Studies on the growth and physical properties of nonlinear optical crystal: 2-Amino-5-nitropyridinium-toluenesulfonate

Article abstract of DOI:10.1016/j.materresbull.2011.02.010

Single crystals of 2-amino-5-nitropyridinium-toluenesulfonate (2A5NPT) were grown by the slow cooling method. The unit cell dimensions were determined from single crystal X-ray diffraction studies. The thermal parameters - thermal diffusivity (α), thermal effusivity (e), thermal conductivity (K) and heat capacity (C_p) of 2A5NPT were measured by an improved photopyroelectric technique at room temperature. Single and multiple shot experiments performed on the grown crystals for the second harmonic of pulsed Nd:YAG laser (532 nm) show that it exhibits a high laser damage threshold which is a favorable property for nonlinear optical applications. Dielectric constant and dielectric loss of the grown crystal were evaluated for the frequency range 1 kHz-1 MHz in the temperature region 40-130 °C. Hardness values were measured using Vickers hardness measurement.

Full text of DOI:10.1016/j.materresbull.2011.02.010

Materials Research Bulletin 46 (2011) 631–634
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Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Studies on the growth and physical properties of nonlinear optical crystal:
2-Amino-5-nitropyridinium-toluenesulfonate
a
b
G. Anandha Babu a, , P. Ramasamy , J. Philip
*
a Centre for Crystal Growth, SSN College of Engineering, SSN Nagar 603 110, Tamilnadu, India
b Department of Instrumentation, Cochin University of Science and Technology, Cochin 682 022, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Single crystals of 2-amino-5-nitropyridinium-toluenesulfonate (2A5NPT) were grown by the slow
cooling method. The unit cell dimensions were determined from single crystal X-ray diffraction studies.
Received 14 December 2010
Received in revised form 28 January 2011
Accepted 11 February 2011
Available online 18 February 2011
The thermal parameters – thermal diffusivity (a), thermal effusivity (e), thermal conductivity (K) and
heat capacity (Cp) of 2A5NPT were measured by an improved photopyroelectric technique at room
temperature. Single and multiple shot experiments performed on the grown crystals for the second
harmonic of pulsed Nd:YAG laser (532 nm) show that it exhibits a high laser damage threshold which is a
favorable property for nonlinear optical applications. Dielectric constant and dielectric loss of the grown
crystal were evaluated for the frequency range 1 kHz–1 MHz in the temperature region 40–130 8C.
Hardness values were measured using Vickers hardness measurement.
PACS:
61.66.Hq
42.70.Nq
81.10.Dn
61.10.Ài
ß 2011 Elsevier Ltd. All rights reserved.
Keywords:
A. optical materials
A. organic compounds
B. crystal growth
D. optical properties
1. Introduction
potential nonlinear optical crystal which is reported to exhibit
second harmonic efficiency 90 times that of the widely used
Organic crystal exhibiting nonlinear optical effects have been
explored with a view to developing optical devices, for example
optical modulators and frequency-doubling devices [1]. But these
are generally difficult to grow as large single crystals owing to the
weak van der Waals and/or hydrogen bonds [2–6]. Also, the
softness of the crystal makes them less advantageous from a device
point of view compared to inorganic crystals. Considerable effort is
being made to find new materials that have the optimal
characteristics needed for use as a nonlinear optical (NLO) element
[7]. Masse et al. have published a series of organic–inorganic
crystals of 2-amino-5-nitropyridine (2A5NP) and inorganic acids
such as arsenic acid, which were designed for NLO materials.
2A5NP allows for the growth of numerous salts, like 2-amino-5-
nitropyridinium dihydrogen phosphate, 2-amino-5-nitropyridi-
nium dihydrogen arsenate, 2-amino-5-nitropyridinium acetopho-
sphate, 2-amino-5-nitropyridinium chloride (2A5NPCl), 2-amino-
5-nitropyridinium bromide (2A5NPBr) and 2-amino-5-nitropyr-
inorganic crystal, potassium dihydrogen phosphate at the funda-
mental wavelength of 1064 nm [12]. This organic material
crystallizes in the monoclinic crystal system with noncentro
symmetric space group Pc, and the unit cell parameters are
˚
˚
˚
a = 11.529(5) A, b = 7.914(7) A, c = 15.213(5) A, and
b = 92.10(3)8
[12]. The crystal structure consists of 2-amino-5-nitropyridinium
and toluenesulfonate molecules held together by a network of
hydrogen bonds resulting in two-dimensional hydrogen bond
framework. Single crystals of 2A5NPT of reasonable size are grown
from solution of 2A5NPT by slow cooling technique. The synthesis,
growth details, and preliminary characterization of this crystal
have already been reported elsewhere [12], however optical
quality of the crystals was poor. In this work, single crystals of
2A5NPT have been grown from saturated solution of the
synthesized salt of 2A5NPT by the cooling technique using mixed
solvent of methanol and dimethyl formamide as solvent. In this
paper, we report bulk growth, laser induced surface damage
threshold, dielectric properties and Vicker’s micro hardness of the
grown crystals. The thermal transport properties, namely thermal
idinium-L-monohydrogentartrate (2A5NPLT) [8–11]. 2A5NPT is a
diffusivity (a), thermal effusivity (e), thermal conductivity (K) and
heat capacity (Cp) of 2A5NPT single crystal are measured using an
* Corresponding author. Tel.: +91 4427475166; fax: +91 4427475166.
E-mail address: anandcgc@gmail.com (G. Anandha Babu).
improved photopyroelectric (PPE) technique.
0025-5408/$ – see front matter ß 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2011.02.010

