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PAPER
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9245
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Growth, structural and mechanical analysis of a
single crystal of L-prolinium tartrate: a promising
material for nonlinear optical applications
Kanika Thukral,ab N. Vijayan,*b Budhendra Singh,c Igor Bdikin,c D. Haranath,b
K. K. Maurya,b J. Philip,d H. Soumya,e P. Sreekanthf and G. Bhagavannarayanab
A single crystal of L-prolinium tartrate (LPT), which is an organic non linear optical material, was
successfully synthesized and grown using a slow evaporation solution growth technique (SEST). The crystal
structure and lattice parameters of the crystal were confirmed by powder X-ray diffraction and it was found
that it belongs to the monoclinic crystal system with β = 100.380 and a noncentrosymmetric space group.
The presence of strain in the grown ingot was calculated from powder X-ray diffraction measurements.
The crystalline perfection was examined by high resolution X-ray diffractometry, which revealed that the
crystal contained structural grain boundaries. The optical behavior of the grown specimen was analyzed by
photoluminescence (PL) spectroscopy and its time resolved PL decay was calculated. The grown crystal
Received 16th June 2014,
Accepted 23rd July 2014
DOI: 10.1039/c4ce01232a
adopted a step wise growth pattern with parallel striations, which was confirmed from the etching
technique. Its ferroelectric and piezoelectric properties were also assessed. Its third order non linearity was
assessed using an open aperture Z-scan technique. The thermal parameters of the LPT single crystal were
calculated using a photopyroelectric technique. The mechanical strength of the single crystal at the micro
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level was observed by nanoindentation using the Oliver–Pharr method.
1. Introduction
In the past few decades it has been found that amino acid
based compounds have potential nonlinear optical (NLO)
behavior and show good mechanical strength. Nonlinear
optical materials are proven to be suitable candidates for
photonics and optoelectronic industries. These materials
have been efficiently utilized for frequency conversion; optical
switching and device fabrication which further depend
on various properties of the material like transparency,
birefringence, dielectric constant, refractive index and many
more. Among organic and inorganic materials, the organic
ones show excellent characteristics of NLO behavior due
to weak van der Waals forces and hydrogen bonds with
a
Academy of Scientific and Innovative Research, CSIR- National Physical Laboratory,
New Delhi-110 012, India
b
CSIR-National Physical Laboratory, Crystal Growth and X-ray Analysis Section,
Dr. K. S. Krishnan Road, New Delhi – 110 012, India. E-mail: nvijayan@nplindia.org,
vjnphy@yahoo.com; Fax: +91 11 45609310; Tel: +91 11 45608263
c
TEMA-NRD, Mechanical Engineering Department and Aveiro Institute of
Nanotechnology (AIN), University of Aveiro, 3810–193 Aveiro, Portugal
d
Department of Basic Sciences, Amal Jyothi College of Engineering, Kanjirappally,
Kottayam 686518, Kerala, India
e
Department of Instrumentation and STIC, Cochin University of Science and
Technology, Cochin-682 022, India
f
Light and Matter Physics Group, Raman Research Institute,
Bangalore-560080, India
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conjugated π electrons.1–6 L-Prolinium tartrate is an organic
NLO compound with an SHG efficiency of 95% of potassium
dihydrogen phosphate (KDP) which is treated as the standard
material for NLO measurements.7 There are many reports
available on organic materials like L-tartaric acid, L-alanine,
L -arginine phosphate, L-prolinium picrate etc. that show
higher orders of SHG.8–11 In the present article, we report
the crystalline perfection, strain calculations using the
Hall–Williamson equation with the help of powder X-ray
diffraction (PXRD), photopyroelectric (PPE) measurements
and surface morphological studies of LPT. Nanoindentation
studies were performed to find the load dependence and
independence of hardness and to measure the Young's modulus using the Oliver–Pharr method. Various characterizations
were performed for the grown compound that are explained
in detail in the forthcoming sections.
2. Growth of a single crystal of
L-prolinium tartrate
The slow evaporation solution growth technique (SEST)
was adopted to grow a single crystal of L-prolinium tartrate
(C5H10NO2+·C4H5O6−). The commercially available raw materials i.e. L-proline and L-tartaric acid were taken as received
and their purity improved by repeated recrystallization
processes. The purified salt was used in an equimolar ratio
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Fig. 1 Single crystal of
7 × 4 × 3 mm3.
