Journal of Minerals & Materials Characterization & Engineering, Vol. 8, No.4, pp 393-403, 2009
Printed in the USA. All rights reserved
Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal
T. Uma Devi*
, N. Lawrence
R. Ramesh Babu
K. Ramamurthi
G. Bhagavannarayana
Department of Physics, Cauvery College for Women, Tiruchirappalli –620018, India
Department of Physics, St. Joseph’s College (Autonomous), Tiruchirappalli –620002, India
Crystal Growth and Thin Film Laboratory, School of Physics, Bharathidasan University,Tiruchirappalli
–620024 India
Materials Characterization Division, National Physical Laboratory, Dr. K. S. Krishnan
Marg, New Delhi-110 012, India
*Corresponding Author, contact:
Phone: 91-431-2751232, Fax: 91-431-2407045
L-valinium picrate, an organic material, was synthesized and grown by solution growth method.
Cell parameters of the grown crystals were obtained from the single crystal X-ray diffraction
analysis and the presence of functional groups was identified by FTIR study. The quality of the
crystal was also examined by high-resolution X-ray diffraction study. Its optical properties were
examined by UV-Vis-NIR analysis, which shows that the crystal is transparent between the
wavelengths 400 and 1000 nm. Thermal analysis carried out for the L-valinium picrate exhibits
a single sharp melting point at 269 °C. The Vicker’s microhardness values were measured for
the grown crystal. Relative powder second harmonic generation efficiency tested by high
intensity Nd: YAG laser as a source is about 60 times greater than that of potassium dihydrogen
Key Words: X-ray diffraction, Growth from solutions, Organic compounds, Non-linear optical
Materials with excellent nonlinearities have been studied extensively for their possible
applications in various fields like telecommunication, optical computing, optical data storage and
optical information processing [1-3]. Acentric molecules consisting of highly delocalized π
394 T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, G. Bhagavannarayana Vol.8, No.5
electron systems interacting with suitably substituted electron donor and acceptor groups exhibit
a high value of second-order polarizability (β). Amino acids, except glycine, are characterized by
chiral carbons, a proton donating carboxyl (–COOH) group and the proton-accepting amino (–
) group [4-6]. L-valine is an important aminoacid which has been exploited for the formation
of salts with different organic acids [7-10]. Di-L-valine selenate monohydrate and tri-L-valine
selenate are reported to be novel materials for second harmonic generation [11]. Investigations of
amino acid picrates have attracted the attention of researchers in the recent past [12-14]. Some of
such new compounds reported recently exhibit relative second harmonic efficiencies of 50-70
times that of potassium dihydrogen phosphate [10, 14]. L-valinium picrate (C
), is a promising NLO material in which L-valine acts as donor and the picric acid as
electron acceptor which provide the ground state charge asymmetry of the molecule required for
second-order nonlinearity. In the present investigation a systematic study is carried out on the
growth and some of the characterization of L-valinium picrate (LVP). The reaction mechanism
of LVP is shown in Fig. 1.
Fig. 1 The Reaction mechanism of LVP
2.1. Material Synthesis and Purification
LVP was synthesized by the reaction between the equimolar picric acid (Loba Chemie) and the
aminoacid, L-valine (Loba Chemie). The reactants were thoroughly dissolved in double distilled
water and stirred well using a temperature controlled magnetic stirrer at 45°C to yield a
homogeneous mixture of solution. Then the solution was allowed to evaporate at room
Vol.8, No.5 Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal 395
temperature, which yielded yellow crystalline salt of LVP. The process of recrystallization was
carried out to purify the synthesized salt.
2.2. Choice of Solvent
Growth of organic crystals having well-developed faces and good optical quality mainly depends
on the selection of suitable solvents. Solvents offering moderate solubility-temperature gradient
for a material and yielding prismatic growth habit will be considered as suitable solvents for
growing crystal of that material. The water solvent yielded small transparent needle shaped
crystals whereas the mixed solvent of water and acetone produced transparent well shaped
crystals. Hence attempts were made to grow LVP crystals from the mixed solvents. From several
trials it was observed that the mixture of equal volume of acetone and water yielded relatively
transparent and prismatic crystals of LVP and hence it was selected as the suitable mixed solvent
to grow LVP crystal.
2.3. Solubility
The solubility of LVP in the 100 ml of mixed solvent of acetone and water (1:1 volume) was
determined. The amount of LVP required to prepare saturated solution at 30 °C was estimated
and this process was repeated for different temperatures in the range of 30-50 °C. From the
solubility curve presented in Fig.2, it is found that the solubility of LVP increases with increase
of temperature.