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G. Anandha Babu et al. / Materials Research Bulletin 46 (2011) 631–634
2. Experimental procedure
2.1. Material synthesis
medium were all thermally thick during measurements. The
thermal thickness of each sample in this experiment was
estimated by plotting the PPE amplitude and phase with frequency
at room temperature. Measurement of the PPE signal phase and
The commercially available 2A5NP (Merck, purity >98%) is a
weak Bronsted base and can acquire a proton in a strongly acidic
aqueous medium (pH < 2). This induces the dissolution of this
molecule in an aqueous acidic medium by forming the 2A5NP+
cation and leads to the synthesis of hydrogen – bonded salts with
the conjugated bases of strong or medium acids. The organic–
organic 2-amino-5-nitropyridinium-toluenesulfonate salt is
obtained by dissolving the 2A5NP in p-toluenesulfonic acid at
amplitude enabled us to determine the thermal diffusivity (
thermal effusivity (e) of the sample. From these measured values of
and e, thermal conductivity (K) and heat capacity (Cp) were
evaluated.
A Q-switched Nd:YAG (yttrium aluminum garnet) Innolas
a) and
a
laser of pulse width 7 ns and 10 Hz repetition rate operating in
TEM00 mode is used as the source. The energy per pulse of
532 nm laser radiation attenuated using high energy variable
attenuator is measured using an energy power meter (Scientech
Inc.) which is externally triggered by the Nd:YAG laser. Since the
surface damage is affected by the energy absorbing defects such
as polishing contaminants and surface scratches, which get
incorporated during mechanical polishing, all the experiments
are performed on the highly polished crystals (uniformly
polished with high quality polishing powder) thus minimising
the strain and incorporation of impurities. For both single and
multiple shot experiments, the sample is mounted on an X–Y
translator which facilitates in bringing different areas of the
sample for exposure precisely. For surface damage, the sample is
placed at the focus of a plano-convex lens of focal length 80 mm.
The onset of damage can be determined by visual damage and
audible cracking.
50 8C in millipore water of resistivity 18.2 M
V cm.
2.2. Crystal growth
In this investigation single crystals are grown by slow cooling
technique using mixed solvent of methanol and dimethyl
formamide as solvent under controlled thermal environment.
Small, optically clear crystal grown by slow cooling method is used
as seed to grow larger single crystal in a beaker. A programmable
PID temperature controller (Eurotherm, Model 3216) with the
lowest cooling rate of 0.01 8C/h is employed for the slow cooling
process. In all the growth runs, a cooling rate of 0.01 8C/h is
adopted. Transparent single crystals of dimensions up to
10 Â 10 Â 8 mm3 could be grown from 200 ml of solution in
about 30 days. Fig. 1 shows as grown crystals of 2A5NPT. The
grown crystals are inclusion free and chemically stable.
The capacitance (Ccrys) and dielectric loss (tan d) were measured
using the conventional parallel plate capacitor method with
frequency range (1 kHz–1 MHz) using Agilent 4284A LCR meter at
various temperatures ranging from 323 K to 403 K.
3. Characterization
The load dependence of Vickers microhardness is measured on
plate of 2A5NPT. Microhardness indentations are made on 2A5NPT
crystal plates of thickness 3 mm using SHIMADZU HMV-2000 for
loads varying from 10 to 70 g. The indentation time is maintained
constant at 10 s.
The grown crystals were subjected to X-ray diffraction studies
using Nonius CAD4/MACH 3 single crystal X-ray difffractometer,
˚
using Mo Ka (l = 0.71073 A). An improved photopyroelectric (PPE)
technique was used to determine the thermal transport properties
of the crystals. The 2A5NPT has no phase transition before melting,
as is evident from TG/DTA analysis (NETZSCH STA 409) performed
in the temperature range 30–600 8C. So these measurements were
limited to room temperature only. A detailed description of the
experimental set up and the method to determine the thermal
parameters of crystals by this technique have been reported
elsewhere [13]. In order to obtain good uniform surface finish, each
crystal having thickness < 2 mm was carefully polished using a
polishing sheet. In order to enhance its optical absorption, a very
thin layer of carbon black from a benzene flame was carefully
coated onto the surface of the prepared samples to be illuminated.
In this measurement, a thermally thick polyvinylidene difluoride
4. Results and discussion
From the single crystal X-ray diffraction studies, it is
observed that the crystal belongs to the monoclinic system
with the space group of Pc and the unit cell parameters are
˚
˚
˚
a = 11.529(5) A, b = 7.914(7) A, c = 15.213(5) A, and
b = 92.10(3)8
[12]. The thermal properties of crystals are of basic importance,
and are relevant for various applications. When a laser beam
passes through the crystal, part of the light will be absorbed by
(PVDF) film with thickness 28
mm, coated with Ni–Cr on both
sides, with a pyroelectric coefficient P = 0.25 Â 10À8 V cmÀ1 KÀ1
,
was used as the pyroelectric detector. The thermally thick
pyroelectric detector film was attached to one side of the sample,
which was also thermally thick, and the combination was mounted
on a thermally thick backing medium made of a copper plate. The
other side of the sample was illuminated with an intensity
modulated beam of light, which gave rise to periodic temperature
variations by optical absorption. The thermal waves so generated
propagated through the sample and were detected by the
pyroelectric detector. A 120 mW He–Cd laser of wavelength
442 nm, intensity modulated by a mechanical chopper, was used
as the optical heating source. The sample-detector-backing
assembly was enclosed in a chamber, and was kept at room
temperature (28 8C). The signal output was measured with a lock-
in amplifier (Stanford Research Systems, Model: SR830) with
10 M
V input impedance and 50 pF input capacitance. The
frequency of modulation of the laser source was kept in the range
60–80 Hz to ensure that the detector, the sample and the backing
Fig. 1. Grown single crystals of 2A5NPT.