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L -prolinium
tartrate with dimensions of
and dissolved in double distilled water to make a saturated
solution. This saturated solution was filtered using Whatmann
filter paper. The purified solution was then placed in a constant temperature bath (CTB) at 34 °C. After the time span of
30 days a good quality crystal of dimensions 7 × 4 × 3 mm3
was harvested from the mother solution (shown in Fig. 1).
3. Characterization analyses
3.1. Powder X-Ray diffraction analysis (PXRD)
To determine the lattice parameters and space group of the
crystal, powder X-Ray diffraction analysis was performed. The
defect free single crystal was crushed into a fine powder and
subjected to analysis by a Rigaku X-ray diffractometer with
CuKα radiation (λ ~ 1.54 Å) and a scan speed of 4° min−1 over
a range of 20°–70° and the recorded pattern is shown in
Fig. 2a. Its lattice parameters, calculated using CheckCell
software, were a = 5.0083 (Å), b = 17.6891 (Å), c = 6.5241 (Å),
monoclinic system β = 100.381 with space group P2 1, all
of which are in good agreement with the literature.12–16
The strain in the lattice of the crystal was calculated using
the Hall–Williamson equation β cos θ = Kλ/τ + η sin θ, where
β, θ, K, λ and τ are the full width at half maximum (FWHM)
of the diffraction peak, Bragg diffraction angle of the peak,
Scherrer constant, wavelength of X-rays and crystallite size
respectively. The plot is drawn between β cos θ and sin θ, such
that the slope of the curve gives the value of the strain
(Fig. 2b). The value of the strain (η) is 0.0012 with a very
slight error of 0.00146. This positive value suggests that the
crystal contains some compressive strain which results in the
interstitial defects in the crystal.17
Fig. 2 (a) Powder X-Ray diffraction pattern of L-prolinium tartrate.
(b) Strain calculation of L-prolinium tartrate.
without using any analyzer at the receiving stage (i.e. before
the detector) to get all of the possible information like the
individual peaks from structural grain boundaries, scattered
intensity from the dislocations and other defects from the
specimen crystal.
3.2. Assessment of crystalline perfection
A PANalytical X'Pert PRO MRD high-resolution XRD system,
with CuKα1 radiation, was employed to assess the crystalline
perfection of the crystal. The rocking curve of the crystal for
the diffraction planes was recorded in symmetrical Bragg
geometry using the natural facets by performing the ω scan18
with a double-axis geometry. The monochromated X-ray beam
incident on the specimen was obtained using a high-resolution
four-bounce Ge (220) monochromator. The diffracted beam
from the specimen was detected using a scintillation detector
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Fig. 3 Diffraction curve recorded for a typical LPT single crystal for (040)
diffracting planes. Inset shows the schematic of a structural grain boundary.
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Fig. 3 shows the high-resolution X-ray diffraction curve
(DC) recorded for a typical specimen crystal (LPT) using (040)
diffracting planes in symmetrical Bragg geometry by employing
the MRD X-ray diffractometer described above with CuKα1
radiation. As seen in the figure, the curve does not have
only a single diffraction peak. The solid line, which fits well
with the experimental points (filled circles), is the convoluted
curve of three peaks using the Lorentzian fit. These multiple
peaks depict the structural grain boundaries. According
to our earlier convention,19 depending on the tilt angle i.e.
the misorientation angle (α) of the boundary with respect
to the two adjacent crystalline regions on both sides of
the boundary, the structural boundaries may be named as
grain boundaries when α ≥ 1 deg, sub-grain or low angle
boundaries when α > 1 arc min but less than a deg and
very low angle boundaries when α ≤ 1 arc min. The tilt angles
of the observed structural grain boundaries are 176 and
136 arc sec, equivalent to 2.93 and 2.27 arc min respectively
from their adjoining regions and hence all of the observed
boundaries fall under the category of low angle boundaries.