Fig. 2 The solubility curve of LVP
396 T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, G. Bhagavannarayana Vol.8, No.5
2.4. Crystal Growth
Saturated solution of 100 ml was prepared at 35 °C using recrystallized LVP salt and was filtered
with microfilters and taken in a glass beaker of 100 ml capacity. Then the beaker was sealed and
placed at room temperature. LVP crystal of dimension about 9x7x3 mm
was harvested in a
growth period of twenty days by slow evaporation of the solvent at room temperature and is
given in Fig.3.
Fig. 3 As grown LVP crystal
3.1. X-ray Diffraction Studies
3.1.1. Single crystal X-ray diffraction
Single crystal X-ray diffraction study was carried out on the as grown LVP crystal. The
calculated unit cell parameters, presented in Table 1 are in good agreement with the
corresponding reported value of Anitha et al. [15].
3.1.2. High-resolution X-ray diffractometry study on LVP
Before recording the diffraction curve, to remove the non-crystallized solute atoms 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 etchent of water and acetone mixture in 1:2 volume
ratio. Fig. 4 shows the high-resolution rocking or diffraction curve (DC) recorded for a typical
LVP specimen grown by slow evaporation solution technique using (002) diffracting planes in
symmetrical Bragg geometry by employing the multicrystal X-ray diffractometer described
Vol.8, No.5 Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal 397
above with MoKα
radiation. As seen in the figure the DC contains a single peak without any
satellite peaks which may otherwise be observed due to internal structural grain boundaries [17].
The full width at half maximum (FWHM) of the diffraction curves is 30 arc s, which is close to
that expected from the plane wave dynamical theory of X-ray diffraction [18] for an ideally
perfect crystal. The relatively less angular spread i.e. around 200 arc s, lesser FWHM value and
the single peak of the DC indicate that the crystalline quality is reasonably good without having
any internal structural grain boundaries.
Table 1. Comparison of single crystal X-ray data of LVP crystal
Present work
Anitha et al. [15]
a (Å) 9.9770 9.9714
b (Å) 6.2394 6.2930
c (Å) 12.6420 12.6480
System monoclinic monoclinic
( ˚) 110.16 110.50
Space group - P2
Fig. 4 Diffraction curve recorded for LVP single crystal using (002) diffracting planes by
employing the multicrystal X-ray diffractometer with MoKα
3.2. FT-IR Spectral Analysis
The FTIR spectrum of the LVP crystal, recorded in the KBr phase in the frequency region
– 4000 cm
using Perkin–Elmer spectrometer, is shown in Fig. 5. The phenolic O
398 T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, G. Bhagavannarayana Vol.8, No.5
vibration produces peak at 1136 cm
reveals that picric acid necessarily protanates the carboxyl
group [19]. The observed vibrational frequencies and the tentative frequency assignments of
LVP are given in Table 2.
Fig. 5 FT-IR spectrum of LVP
3.3. Optical Transmittance of LVP
The optical transmittance spectrum of LVP recorded using Shimadzu model 1601 in the range of
300–1000 nm is shown in Fig. 6a. Optically clear single crystal of thickness about 2 mm was
used for this study. There is no appreciable absorption of light in the entire visible range as is the
case of other amino acids [14]. The transmittance between 500 and 1000 nm is approximately
65 %. The short wavelength cutoff occurs at 480 nm. The peaks in the range between 300 and
350 nm, are due to n to π* transitions of carbonyl group. The optical transmission range shows
that LVP is a potential candidate for SHG. The value of band gap energy was estimated from the
graph between hv and (
by extrapolating the linear portion of the curve to zero absorption
(Fig. 6b). The bandgap energy calculated is about 2.24 eV for the LVP single crystal.
Vol.8, No.5 Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal 399
Table 2 Comparison of vibrational frequencies of LVP
[18] Assignments [19]
1636 1630 NH
asymmetric deformation
1509 1535 NH
symmetric deformation
1394 1365 CH
symmetric deformation
1330 1349 (NO
) symmetric stretching
1271 1267 phenolic C–O stretching .