G. Anandha Babu et al. / Materials Research Bulletin 46 (2011) 631–634
633
the crystal and get converted into heat, forming a temperature
gradient and resulting in consequent thermal expansion. If the
thermal expansion coefficients of the crystal are anisotropic, the
crystal may crack when a high enough thermal energy gets
deposited due to high intensity of the incident light. A crystal
with a high specific heat will have a lower temperature gradient
than a crystal with a low specific heat while absorbing the same
quantity of heat.
The thermal conductivity of 2A5NPT is comparatively higher
than the inorganic crystals potassium dihydrogen phosphate
(KDP), ammonium dihydrogen phosphate (ADP), beta-barium
borate (BBO) and lithium iodate (LiIO3). Table 1 shows the thermal
parameters of 2A5NPT and a few other known crystals [14–21].
The specific heat of a crystal is a basic property that is important
for device applications. If a crystal possesses a large specific heat,
it may have large damage threshold. The Cp of the 2A5NPT crystal
is 1283 J kgÀ1KÀ1 and is comparatively higher than the well
known inorganic crystals potassium dihydrogen phosphate
(KDP), ammonium dihydrogen phosphate (ADP), beta-barium
borate (BBO) and lithium iodate (LiIO3). This suggests that the
2A5NPT could exhibit more resistance to damage caused by laser
irradiation.
For harmonic generation, the efficiency of conversion is
strongly dependent on the incident power level. The operation
of nonlinear optical devices obviously involves the exposure of
these materials to high intensity laser light. Hence, the utility of
NLO crystal depends not only on the linear and nonlinear optical
properties but also largely on its ability to withstand high power
lasers [22]. The knowledge of laser damage studies on NLO
crystals is extremely important in NLO applications. Single shot
and multiple shot (30 pulses) surface laser damage thresholds
are determined to be 54.92 GW/cm2 and 41.19 GW/cm2
respectively at 532 nm laser radiation. Fig. 2(a) and (b) shows
the optical micrograph of the single shot damage profile of
532 nm laser radiation of 2A5NPT. The damage pattern of
2A5NPT (Fig. 2) shows blobs surrounding the core of the
damage. Such blobs are generally seen in crystals where the
damage is mainly due to thermal effects resulting in melting and
solidification or decomposition of the material [22]. Recent
investigations into laser damage in various optical materials by
nanosecond pulses have shown that the temperature reached at
the damage site could be as high as 12,000 K [23]. Since 2A5NPT
decomposes at around 204 8C it is most likely that in the present
case damage occurs due to decomposition of the crystal. Hence,
in 2A5NPT we can expect the damage to be of thermal origin.
Fig. 2. (a) and (b) Laser damage profiles of 2A5NPT.
However, one cannot rule out other mechanisms being operative
simultaneously, as the damage mechanism is quite complex and
depends on the nature of the material and various experimental
parameters.
Table 1
Comparison of the thermal parameters of 2A5NPT with a few known crystals.
Sample
Thermal effusivity (e) (W s1/2/m2 K) Thermal diffusivity (
a
) (Â10À6 m2/s) Thermal conductivity (K) (W/mK) Specific heat capacity (Cp) (J/kg K)
Literature [14–21]
KDP
–
–
–
–
–
–
–
–
1.21 (302 K)
1.86
857 (298 K)
DKDP
–
–
ADP
–
1.26 (315 K)
2
1236 (298 K)
KTP
–
688
BBO
–
1.20
490 (298 K)
365
LiIO3
–
1.47
LAP
–
0.59
–
EMPO
660
669
661
663
663
2.773
3.131
1.744
1.804
2.951
2.561
1.099
1.185
0.873
0.891
1.140
5.143
443
BEDO
322
EDMP.3H2O
MDMP.3H2O
DMAPDP
2A5NPT
(Present study)
716
706
332
3216
1283
Abbreviations: KDP, potassium dihydrogen orthophosphate; DKDP, deuterated potassium dihydrogen phosphate; ADP, ammonium dihydrogen orthophosphate; KTP,
potassium titanyl phosphate; BBO, beta-barium borate; LiIO3, lithium iodate; LAP, -arginine phosphate monohydrate; EMPO, 3-[(1E)-N-ethylethanimidoyl]-4-hydroxy-6-
L
methyl-2H-pyran-2-one; BEDO, bis-2,7-diethylaminohepta-2,5-dien-4-one; EDMPÁ3H2O, 1-ethyl-2,6-dimethyl-4(1H)-pyridinone trihydrate; MDMPÁ3H2O, 1-methyl-2,6-
dimethyl-4(1H)-pyridinone trihydrate; DMAPDP, 4-dimethylaminopyridinium dihydrogen phosphate.

