An extensive investigation on nucleation, growth parameters, crystalline perfection, spectroscopy, thermal, optical, microhardness, dielectric and SHG studies on potential NLO crystal-Ammonium Hydrogen l-tartarte

Article abstract of DOI:10.1016/j.saa.2012.10.044

Ammonium Hydrogen l-tartarte (AMT), an organic nonlinear optical crystal was grown by slow evaporation method at ambient temperature. Solubility, metastable zone width and induction period of Ammonium Hydrogen l-tartarte in aqueous solution were determined. Good quality crystals were selected and characterized by Single crystal XRD, HR-XRD, FT-IR, ¹H NMR, Mass, TGA-DTA, SEM, EDAX, optical and NLO studies. Single crystal XRD analysis revealed that the crystal system belongs to orthorhombic with cell parameters a = 7.65 A, b = 7.85 A and c = 11.07 A. High-resolution-X-ray diffraction (HR-XRD) analysis was carried out to study the crystalline perfection of the grown crystal. ¹H NMR and FTIR spectrum thus confirmed the presence of functional groups of the grown crystal. Molecular mass of AMT was measured accurately by mass spectroscopic analysis. Surface features of the grown crystal were analyzed by SEM, AFM, chemical etching and the presence of elements in the compound was identified by EDAX analysis. Thermal behavior of the grown crystal has been studied by TG/DTA analysis. The recorded UV-Vis-NIR spectrum shows excellent transmission in the region of 190-1100 nm. The Vickers and Knoop's microhardness studies have been carried out on AMT crystals over a range of 10-50 g. Hardness anisotropy has been observed in accordance with the orientation of the crystal. Fluorescence spectral studies were carried in the range of 280-500 nm for the grown crystal. The SHG conversion efficiency and laser damage threshold were measured using an Nd: YAG laser (1064 nm).

Full text of DOI:10.1016/j.saa.2012.10.044

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
An extensive investigation on nucleation, growth parameters, crystalline
perfection, spectroscopy, thermal, optical, microhardness, dielectric and SHG
studies on potential NLO crystal – Ammonium Hydrogen L-tartarte
Redrothu Hanumantharao , S. Kalainathan , G. Bhagavannarayana , U. Madhusoodanan c
a
a,
⇑
b
a
School of Advanced Sciences, Vellore Institute of Technology University, Vellore, Tamilnadu 632 014, India
CGC Section, National Physical Laboratory, New Delhi 110 012, India
HSL, IGCAR, Kalpakkam, Tamilnadu 603 102, India
b
c
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
"
"
"
"
Spectral studies of novel AMT crystal
reporting first time in literature.
Dielectric studies for AMT show low
loss values at high frequencies.
Photoluminescence studies show
blue fluorescence for AMT crystal.
From SHG studies, it is observed that
AMT is potential NLO crystal.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2012
Received in revised form 27 September 2012
Accepted 23 October 2012
Available online 1 November 2012
Ammonium Hydrogen
tion method at ambient temperature. Solubility, metastable zone width and induction period of Ammo-
nium Hydrogen -tartarte in aqueous solution were determined. Good quality crystals were selected and
characterized by Single crystal XRD, HR-XRD, FT-IR, H NMR, Mass, TGA-DTA, SEM, EDAX, optical and NLO
studies. Single crystal XRD analysis revealed that the crystal system belongs to orthorhombic with cell
L-tartarte (AMT), an organic nonlinear optical crystal was grown by slow evapora-
L
1
parameters a = 7.65 Å, b = 7.85 Å and c = 11.07 Å. High-resolution-X-ray diffraction (HR-XRD) analysis
Keywords:
Spectroscopy
NMR
Crystal growth
Infrared spectroscopy
X-ray diffraction
Luminescence
was carried out to study the crystalline perfection of the grown crystal. 1H NMR and FTIR spectrum thus
confirmed the presence of functional groups of the grown crystal. Molecular mass of AMT was measured
accurately by mass spectroscopic analysis. Surface features of the grown crystal were analyzed by SEM,
AFM, chemical etching and the presence of elements in the compound was identified by EDAX analysis.