1064 1060 C–C stretching
1030 1017 –(CH
sym stretching
944 915 [NH
rocking ;
C–C stretching
893 872 C-C-N symmetric stretching
821 820 NO
767 747 NO
713 706 CH
662 676 C–C out-of-plane ring deformation
542 521 NO
Fig. 6a Transmittance of LVP
400 T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, G. Bhagavannarayana Vol.8, No.5
Fig. 6b Plot of (
vs hv
3.4. Powder SHG Measurement
The study of nonlinear optical conversion efficiency was carried out using the experimental
setup of Kurtz and Perry [20]. A Q-switched Nd: YAG laser beam of wavelength 1064 nm, with
an input power of 5.6 mJ, and pulse of width 8 ns with a repetition rate of 10 Hz were used. The
grown single crystal of LVP was powdered with a uniform particle size and then packed in a
microcapillary of uniform bore and exposed to laser radiations. The generation of the second
harmonics was confirmed by the emission of green light. A sample of potassium dihydrogen
phosphate (KDP), also powdered to the same particle size as the experimental sample, was used
as a reference material in the present measurement. The relative SHG conversion efficiency of
LVP is found to be about 60 times that of KDP. This may be attributed to the molecular structure
of LVP in which the carboxyl group of the valinium residue is engaged in a strong hydrogen
bond with the picrate anion [16]. The amino group of the L-valinium cation and the picrate anion
are held together by an intermolecular hydrogen bond. Table 3 shows comparison of SHG signal
energy output of LVP Table 4 compares the relative SHG efficiency of some of the aminoacid
Table 3.Comparison of SHG signal energy output
Input power
5.6 22 1.32
Vol.8, No.5 Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal 401
Table 4 Comparison of SHG efficiency of some of the aminoacid picrates
S.NO NLO crystals Reference SHG efficiency
1 L- prolinium picrate [14] 52
2 L-
asparaginium picrate [12] 66.5
3 L-valinium picrate [present work] 60
3.5. Thermal Studies
Thermogravimetric analysis (TGA) of LVP was carried out using a simultaneous thermal
analyzer PL-STA 1500. A ceramic crucible was employed for heating the sample and the
analyses were carried out in an atmosphere of nitrogen at a heating rate of 20
C in the
temperature range 30–800
C. The initial mass of the material subjected to analyses was 2.032 mg
and the final mass left out after the experiment was only 5.3 % of the initial mass at a
temperature of about 800
C indicating the bulk decomposition occurring in the sample. From the
curves (Fig. 7) it is inferred that the melting of the material takes place in the vicinity of 269
The sharpness of this peak shows the good degree of crystallinity of the sample [21]. Further, it
indicates no phase transition before melting. There is a gradual and significant weight-loss as the
temperature is increased above the melting point. It is seen that at different stages various
gaseous fractions like CO, CO
, NH
etc. are liberated, and a total decomposition of the
compound takes place at a temperature of about 800
Fig. 7 TG/DTA of LVP
402 T. Uma Devi, N. Lawrence, R. Ramesh Babu, K. Ramamurthi, G. Bhagavannarayana Vol.8, No.5
3.6. Vickers Microhardness Study
The mechanical strength of the LVP crystal was measured using a Leitz hardness tester fitted
with a diamond indenter attached to Leitz incident light microscope. Indentations were made for
various loads from 5 g to 50 g. Several trials of indentation were carried out on the prominent
face and the average diagonal length was calculated for an indentation time of 5 seconds. The
Vickers hardness number (H
) of the crystal was calculated using the relation H
= 1.8544 P/d
where P is the applied load in Kg and d is the average diagonal length of impression in mm.
Fig.8 shows the variation of Vickers hardness number with load for LVP. Cracks were observed
for loads more than 50 g.
Fig.8 Vickers Microhardness values of LVP
The potential organic NLO crystal of L-valinium picrate (LVP) was grown by slow evaporation
at room temperature. Single crystal X-ray diffraction study revealed that the LVP crystal grown
at room temperature belongs to monoclinic system. The quality of the crystal examined by high-
resolution X-ray diffraction study indicates that the crystalline quality is reasonably good without
having any internal structural grain boundaries. Vibrational frequencies were assigned from FT-
IR spectral analysis, which confirm the presence of functional groups of the LVP. The UV-
Visible-NIR spectrum of LVP showed that the crystal is transparent in the range of 500 – 1000
nm. The band gap energy of LVP estimated from the optical transmittance spectrum is about
2.24 eV. TGA shows the good thermal stability of the material. The hardness measurement
shows that the grown LVP crystal is mechanically stable only up to 50g. Its relative SHG
Vol.8, No.5 Studies on L-valinium Picrate Single Crystal: A Promising NLO Crystal 403
efficiency tested by high intensity Nd: YAG laser as a source is about 60 times greater than that
of KDP. Owing to these properties LVP could be a promising material for NLO applications.
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