The FWHM (full width at half maximum) of the peaks
corresponding to the observed boundaries are 52, 92 and
28 arc sec. It shows that the crystalline perfection is better
than a previously reported example.20 The relatively higher
values of FWHM of the grains in comparison with those
of the good crystals with only a few arc sec (ref. 21) indicate
that impurities or solvent molecules22 entrapped during the
growth process could be responsible for the formation of
these grain boundaries, where these may be segregated due
to self-generated strains.23 It may be mentioned here that
such low angle boundaries could be detected with wellresolved peaks in the diffraction curve only because of the
high-resolution of the diffractometer with a four bounce
monochromator. Such defects may not have much influence
on the NLO properties. However, a quantitative analysis of
such unavoidable defects is of great importance, particularly
in the case of phase matching applications as described in
our recent article.24
Paper
Fig. 4 (a) PL spectrum for a single crystal of L-prolinium tartrate;
(b) time resolved PL decay and residual recorded at room temperature.
3.3. Time-resolved photoluminescence (PL)
Photoluminescence (PL) is another technique used to find
out about the concentration of defects/imperfections present
in a crystal by exciting a sample and recording its emission
spectrum. Apart from PL, information about the decay kinetics
of a material is highly important for the fabrication of laser
devices from a crystal. The time-resolved PL (TRPL) decay
curve indicates the life time of the exciton, which means the
efficiency of radiative recombination after excitation is
ceased. By calculating the decay time one is able to obtain the
defect concentration as well. The PL and TRPL behavior of the
title compound were recorded on an Edinburgh luminescence
spectrometer (Model: F900) equipped with a microsecond
xenon flash lamp as the source of excitation operating at
ambient conditions. The PL spectrum is shown in Fig. 4a,
which was recorded to the corresponding excitation
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wavelength of 225 nm. The peak maximum was observed at
385 nm, which is near to the visible region in accordance with
the electromagnetic spectrum. The remaining peaks located
at 461 and 692 nm were assigned to the defects present inside
the crystal that were further confirmed by HRXRD. Usually,
the TRPL decay curve is a multiexponential process with a
series of long and short decay components but in the current
situation it is observed to be biexponential. The actual behavior
of TRPL decay curves can be expressed as
y = A + B1e−t/τ1 + B2e−t/τ1
(1)
where τ1 and τ2 represent the fast and slow decay components
of lifetime, and A, B1 and B 2 are constants which determine
the contributions of the fast and slow decay components.25
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The decay profile of L-prolinium tartrate recorded in the range
of 0–60 μs is shown in Fig. 4b. The curve obtained from the
PL decay plot was exponential fitted and it shows a biexponential
nature. The values of the decay constants are 1.0056 μs with
88% decay and 11.6 μs with 12% decay for the fast (τ 1) and
slow (τ2) decay components, respectively. This shows that 88%
of the atoms in the material have a faster decay. The values
of coefficients A, B 1, B 2 are −0.901, 2244.768, and 27.062
respectively. The residuals shown in Fig. 4b (inset) indicate
the extent of best fitting performed on the actual decay curve.
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on the surface of a crystal which is helpful in assessing the
surface quality of the crystal. In the present study doubly distilled water was used as the etchant for etching the (040)
plane of the LPT crystal. The single crystal was dipped into
doubly distilled water for 5 to 10 minutes for dissolution in
order to ensure that the growth pattern, defects, hillocks,
morphology etc. were more visible. Fig. 6a shows the surface
of the crystal before etching which is completely smooth
without any roughness visible. After 5–10 minutes Fig. 6b and c
3.4. Ferroelectric and piezoelectric measurements
The ferroelectric behavior of the sample was assessed using a
ferroelectric loop tracer (make: Marine India). A defect free
single crystal was taken for analysis and then coated with
silver paste on both sides, which acted as an electrode. The
area of the sample was 28 mm2 and the thickness of the sample was 3 mm. Then the sample was placed in the holder and
an electric field applied. The graph was then plotted between
polarization and electric field as shown in Fig. 5. From the
plot it is clear that the material is suitable for use in a lossy
capacitor.