Thermal behavior of the grown crystal has been studied by TG/DTA analysis. The recorded UV–Vis-NIR
spectrum shows excellent transmission in the region of 190–1100 nm. The Vickers and Knoop’s microh-
ardness studies have been carried out on AMT crystals over a range of 10–50 g. Hardness anisotropy has
been observed in accordance with the orientation of the crystal. Fluorescence spectral studies were
carried in the range of 280–500 nm for the grown crystal. The SHG conversion efficiency and laser
damage threshold were measured using an Nd: YAG laser (1064 nm).
Ó 2012 Elsevier B.V. All rights reserved.
Introduction
Materials that possess optical nonlinearities have been studied
⇑
Corresponding author. Tel.: +91 416 2202350; fax: +91 416 2243092.
E-mail address: kalainathan@yahoo.com (S. Kalainathan).
extensively for their possible applications in various fields like
telecommunication, optical computing, optical data storage and
1
386-1425/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.10.044

R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
389
optical information processing [1,2]. Organic nonlinear materials
are attracting a great deal of attention, as they have large optical
susceptibilities, inherent ultra fast response times and high optical
thresholds for laser power as compared to inorganic materials [3].
In the last decades, many researchers have tried to find varieties of
new NLO materials for laser applications [4,5].
Amino acid crystals have subjected to extensive investigation
by several researches for their excellent characteristics. Amino
acids are interesting organic materials for NLO applications as they
contain donor carboxylic (COOH) group and the proton acceptor
2
amino (NH ) group in them, known as zwitter ions which create
hydrogen bonds. Due to this dipolar nature, amino acids have
physical properties which make them ideal candidates for applica-
tions [6].Taking advantage of the properties of amino acid ligands,
Fig. 2. As grown AMT single crystals.
Crystal growth
researchers reported on several amino acid compounds
phosphate, -histidine tetrafluoroborate and -alanine acetate, etc.
7–11], or their high conversion efficiency, good thermal and
L-arginine
L
L
The saturated solution of AMT was prepared at room tempera-
ture from the recrystallized salt. The solution was filtered twice
using microfilter paper and the solution was optimally closed using
a perforated polythene sheet with few holes for slow evaporation
and placed in a constant temperature water bath. Seed crystals
are harvested with in a period of 2 weeks. Single crystals of Ammo-
[
mechanical strength and these are considered as crucial factors
for device fabrication.
In this paper we report on Ammonium Hydrogen
L-tartarte
(
AMT), an organic nonlinear optical crystal and its growth and
characterization.
nium Hydrogen L-tartarte (AMT) were prepared by slow evapora-
tion technique shown in Fig. 2.
Experimental
Metastable zone width and Induction period measurements
Material synthesis
The metastable zone width was measured by conventional
polythermal method [12]. In the Polythermal method, the equilib-
rium – saturated solution is cooled from the overheated tempera-
ture until the first visible crystal is observed [12]. The experiments
were carried out using a constant temperature water bath con-
trolled to an accuracy of ±0.01 °C, provided with a cryostat for cool-
ing below room temperature. The metastable zone width of AMT
crystal in aqueous solution as a function of temperature (Fig. 3)
indicated that the zone width increase with temperature.
Induction period of AMT crystal was measured by means of iso-
thermal method [13] at a temperature of 35 °C shown in Fig. 4. The
time required for the growth of critical nucleus to a detectable size
is negligibly small compared with the time needed between the
attainment of super saturation and the appearance of crystal of
detectable size and hence this can be taken as induction period.
The appearance of first speck of the nucleus was seen at the bottom
of the container
The starting material was synthesized by taking L-glutamine
(
Qualigens) and
The required amount of starting materials for the synthesis of
Ammonium Hydrogen -tartarte (C NO ) salt was calculated
L-tartaric acid (Acros organics) in 1:1 M ratio.
L
H
4 9
6
and dissolved in triple distilled water. The prepared solution was
allowed to dry at room temperature and the salts were obtained
by slow evaporation technique. The purity of the synthesized salt
was further improved by successive recrystallization process.
Solubility studies
Solubility being the key factor of bulk crystal growth, it was
determined by gravimetric method [12] in the temperature ranges
0–50 °C in steps of 5 °C and the solubility curve is shown in Fig. 1.
Solubility determination shows that the solubility increases with
temperature. Hence, it is found to have positive slope.
