defects is limited to the very defect core and the long range order could not be
Vol.10, No.10 Influence of Picric Acid on the SHG Efficiency 917
expected and hence the change in the lattice parameter of the overall crystal may not be
possible. It may be mentioned here that the minute information like the asymmetry in the DC
could be possible as in the present sample only because of the high-resolution of the
multicrystal X-ray diffractometer used in the present investigation.
From the above discussion and comparison of curves (a) and (b), it is evident that the as-
grown crystal of LPT has a tendency to grow with grain boundaries and when grown with
picric acid (as dopant), it seems the crystalline perfection improved to a significant extent.
However, we cannot reduce completely the formation of low angle boundaries.
3.3 TGA/DTA Analysis
Thermo gravimetric analysis has been carried out to investigate the changes in the thermal
behavior of the LPT and PLPT. The crystals of PLPT also showed the same trend of
decomposition as that of LPT. The interpretation of the thermal analysis has already been
reported earlier [10].
3.4 Fourier Transform Infra-red and Fourier Transform Raman Spectroscopic Analysis
The FTIR and FT-Raman spectroscopic studies were carried out to analyze qualitatively the
presence of functional groups in the LPT and PLPT crystals. The spectra of LPT and PLPT
are shown in Fig. 3 and Fig. 4 The interpretation of FTIR is described in our earlier report
[10]. Fig. 3 and Fig. 4 show that the FTIR spectra of LPT and PLPT are identical as is the
case with their FT-Raman spectra.
Fig. 1. High-resolution X-
ray diffraction curves recorded for (040) diffracting
planes in symmetrical Bragg geometry using MoKα
radiation for LPT single
crystals: (a) LPT and (b) PLPT
-1000 -50005001000
Diffracted X-ray intensity [c/s]
Glancing angle [arc s]
302" 326"
05001000 1500
Diffracted X-ray intensity [c/s]
Glancing angle [arc s]
LPT (1P)
918 S. Natarajan, et al Vol.10, No.10
Fig 2. TGA/DTA of PLPT crystal
Fig. 3 FTIR spectra of LPT and PLPT
Vol.10, No.10 Influence of Picric Acid on the SHG Efficiency 919
Fig. 4 FT-Raman spectra of LPT and PLPT
The Raman bands of the compound around 3010 cm
are attributed to O–H stretching
vibration of the carboxyl group. The very strong peak at 2935 cm
and the shoulder peak at
2883 cm
are due to CH
stretching. The medium peak at 1712 cm
is assigned to the
stretching vibration of C=O group. The weak peak at 1560 cm
is attributed to the in-plane
bending of the amino group. The medium intense peaks at 1321, 1227 (wagging), 1188
(twisting), 828 cm
and the strong peak 784 cm
(rocking) are assigned for the different
modes of vibrations of CH
. The weak peaks at 993 cm
and 902 cm
are assigned to proline
ring stretching. The strong peak at 884 cm
is assigned to the combination of ring stretching
and CH
rocking. The peaks at 553 and 472 cm
are due to the skeletal deformation and
COO rocking, respectively. No peaks are observed due to the functional groups such as nitro
group or aromatic vibrations; which reveal the absence of picric acid molecules in the
3.5 SHG Measurement
The preliminary testing of materials for second harmonic generation was carried out by using
a modified setup of Kurtz and Perry [14]. The SHG conversion efficiency of PLPT was found
to have enhanced compared to that of LPT. The measured output for pure LPT and PLPT
were 15 mv and 34 mv, respectively. This shows that the SHG conversion efficiency of PLPT
is 2.3 times greater than that of LPT. The enhancement in SHG efficiency of PLPT may be
due to the crystalline perfection.
920 S. Natarajan, et al Vol.10, No.10
Single crystals of L-Prolinium tartrate, a new organic nonlinear optical crystal have been
grown in the presence and absence of picric acid by slow evaporation technique. The grown
crystals were characterized by single crystal X-ray diffraction. The HRXRD reveals that the
as-grown crystal of LPT has a tendency to grow with grain boundaries and when dopants
(picric acid) are added, it seems that the crystalline perfection improved to a significant level.
The crystals of PLPT also showed the same trend of thermal decomposition as that of LPT.
Fourier transform infrared and FT-Raman spectroscopic studies showed the absence of picric
acid in the crystals of L-Prolinium tartrate. Also, the vibrational modes of the functional
groups of both the crystals have been identified in the Raman spectra. The Kurtz and Perry
method revealed that the SHG efficiency increased significantly. It may be due to the
crystalline perfection.
One of the authors (SN) thanks the CSIR for the funding provided under the Emeritus
Scientist Scheme.
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