Piezoelectric measurements were carried out on a single
crystal of L-prolinium tartrate using a PM-200 piezometer
system with a frequency of 110 Hz and a dynamic force of
0.25 N at ambient conditions. The (040) surface of the sample was polished and then coated with silver paste for d 33
analysis. The thickness of the crystal was 3.1 mm which
was placed between the electrodes to make proper electrical
contact. The force was gently applied to the sample and the
observed value of piezoelectric constant was 4.4 pc N−1.
3.5. Surface morphological studies
Etching is a revolutionary technique that reveals the crystal
defects and growth mechanism. The etching technique provides information on the density and distribution of defects
Fig. 5 PE loop for L-prolinium tartrate.
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Fig. 6 Micrographs of the surface of (a) the as grown LPT crystal;
(b) after 5 minutes of etching; (c) after 10 minutes of etching.
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suggest that it has a step wise growth pattern. Such parallel
striations shown in Fig. 6b and c are due to the high growth
rate and two dimensional nucleation mechanism which
occur when the solution becomes supersaturated.26,27 It also
confirms, with regards to the structure, that the semi-tartrate
ions form hydrogen bonded strings and these strings are
further connected to proline ions by forming a parallel layered
network.12
3.6. Measurement of thermal transport parameters
The photopyroelectric (PPE) technique is a convenient nondestructive technique used to measure thermal properties,
such as the thermal conductivity and specific heat capacity of
solids, including small single crystals. The PPE technique is
essentially a photothermal one in which the sample is irradiated by an intensity modulated beam of light (usually a
laser), which generates thermal waves in the medium due to
optical absorption which propagates through the medium.
The corresponding periodic temperature rise on the opposite
side of the sample is detected by a sensitive pyroelectric
detector. The detector is usually supported by a backing
medium. The sample thickness and modulation frequencies
are selected so that the sample is thermally thick during the
measurements. This ensures good thermal exchange between
the sample and detector so that the detected signal is free of
fluctuations.
The detected PPE signals are amplified and processed
with a lock-in-amplifier. Measurement of the PPE signal
amplitude and phase as a function of modulation frequency
of the incident radiation enables one to determine the
thermal effusivity and diffusivity of the sample, from which
its thermal conductivity and specific heat capacity are determined.28 In the present experiments we used a 120 mW
He–Cd laser (λ = 442 nm) as the optical heating source and
a 28 μm thick Ni–Cr coated PVDF film as the pyroelectric
detector. The PPE signal amplitude and phase plotted against
modulation frequency for the LPT crystal are shown in Fig. 7.
The thermal diffusivity and effusivity determined from these
Paper
plots as well as the thermal conductivity and specific heat
capacity are tabulated in Table 1.
3.7. Third order non linearity studies
To investigate the non linear optical properties we employed
the open aperture Z-scan technique 29 using the second
harmonic output of a Q-switched Nd:YAG laser (Minilite
continuum, 532 nm, 5 ns (FWHM)). The Gaussian beam
was focused through a plano- convex lens of focal length
10.75 and the focal point is taken as Z = 0, since the beam is
propagating in the Z-direction. At the focus point the density
of energy is at its maximum and on either side it decreases.
Therefore, the light fluence can easily be varied just by translating the sample stage along the z-axis. The sample was
dissolved in distilled water and placed in a 1 mm cuvette.
The cuvette was mounted on a stepper motor controlled
translational stage and translated along the axis through the
focus. The sample faces a different fluence at each position
due to translation and the corresponding transmission is
measured using a pyroelectric detector (laser probe Rj 7620)
placed after the sample.30 From the data obtained the nonlinear absorption coefficient could be calculated. Fig. 8 shows
the open aperture Z-scans (inset) and the corresponding
fluence dependent nonlinear transmissions obtained for
L-prolinium tartrate at input laser pulse energies of 150 and
230 μJ. In general, the optical limiting behaviour of a
medium can have contributions from effects such ESA
(including excited singlet or triplet absorption, free-carrier
absorption etc.), two- or three-photon absorption (2PA, 3PA),
self-focusing/defocusing, thermal blooming, and nonlinear
scattering. The two photon absorption coefficient can be calculated by numerically fitting the observed data using nonlinear transmission equations. Here the best fit is obtained
for a process involving a two photon absorption (2PA) along
with saturable absorption (SA). The obtained nonlinear
parameters are presented in Table 2. The intensity dependent
effective nonlinear absorption coefficient α (I) is given by
I
0
I
1 I /I s
(2)
where α 0 is the linear absorption coefficient at the wavelength of excitation, I is the input laser intensity, I s is the
saturation intensity, and β is the effective 2PA coefficient. For
a given input intensity, the transmitted intensity is calculated
by numerically solving the following propagation equation
dI 0
II
dz 1 I /I s
Fig. 7 PPE amplitude vs. frequency curve along with phase vs.
frequency curve for a single crystal of LPT.