3
Fig. 1. Solubility curve for AMT single crystals.
Fig. 3. Meta stable zone width for AMT crystal.

3
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R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Results and discussion
Single crystal X-ray diffraction
The single crystals were subjected to single crystal X-ray dif-
fractometer with Mo K radiation of wavelength k = 0.71073 Å is
a
to determine the unit cell parameters and space group. Details of
cell parameters and space group are presented in Table 1. The re-
sults shown from single crystal diffraction studies were exactly
correlated with the reported ones [14].
4.2. High resolution X-ray diffractometry (HR-XRD) analysis
To evaluate the crystalline perfection of the specimen crystals,
high-resolution X-ray diffraction (HRXRD) analysis was carried out
for AMT single crystals. A multicrystal X-ray diffractometer devel-
oped at National Physical Laboratory (NPL), New Delhi [15] was used
to record high-resolution rocking or diffraction curves (DCs).
Fig. 5 shows the RC recorded for a typical AMT specimen crystal
using (200) diffracting planes in symmetrical Bragg geometry with
Fig. 4. Induction period vs. super saturation plot for AMT crystal.
1
Mo Ka radiation. As seen in the figure, the curve is not having a
single diffraction peak. The solid line, which follows well with
the experimental points (filled circles), is the convoluted curve of
two peaks using the Lorentzian fit. The additional peak depicts
an internal structural very low angle (tilt angle < 1 arcmin) bound-
ary [16] whose tilt angle (misorientation angle between the two
crystalline regions on both sides of the structural grain boundary)
is 49 arc sec from its adjoining region. The FWHM (full width at
half maximum) of the main peak and the low angle boundary are
respectively 44 and 52 arcsec. The area under the curve of the
boundary is much less than that of the main peak which reveals
that the size of the very low angle boundary is very small. The
small area under the curve of boundary and the low angular spread
of ꢂ100 arcsec of the RC indicates that the quality of the crystal is
fairly good. It may be mentioned here that such a very low angle
boundary could be detected with well-resolved peak in the
diffraction curve only because of the high-resolution of the
multicrystal X-ray diffractometer used in the present studies. The
influence of such defects may not influence much on the NLO
properties. However, a quantitative analysis of such unavoidable
defects is of great importance, particularly in case of phase match-
ing applications [17].