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G. Anandha Babu et al. / Materials Research Bulletin 46 (2011) 631–634
24
22
20
18
16
14
12
10
8
52
50
48
46
44
42
40
38
36
34
1 KHz
10 KHz
100 KHz
1 MHz
10
20
30
40
50
60
70
20
40
60
80
100
120
140
Temperature (oC)
load, P(gm)
Fig. 5. Load dependence of Vickers hardness number of 2A5NPT.
Fig. 3. Temperature dependence of dielectric constant of 2A5NPT for various
frequencies.
5. Conclusion
Single crystals of 2A5NPT were grown from solution by the
temperature-lowering method. The unit-cell parameters of 2A5NPT
were confirmed by single crystal X-ray diffraction analysis. The laser
induced surface damage threshold values of a recently developed
nonlinear optical crystal, 2A5NPT, are reported. The single shot and
multiple shot damage threshold experiments performed on the
plate of 2A5NPT are evaluated as 54.92 GW/cm2 and 41.19 GW/cm2,
respectively, for 532 nm Nd:YAG laser radiation. The high value of
damage threshold is related to the specific heat of the crystal which
is reasonably high. The measured thermal conductivity and heat
capacity by photopyroelectric technique, suggest that the 2A5NPT
may have high optical damage threshold value, which is an essential
requirement for the fabrication of nonlinear optical devices. The
dielectric constant and dielectric loss studies of 2A5NPT establish
the normal behavior. From the mechanical measurements, it was
observed that the hardness increases with increase of load.
1.6
1.2
1 KHz
10 KHz
100 KHz
1 MHz
0.8
0.4
0.0
20
40
60
80
100
120
140
References
Temperature (oC)
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Fig. 4. Temperature dependence of dielectric loss of 2A5NPT for various frequencies.
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clear that Vickers hardness number (VHN) increases with load.
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generated locally by indentation. The increase in VHN at low
loads can be attributed to the work hardening of surface layers.
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crystals.
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