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(3)
where z′ is the propagation distance within the sample.
The value of β numerically obtained from the fits indicates
that the nonlinear absorption in the present case is arising
from an effective two photon absorption, originating from
a sequential excited state absorption involving real intermediate states. In the case of a genuine TPA, where the
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Table 1 Thermal parameters of a single crystal of LPT
Sample
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L-Prolinium
tartrate
Thermal diffusivity,
α (×10−7 m2 s−1)
Thermal effusivity,
e (W m−2 K−1 s+1/2)
Specific heat capacity,
Cp ( J kg−1 K)
Thermal conductivity,
K (W mK−1)
9.16 ± 0.45
532 ± 8
469 ± 7
0.51 ± 0.01
Fig. 8 Fluence dependent nonlinear transmission curve of L-prolinium
tartrate, for an input laser pulse energy of 150 μJ. The open aperture
Z-scan curve is shown in the inset.
transition states involved are virtual, the TPA coefficient is
a constant, which is independent of incident laser fluence
which is a very weak process. But a strong excited state
absorption during an effective TPA depletes the ground
state population significantly, so that the absorption coefficient is no longer a constant. It is clear from the Z-scan
curves that the optical transmittance falls as input intensity
increases, indicating the optical limiting behavior of the
present sample. For an optical limiter the depth of the
valley is a direct measure of the extent of optical limiting.
The optical limiting material can be used as an optical sensor,
also for protecting the human eye. The optical limiter should
have a low limiting threshold, high optical damage threshold
and fast response time with stability.
various parameters relating to the mechanical behaviour of
the materials are evaluated. Nanoindentation is a technique
used to evaluate the hardness of a material at the nanoscale.
This technique is quite helpful for evaluating and understanding the mechanical behavior of a material in relation to its
structure and/or bond strengths. In addition, surface texture,
which is an important issue when the main interest is to
understand the nature of the surfaces of a material, plays
an important role in the functional performance of many
engineering components. Taking advantage of high resolution
atomic force microscopy (AFM), which allows precise topography imaging in a controlled way, we can visualize the surface
and analyze the deformation beneath the indentation area.
The nanoindentation analyses were performed using a
sharp Berkovich tip with a nominal edge radius of ~20 nm
attached to a fully calibrated nanoindenter (TTX-NHT, CSM
instruments). The indentation impressions were captured
after unloading using atomic force microscopy (AFM). The
experiments were performed such that the indentation axis
was aligned normal to the (010) plane with different loads
ranging from 5 to 150 mN. The approach speed, dwelling
time and load/unload speed were kept at 2000 nm min−1,
10 s and 20 mN min−1, respectively. In a typical procedure
9 indentations at different locations on the specimen with
different loads were made along the (010) direction. During
each indentation run, a fully interfaced personal computer
collected and stored data for the load and displacement as
the indenter was driven into the sample (loading segment)
and then withdrawn from it (unloading segment). The obtained
results were analyzed and interpreted using the standard
Oliver and Pharr method.33 The hardness was calculated
using the relationship
H
3.8. Mechanical strength analysis of LPT at the nanoscale
There is a constant interest in assessing the mechanical
properties of crystals as they are closely related to their other
physical and electrical properties,31,32 and determine their
performance when used in various device applications. For
this, a hard indenter of a specific geometry is allowed to
press the surface and based on the deformation beneath
and around the indentation area (confined to a small volume)
Pmax
Ap
(4)
where P max is the maximum load applied by the indenter,
and A p is the projected contact area of indentation. For the
present study, a proper calibration was made as per the standard procedure34 given with the machine manual on a reference steel sample (provided with the system) with a known
hardness using a 6 order polynomial area function given as33
Table 2 Nonlinear optical parameters calculated from the open aperture Z-scan measurements
Sample name
Input pulse energy (μJ) Linear transmission (%) β (×10−11 m W−1) Is (×1012 W m−2) Optical limiting threshold (×1013 W m−2)
L-Prolinium
tartrate
150
230
75
75
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0.6
2.7
40
60
5.201
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Ap = 24.5h 2 + a1hc − a2h 1/2 + a3h 1/4 − a4h 1/8 + a5h 1/16.