Characterization studies
The single crystal X-ray diffraction (XRD) analysis of AMT was
carried out using a computer controlled Enraf Nonius CAD4-
MV31 diffractometer and the unit cell dimensions and space group
were determined. The crystalline quality was found by HR-XRD,
developed at NPL, New Delhi using Philips (PW 1743) X-ray gener-
2
ator of 0.4 ꢀ 8 mm ; 2 kW Mo X-ray source. Fourier transform
infrared (FTIR) spectrum was recorded by the KBr pellet technique
using AVATAR 330 FTIR thermo Nicollet spectrometer for the range
ꢁ
1
1
5
00–4000 cm to confirm the functional groups. H NMR spec-
trum of the grown crystal has been recorded with a Bruker
AVANCE III 500 MHz (AV 500) multi nuclei solution NMR. The mass
spectral analysis was carried out for AMT using JEOL GCMATE II
GC–MS high resolution, double focusing instrument. The crushed
samples of AMT have been used for this analysis. Energy dispersive
spectrometry is a chemical microanalysis technique performed in
combination with Quanta 200 FEG scanning electron microscope
(
SEM) capable of detecting the presence of elements down to bor-
on (B) on any solid conducting material. Thermal analysis was car-
ried out using a TA Instruments SBT Q600 apparatus at a heating
rate of 20 °C/min in inert nitrogen atmosphere. Optical studies
has been carried out using ELICO SL 218 double beam UV–Vis spec-
trophotometer which records optical absorption spectrum in the
range of 200–1100 nm. Etch patterns was observed and photo-
graphed under a Carl Zeiss metallurgical microscope – (Axios kop
FT-IR spectral analysis
The infrared spectral analysis was carried out to understand the
chemical bonding and it provides useful information regarding the
molecular structure of the compound. Fourier transform infrared
spectrum was taken for the powdered sample of AMT in the wave-
4
0 MAT) provided by clemex vision PE software in reflected light.
The surface characteristics were studied with the aid of an AFM
Nano surf easy scan 2, Switzerland) with a maximum XY-scan
range of 70 m. The mechanical studies of AMT single crystals
ꢁ1
length of 400–4000 cm following KBr pellet technique as shown
ꢁ1
(
in Fig. 6. The sharp peak at 2953 cm is due to CH stretching
ꢁ1
l
modes of vibration [18]. The peak at 1726 cm is due to C@O
were made by Vickers and Knoop’s hardness microhardness tests
at room temperature. In order to determine the laser induced sur-
face damage a Q-switched Nd: YAG laser (Litron model no:
LPY704G-10) (1064 nm, 10 ns, 5 Hz) was used. Dielectric measure-
ments, was measured using two -probe set up. Temperature varia-
tion of dielectric response was carried out at various frequency
ranges from 50 Hz to 5 MHz using HIOKI 3532–50 LCR HITESTER.
The photoluminescence measurements were carried out using
the Jobin Yvon-spex make spectrofluorometer (Fluorolog version-
stretching modes of vibration [19]. The intense peak observed at
ꢁ1
1402 cm is due to the combination of CAO stretching and OAH
deformation [20]. The CAO stretching mode of vibration and
OAH in-plane bending modes of vibration is observed at
Table 1
Crystalline data of Ammonium Hydrogen L-tartarte (AMT).
Sample name
Crystal system
Lattice type
Lattice
Ammonium Hydrogen
Orthorhombic
L-tartarte
3
: Model FL3-11), 450 W high pressure Xenon lamp was used as
‘‘P2
a = 7.65 ± 0.01 Å, b = 7.85 ± 0.01 Å, c = 11.07 ± 0.01 Å,
= b = = 90°
1 1 1
2 2 ’’
an excitation source.SHG studies were carried out using Q-
switched Nd: YAG laser (1064 nm.Quanta ray Series, USA) emitting
a fundamental wavelength of 1064 nm.
parameter
a
c

R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
391
important tool for the identification of molecules and the examina-
1
tion of their electronic structure [22]. The proton NMR ( H)
spectral analysis was carried out on the AMT crystal in DMSO as
solvent and shown in Fig. 7. The sample in the presence of solvent
may absorb small quantity of water appeared in the spectrum at
d = 3.357 ppm. The peak at d = 2.508 ppm is due to methyl group
of DMSO.
þ
The signal at d = 3.962 ppm is a quartet assigned to NH4 proton
of AMT compound. The signal at d = 2.369 ppm is due to OH group
and signal at d = 2.106 ppm is due to CH proton of AMT compound.
The peak observed at d = 1.239 ppm is due to CAC bonding of
L-tartaric acid. The signal at d = 7.800 ppm is due to proton of the
carboxyl group (OH) in the compound [23]. Thus the molecular
1
structure of AMT compound was confirmed by H NMR spectral
analysis.
Mass spectrum
Fig. 5. Rocking curve recorded for a typical AMT single crystal for (200) diffraction
1
planes by employing the multicrystal X-ray diffractometer with Mo Ka radiation.
Mass spectroscopy is much more precise technique than chem-
ical methods of determining molecular weights. The ion have larg-
est mass are often the parent molecular ion in ionized molecule
which is not fragmented. A high-resolution mass spectrum shown
in Fig. 8 helps to determine the exact mass of the molecular ion.
The calculated molecular weight of AMT is 168. From the spectrum
it is observed that the parent ion peak representing precise mass of
AMT is observed to be at 167.20 (m/z). Depending on the atoms
contained in a molecule, it is possible for particles of the same
nominal mass to have a slightly different measured mass when
precise determinations are possible. Other peaks have smaller
mass may represent charged fragments of this ion. The mass at
these fragments may give clues as to the structure of the molecule.
Each particular line in the mass spectrum count the peak repre-
sents the different ion. Hence, compound mass has been confirmed
by high resolution mass spectrum.
SEM and EDX analysis
The external morphology of the sample is derived from signals
received from electron sample interactions. A two dimensional im-
age is generated over a selected area of the surface of the sample
for analysis. The sample is coated with carbon and the images
are recorded in the mode of operation of low vacuum (LV). The re-
corded SEM image is shown in Fig. 9. From the figure it is observed
that surface is smooth and free from cracks. However, very few mi-
cro crystals are seen on the surface.
Fig. 6. FTIR spectrum of AMT.
Table 2
FTIR assignments of Ammonium Hydrogen L-tartarte.
Ammonium Hydrogen L-tartarte
Energy dispersive X-ray spectrometry (EDX) providing crystal-
line information from the few nanometer depth of the material
surface. A spectrum of the energy vs. relative counts of the de-
tected X-rays is obtained and evaluated for qualitative and quanti-
tative determinations of the elements present in the sample
volume. The EDAX spectrum of AMT is shown in Fig. 10.
Wave number (cm 1
ꢁ
)
Assignments
ꢁ
523.579
709.707
841.776
COO wagging
ꢁ
COO in-plane-deformation
CAC stretching
þ
1
1
1
1
1
3
067.41
213.01
334.5
402
726.94
323.71
NH rocking
4
ꢁ
CACOO stretching
CACAH in-plane deformation
OAH in-plane deformation
ꢁ
Thermal analysis
COO stretching
NAH stretching
The thermal stability of the title compound was identified by
thermo gravimetric (TGA) and differential thermal analysis (DTA)
studies simultaneously. The TG and DTA thermogram of AMT is
shown in Fig. 11. Thermo gravimetric analysis shows that AMT is
very stable up to 213.66 °C and the weight loss starts at this
temperature. It is observed from DTA thermogram that an endo-
thermic event begins at 201.77 °C and then sharp peak appears
at 249.53 °C. An endothermic event is observed at 249.53 °C which
is due to melting of the sample. Good crystalline nature of the title
compound is clearly noticeable from the sharpness of the endo-
thermic peak.
ꢁ
1
ꢁ1
1
213 cm [18]. The band observed at 709 cm is due to C@O in-
plane bending of COOH [21]. This observation confirms the forma-
tion of AMT compound. The detailed FTIR spectral band assign-
ments of AMT are presented in Table 2.
1
H NMR analysis
The NMR technique is used to detect the presence of particular
nuclei in a compound for a given nuclear species. It is also an

3
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R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Fig. 7. 1H NMR spectrum of AMT compound.
Fig. 9. SEM image of AMT crystal.
Optical studies
The optical transmission spectrum of AMT is shown in Fig. 12. A
sample of 1.4 mm thickness is measured without polishing. It is
observed that UV cut off wavelength is at 254 nm. The crystal
has low absorption in the entire visible and NIR region. The large
transmittance window in the visible and NIR region enables very
good optical transmission of the second harmonic frequencies of
Nd:YAG laser. Estimated band gap at cutoff wavelength was calcu-
lated to be 5.065 eV.
Fig. 8. Mass spectrum of AMT crystal.
From TGA thermogram, the grown crystal is thermally stable up
to 213.66 °C, which confirms that the absence of physically ad-
sorbed water or lattice water in the grown crystal. The first stage
of decomposition ends at 270.77 °C with a weight loss of 58.64%.
The TGA thermogram shows complete weight loss and the reduced
weight loss obtained at 644.33 °C is only 1.08%. This is due to step-
by-step decomposition and release of volatile substances in the
compound, ammonia and carbon dioxide [23].
Determination of optical band gap and optical constants
The measured transmittance (T) using optical absorption spec-
trum was used to calculate the absorption coefficient (
the formula
a) using

R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
393
Fig. 10. EDS spectrum of AMT.
Fig. 11. TGA-DTA curve of AMT.
¼ 2
:3026 logð1=TÞ
ð1Þ
k
a
a
K ¼
ð3Þ
t
4p
The extinction coefficient as a function of photon energy is
shown in Fig. 14. The extinction coefficient decreases with energy
shows inverse dependence with E up to 4.794 eV. There is a steep
rise in ‘K’ observed at 4.794–5.151 eV.
where T is the transmittance and t is the thickness of the crystal.