(5)
Furthermore, we know that indentation size, d, which is
proportional to the value of the contact depth, h c is also
related to the peak load by
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P = a0 + a1hc + a2hc2
(6)
where a 0, a 1 and a 2 are constants. Furthermore, the parameter a 2, which is considered a measure of the loadindependent hardness (H o), and is related to the empirical
formula
H0 = ka2
(7)
(where k is a constant which depends on the indenter geometry, for the Berkovich indenter, the value of k is 1/24.5) was
used for calculating the load independent hardness in the
present case. The stiffness S (dP/dh), was determined using
the initial slope of the unloading curve, which can be
expressed as
hc hmax
Pmax
S
(8)
where ε is the indenter constant and is equal to 0.75 and hmax
is the maximum displacement of the indenter in the crystal
when a load is applied. Different curves of loading and
unloading with respect to the displacement were obtained
using the relationship35
P = α(h − hf)m
(9)
where P is the indentation load on the crystal, h is the displacement, hf is the final displacement after completing the
cycle of loading and unloading and α and m are empirical
parameters. After getting the value of the load the slope value
is calculated by differentiating the value of P
m 1
dP
S m hmax hf .
dh
h hmax
(10)
For the present case, a well polished surface was scanned
using AFM in contact mode for obtaining the roughness
parameters for the surface under nanoindentaion. A nearly
very smooth (Fig. 9a) surface was obtained with various
scratch lines due to polishing seen on the surface with typical
depths in the order of a few 100 nm. Fig. 9b shows the roughness profile of the single crystal of LPT along the (010) plane.
The calculated RMS roughness parameter in the order or
72 nm was obtained. Fig. 10a shows representative P–h curves
obtained on the (010) facets of the single crystals of
L-prolinium tartrate using a Berkovich tip. A smooth loading
part was observed indicating a plastic deformation, which
occurs underneath the indenter so as to accommodate the
strain imposed by the sharp indenter. In addition, a relatively
homogeneous loading curve with no traces of any ‘pop-in’
features (i.e. discrete displacement bursts) except for a few
serrations was observed. However, a pile-up was evident from
the crystal after indentation (typical imprint made by 50 mN
force) as shown in the insets of Fig. 10. Fig. 10b shows the
cross section profile for the depth after indentation with a
50 mN load. A good correlation between the nanoindentation
depth profile and the profile from the AFM were seen
(Fig. 10a and b). The obtained P–h curves were analyzed
using eqn (4–10) for obtaining various parameters associated
with the mechanical behavior of the single crystal of
L -prolinium tartrate, which are listed in Table 3. A decrease
in the value of hardness with increasing load was observed,
suggesting a load dependent hardness parameter. Fig. 11
shows the variation of the contact depth with peak load. The
contact depth was found to increase as the peak load
increased, due to a load dependant hardness, suggesting that
the indentation size can have an effect on the hardness of
the material, which is proportional to the values of contact
depth and peak load. For calculating the load independent
hardness parameter using eqn (6), the variation of contact
Fig. 9 (a) AFM topography image of the LPT crystal after polishing along the (010) plane and (b) roughness profile of the AFM image.
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Fig. 10 (a) Load–displacement curves of a single crystal of L-prolinium tartrate along the (010) plane and (b) depth profile of the indentation
imprint after applying a 50 mN load. The insets of figures show a typical imprint made after indentation along the (010) plane.