The optical band gap (E ) has been evaluated from the transmis-
g
sion spectra and the optical absorption coefficient (
absorption edge is given by
a
) near the
The reflectance (R) in terms of the absorption coefficient and
refractive index (n) can be derived from the relations:
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
1
=2
h
ma
¼ Aðh
m
ꢁ E
g
Þ
ð2Þ
R ¼ 1
ꢃ
ð1 ꢁ expðꢁ
a
tÞ þ expð
atÞÞ
ð4Þ
where A is a constant, E
constant and the frequency of incident photons. The band gap
of AMT crystal was estimated by plotting (ahm) vs. hm as shown
in Fig. 13 and extrapolating the linear portion near the onset of
absorption edge to the energy axis. From Fig. 10, the value of
band gap was found to be 5.065 eV. The wide band gap of the
AMT crystals confirms the large transmittance in the visible re-
gion [24].
g
the optical band gap, h the plank’s
ð1 þ expðꢁ tÞÞ
a
m
2
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
2
ꢁ
ðR þ 1Þ ꢃ ðꢁ3R þ 10R ꢁ 3Þ
g
¼
ð5Þ
2
ðR ꢁ 1Þ
The dependence of reflectance (R) and refractive index as a
function of energy is shown in Figs. 15 and 16.
From the optical constants, the electric susceptibility
calculated according to the relation [25]
Extinction coefficient (K) can be obtained from the following
equation
c
v can be

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R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Fig. 14. Plot of photon energy vs. extinction coefficient.
Fig. 12. UV–Vis-NIR transmission spectrum of AMT crystal.
observed, when etched with water and ethanol for 10 s. The
patterns obtained are quite stable and did not degrade for a longer
time. Fast etchants like water and ethanol could produce well
defined etch pits on the surfaces [28].
AFM studies
Fig. 18 shows AFM image recorded on the flat surface using
microfabricated cantilever whose dimensions are length
(
l) = 450
tip height 12
to be 0.15 N/m. The image size is measured to be 15
l
m, width (w) = 45
m. Spring constant of the cantilever is estimated
m. It is ob-
lm, thickness (t) = 1.5 lm, provided
l
l
served from the image that crystal surface possess few valleys
and hillocks. The parameters such as roughness average, the mean
value, root mean square value, the valley depth, the peak height
and peak–valley height has been estimated. Measured area rough-
ness shown from Table 3 determine the mean height differences
between two plateaus.
Microhardness studies
Fig. 13. Plot of (ahm
)2 vs. photon energy for AMT crystal.
The mechanical studies of AMT crystals were made by Vickers
and Knoop’s hardness measurements at room temperature.
g2 ꢁ k ꢁ
2
ꢀ
0
2
2
ꢀ
r
¼
ꢀ
o
þ 4p
vc ¼ n ꢁ K ; vc
¼
ð6Þ
4
p
where e
o
is the dielectric constant. The value of electric susceptibil-
ity is 0.1056 at k = 1100 nm.
v
c
The real part dielectric constant e
constant e
r
and imaginary part dielectric
i
can be calculated following the relations [26]:
2
2
ꢀ
r
¼ n ꢁ K
and
ꢀ
i
¼ 2nK
and imaginary e
ð7Þ
The value of real e
k = 1100 nm are 1.327 and 1.51 ꢀ 10 , respectively.
r
i
dielectric constants, at
ꢁ
4
Chemical etching
Chemical etching is one of the most efficient and powerful
technique for mapping and characterizing the growth defects.
Generally crystals grown from solution suffer by the growth
imperfections such as solvent inclusions, twins, grain boundaries
and dislocations [27]. Etching studies of AMT crystals were carried
out using water and ethanol as etchants. Well defined elliptical
etch pits (Fig. 17a) and striations like pattern (Fig. 17b) were
Fig. 15. Plot of photon energy vs. reflectance.

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395
of the microscope after unloading and the average was found out.
For a particular load at least five well defined indentations were
considered and the average of all the diagonals (d) was considered.
Vickers hardness studies
The H
v
was calculated using the standard formula:
2
H
v
¼ 1:8544 P=d ;
ð8Þ
is in kg mm 2
ꢁ
.
where P is the applied load in kg, d is in mm and H
The graph was plotted against Vickers hardness (H
P). The plot is shown in Fig. 19. It is very clear from the figure that
increases with increase in load. The Meyer’s index number was
v
k
) and load
(
H
v
calculated from the Mayer’s law [29], which relates the load and
indentation diagonal length as
n
P ¼ kd
log P ¼ log k þ n log d
ð9Þ
where k is the material constant and ‘n’ is the Meyer’s index.
Fig. 16. Plot of photon energy vs. refractive index.
In order to find the value of ‘n’, a graph is plotted for logP
against logd (Fig. 20) which gives a straight line (after least square
fitting). From the slope line the Meyer’s index number ‘n’ was cal-
culated to be 3.038 respectively. According to Onitsch and Hann-
eman [30,31] the values of n comes out to be 1–1.6 for hard
materials and more than 1.6 for soft ones. Thus AMT belongs to soft
material category.
The elastic stiffness constant was calculated at loads of 10, 25
and 50 g by using Wooster’s empirical relation [32]
7
=4
C
11 ¼ Hv
ð10Þ
The calculated stiffness constant for different loads were tabu-
lated in Table 4.
Knoop hardness
The Knoop’s hardness value (H
dard formula [32]:
k
) was calculated using the stan-
2
H
k
¼ 14:229P=d
ð11Þ
in kg mm 2
ꢁ
.
where P is the applied load in kg, d in mm and H
k
The Knoop’s indented impressions were approximately rhom-
bohedral in shape.
k
The graph was plotted against Knoop hardness (H ) and load (P).
The plot is shown in Fig. 21. From the Knoop microhardness mea-
surements the Young’s modulus (E) of the crystal was calculated
using the relation [32],
ꢀ
ꢁ
b
E ¼ 0:45H
k
= 0:1406 ꢁ a
ð12Þ
k
where H is Knoop microhardness values at a particular load, and b
and a are the shorter and longer Knoop indentation diagonal respec-
1
1
ꢁ2
tively. The calculated Young’s modulus is 1.959 ꢀ 10 Nm
.
For studying the crystal anisotropy, the microhardness was
measured by varying the crystal orientations over a range of 0–
Fig. 17. Etched surface on AMT crystal by: (a) water and (b) ethanol.
3
60° insteps of 30°. From Fig. 22, it is very clear that variation
was periodic. The variation in hardness number indicates the
anisotropic nature of AMT crystal. The crystal structure and the slip
system play an important role in the observed variation of hard-
ness with crystal orientation.
Crystals, free from cracks, with flat and smooth surfaces were cho-
sen for the static indentation tests. The crystal was mounted prop-
erly on the base of the microscope. Now, the selected faces were
indented gently by loads varying from 10 to 50 g for a dwell period
of 10 s using both Vickers diamond pyramid indenter and Knoop’s
indenter attached to an incident ray research microscope (Mutu-
toyo MH 112, Japan). The Vickers indented impressions were
approximately square in shape. The length of the two diagonals
was measured by a calibrated micrometer attached to the eyepiece
Laser damage threshold (LDT) studies
Laser damage threshold is important for a NLO crystal to per-
form as a device is its resistance to laser damage, since high optical
intensities are involved in nonlinear processes. The surface laser

3
96
R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Fig. 18. AFM image of AMT single crystal.
Table 3
Amplitude parameters of AMT measured by AFM.
S. no.
Amplitude parameter
Value
1
2
3
4
5
6
Roughness average (S
a
) (nm)
) (nm)
) ( m)
37
47
0.32
0.22
ꢁ0.1
0.11
Root mean square (S
Peak–valley height (S
Peak height (S ) ( m)
Valley depth (S ) ( m)
The mean value (S ) (nm)
q
y
l
p
l
v
l
m
Fig. 20. Graph between log P vs. logd.
Table 4
Variation of elastic stiffness constant (c11) with load.