Table 3 Various obtained parameters associated with the mechanical properties of a single crystal of LPT along the (010) plane
F (mN)
HIT (GPa)
EIT (GPa)
hm (nm)
S [mN nm−1]
hc (nm)
hr (nm)
hp (nm)
m
10
20
30
50
75
100
125
150
654.34
507.03
619.93
509.33
492.65
452.34
429.84
402.08
7.96
6.84
5.86
5.49
5.30
4.31
4.42
4.49
982.81
1545.42
1851.32
2551.88
3178.02
3935.97
4436.03
4929.99
0.04
0.055
0.052
0.069
0.084
0.082
0.096
0.111
790.18
1268.26
1404.91
2000.48
2490.45
3001.17
3441.60
3897.89
729.76
1179.86
1273.77
1830.50
2279.34
2714.67
3136.00
3575.41
624.45
1021.72
1092.03
1568.31
1975.26
2300.70
2705.87
3085.47
1.38
1.41
1.30
1.36
1.35
1.35
1.35
1.38
depth with peak load was plotted (Fig. 11) and fitted with
the polynomial function for obtaining the constants a 0, a 1
and a2. The value of a2 for the present case was obtained as
3.224 × 10−6 mN nm−2. Furthermore, using eqn (7), the value
of the load independent hardness (H o) was calculated to be
131 MPa. Fig. 12 shows the variation of the Young's modulus
extracted from the analysis of the load–displacement curves
Fig. 11 Variation of contact depth with peak load.
9252 | CrystEngComm, 2014, 16, 9245–9254
as a function of peak load. A significant decrease in the
Young's modulus was observed with increasing peak load.
This dependence is attributed to the indenter tip rounding. It
is to be noted that the tip imperfection of the indenter does
not affect the hardness results.36 In a recent study, Antunes
et al. concluded that a correction of the geometry of the
Fig. 12 Variation of Young's modulus extracted from the analysis of
the load–displacement curves as a function of peak load along the
(010) plane.
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dependent and independent hardness along the Young's
modulus was calculated by nanoindentation.
Published on 24 July 2014. Downloaded by National Physical Laboratory (NPL) on 13/03/2015 11:54:20.
Acknowledgements
The authors are highly thankful to Dr. R.C. Budhani, Director,
NPL for his constant support and encouragement. One of the
authors (K.T.) is thankful to the U.G.C (University of Grant
Commission) for providing the research fellowship. Budhendra Singh
would like to express their personal thanks to FCT
(Fundaçãopara a Ciência e a Tecnologia) for post-doctoral
research grants with reference numbers SFRH/BPD/76184/2011.
Fig. 13 Variation of initial loading stiffness, S, with the contact depth
at peak load.
Vickers indenters with offset, using the respective area
function, is enough to obtain accurate values of the mechanical properties, namely the Young's modulus and the
hardness.37 However for understanding the degree of tip
rounding, the variation of initial loading stiffness, S, with the
contact depth at peak load, hc for the (010) plane was plotted
(Fig. 13). The relationship of S with hc is given as
S = a + bhc
(11)
where a is a constant related to indentation tip rounding and
the slope b is related to the reduced Young's modulus. A
linear relationship between the initial unloading stiffness,
S and the contact depth at peak load was observed. The
reduced Young's modulus was extracted from the slope of the
linear fitted curve. The constant ‘a’ and the slope for the (010)
plane were found to be 0.0241 and 2.1332 × 10−5 mN nm−2,
respectively. The reduced Young's modulus was estimated
from the slope and found to be 21 GPa for the indentation
made on the (010) plane. However, no data in the literature
are available for direct comparison of the obtained values.
Furthermore, the best fit curve should pass through the
origin, however, a significantly larger value of the intercept,
(‘a’) 0.0241, was observed due to indenter tip rounding.
4. Conclusion
In the present report, a good quality single crystal was grown
by the slow evaporation solution growth technique. The
grown crystal was subjected to powder XRD for primary
identification of its lattice parameters and its strain was also
calculated. The crystalline perfection was assessed by high
resolution X-ray diffraction and found to be reasonably good.
Its optical properties were analyzed by photoluminescence.
The presence of defects on the surface was examined by
etching studies using distilled water as an etchant. Furthermore its third order non linearity, thermal properties, PE loop
and piezoelectric coefficient were determined. The load
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