S. no.
Load (g)
c11 (pa)
15
16
16
1
2
3
10
25
50
4.025 ꢀ 10
1.180 ꢀ 10
2.777 ꢀ 10
v
Fig. 19. Variation of Vickers microhardness number H with load.
time period of 30 s for all measurements. The damage was observed
and the energy of the laser beam was measured by Coherent en-
ergy/power meter (Model No. EPM 200). The estimated damage
threshold value is found to be higher than potassium dihydrogen
orthophosphate (KDP) crystals [34].
damage threshold of the crystal was calculated using the equation
33]
[
E
spr2
P ¼
ð13Þ
Dielectric studies
where ‘‘E’’ is the energy in mJ, ‘‘
s
’’ is the pulse width in ns; ‘‘r’’ is ra-
dius of the spot in mm. LDT studies were carried out by single shot
The measurement of dielectric loss and dielectric constant has
been done as a function of frequency. Good quality crystals
method and multi shot method. In our experiment, the laser dam-
2
3
age threshold of AMT crystal was found to be 0.53 GW/cm laser en-
8 ꢀ 6 ꢀ 2 mm was polished and electronic grade silver paste
ergy by multi shot method. The crystal was exposed to a laser for a
was applied on either side of the sample, which acts as an elec-

R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
397
Fig. 21. Variation of Knoop’s microhardness number H
k
with load.
Fig. 23. Dielectric constant vs. logF of AMT crystal.
than that of the exciting light (i.e., of equal or less energy).This dif-
ference in wavelength is caused by a transformation of the exciting
light, to a greater or lesser extent, to non-radiation vibration en-
ergy of atoms or ions. The emission spectra of AMT was recorded
in the range of 280–500 nm (Fig. 25). Excitation wavelength
chosen for this sample is 254 nm from UV spectral studies. Two
emission peaks were observed in the spectrum at 408.26 nm and
4
30.84 nm respectively. The results indicate that AMT has blue
fluorescence spectrum. The band gap energy for the highest peak
intensity was calculated using the formula E = hc/ke. Here h, c,
g
and e are constants and k is the wavelength of fluorescence. The
band gap energy calculated is about 2.8875 eV (at 430.84 nm).
Peak observed at wavelength of 468.80 nm is due to xenon source
used for spectral studies.
SHG studies
Kurtz powder technique [35,36] was employed for the powder
sample of Ammonium Hydrogen
L-tartarte using 1064 nm
laser radiations. In this technique the powdered sample with an
Fig. 22. Anisotropy nature of the AMT single crystal.
trode. Dielectric studies were carried out in the temperature range
of 313–353 K in the frequency range of 50 Hz to 5 MHz along (001)
orientation. Crystals with high transparency and large surface de-
fect-free were selected and used for dielectric measurements.
The dielectric constant of the crystal was calculated using the
relation
r p o p
e = C d/Ae . Where ‘C ’ is the capacitance of parallel
plate capacitor, ‘A’ is the area of overlap of the two plates and ‘d’
is the separation between the plates.
Figs. 23 and 24 indicate that the dielectric constant and dielec-
tric loss decreases with increase in frequency. The high value of
dielectric constant at low frequencies may be due to the presence
of all the polarizations namely, space charge, orientation, ionic and
electronic polarization [35]. Of all these polarizations only space
charge is active at lower frequencies and there is loss of signifi-
cance of other three polarizations which may leads to low value
of dielectric constant at higher frequencies.
Photoluminescence studies
Photoluminescence in solids is the phenomenon in which elec-
tronic states of solids are excited by light of particular energy and
the excitation energy is released in general is equal to or longer
Fig. 24. Dielectric loss vs. logF of AMT crystal.

3
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R. Hanumantharao et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 103 (2013) 388–399
Fig. 25. Photoluminescence studies of AMT crystal.
average particle sizes range 125–150
capillary tube about 1.5 mm diameter. Q-switched Nd: YAG laser
1064 nm. Quanta ray Series, USA) emitting a fundamental wave-
l
m is filled in a micro-
Kurtz powder technique shows that the SHG sufficiency of AMT
is 1.02 times to that of standard KDP crystal.
(
length of 1064 nm was used. To make relevant comparisons with
known SHG materials, KDP was also ground and sieved into the
same particle size range. The SHG output (532 nm) was emitted
and collected by a photomultiplier tube after being monochromat-
ed to collect only the 532 nm radiation. The input laser energy inci-
dent on the capillary tube was chosen to be 0.68 J, an energy level
optimized not to cause any chemical decomposition of the sample.
The SHG was confirmed by the emission of bright green radiation
Acknowledgements
The author express thanks to SAIF, IIT madras for providing the
spectral data, single crystal XRD studies. The author also express
thanks to VIT University for their constant encouragement and
support.
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