Journal of Minerals and Materials Characterization and Engineering, 2012, 11, 904-907
Published Online September 2012 (http://www.SciRP.org/journal/jmmce)
Growth, Structural, Optical and Hardness Studies of
Lithium Potassium Sulphate Single Crystals—
An Inorganic NLO Material
G. Pasupathi*, P. Philominathan
PG Research and Department of Physics, A. Veeriya Vandayar Memorial Sri Pushpam College, Thanjavur, India
Received May 25, 2012; revised June 27, 2012; accepted July 20, 2012
ABSTRACT
Single crystal of lithium potassium sulphate, a nonlinear optical material, was grown from aqua solution by slow evapo-
ration method at room temperature. The cell parameters were estimated by single crystal X-ray diffraction analysis. The
optical transmittance of the crystal was recorded using the UV-Vis-NIR spectrophotometer and the optical band gap
was calculated using this method. The second harmonic generation efficiency was measured by Kurtz and Perry powder
technique and the phase-matching property was confirmed. The hardness of the material was measured by Vicker’s
hardness test.
Keywords: Crystal Growth; Lithium-Potassium Sulphate; Inorganic NLO Material; X-Ray Diffraction; SHG Efficiency
1. Introduction
In the recent past, extensive studies have been made on
the synthesis and crystal growth of nonlinear optical
(NLO) materials due to their potential application in the
field of telecommunication, photonics and opto electron-
ics technology. Presently, numerous inorganic NLO ma-
terials have been developed to increasing the variety of
these applications by the researchers [1-3]. In this series,
lithium potassium sulphate LiKSO4 (namely, LKS) be-
longs to a family with the general structural formula
4 (M' = Li, Na; M'' = K, Cs, Rb ions, NH4,
N2H5 group and AX4 =
M'M''AX
2
4
SO

, , BeF4). The sig-
nificant attention is currently being paid to these crystals
due to their physical properties such as ferroelectricity,
piezoelectricity and ionic conductivity. During the past
two decades, extensive studies have been carried out for
the growth, structural and phase transition of LKS crystal.
At room temperature, it exhibits a hexagonal system with
P63 space group [4,5]. In addition, it undergoes several
structural phase transitions below [6] and above room
temperature [7].
2
4
SeO
In the present investigation is aimed at the growth of
lithium potassium sulphate single crystals by slow evap-
oration method at room temperature. The grown crystals
have been subjected to single crystal X-ray diffraction
analysis, UV-Vis-NIR spectral analysis, optical band gap
measurements, second harmonic generation (SHG) mea-
surement, phase matching and Vicker’s hardness test.
2. Experimental Procedure
2.1. Synthesis of the Material
Lithium potassium sulphate was synthesized by the che-
mical reaction of commercially available Lithium sul-
phate (Analar grade-E. Merck) with Potassium sulphate
(Analar grade-E. Merck) taken in the stoichiometric ratio
1:1 by dissolving double-distilled water (solvent) at room
temperature and the chemical reaction is given below
242 2442
KSO HOLiSOLiKSO HO

  (1)
2.2. Growth of Single Crystals
A saturated solution of LKS was prepared by double-
distilled water. The solution was allowed to slow evapo-
ration in the vibrational and dust free atmosphere. After
the growth period of 25 days, colourless and transparent
crystals were harvested. The harvested crystals were rec-
rystallized repeatedly to achieve good quality as shown
in Figure 1.
2.3. Characterization Technique
The single crystal X-ray diffraction (XRD) studies of
LKS were carried out using Enraf nonius CAD4 single
X-ray diffractometer with MoKα (λ = 0.717 Å) radiation.
The UV-Vis-NIR spectrum was recorded using Perkin
elmmer Lamda 35 spectrophotometer in the range of 190 -
*Corresponding author.

G. PASUPATHI, P. PHILOMINATHAN 905
Figure 1. As the grown single crystal of lithium potassium
sulphate.
1100 nm. The SHG efficiency and the phasematching
studies were carried out by the Kurtz and Perry powder
technique using a Q-switched, mode locked Nd:YAG
laser. Microhardness behaviour of the grown crystal was
carried out using Vicker’s hardness tester.
3. Results and Discussion
3.1. Structure of the Crystals
The single crystal XRD study indicates that LiKSO4 cry-
stallize in hexagonal system with P63 space group at
room temperature. The unit cell dimensions are a = b =
5.1453(2) Ǻ and c = 8.6342(7) Ǻ. These values are good
agreement with the reported values [4]. There are two
molecules in the unit cell. The structure of LKS consists
of Li+ and lying on threefold axes and K+ ion has
a tetrahedral coordination with Li-O distances 1.909 -
1.923 Ǻ as reported by Karpinnen et al. (Figure 2).
2
4
SO 
3.2. Optical Transmittance Study
The optical transmittance spectrum of LKS crystals is
shown in Figure 3. The optical transmittance study may
be assisted in understanding the electronic structure of
the optical band gap of the crystal. The study of the ab-
sorption edge is essential in connection with the theory of
electronic structure, which leads to the prediction of
whether the band structure is affected near the band ex-
treme. From the transmittance spectrum, it was observed
that the grown crystals have high transmittance in the
entire visible-NIR region and the lower cut-off wave-
length (253 nm) facilitates LKS crystals to be potential
nonlinear optical material for second harmonic genera-
tion of Nd:YAG laser. Using the formula,
g
hc
E




(2)
the value of optical band gap of LKS is calculated to be
4.24 eV. The observed behaviour of the optical spectrum
and band gap value found in this work is in good agree
ment with the spectrum of LKS crystal reported in lit-
erature [8].
Figure 2. Coordination polyhedron of K+ viewed along c-
axis.
Figure 3. Optical transmittance spectrum of LKS.
3.3. Second Harmonic Generation Measurement
and Phase-Matching Studies
The first and the most widely used technique for con-
firming the SHG efficiency from prospective second-
order NLO material is the Kurtz powder technique [9]. In
addition to identifying the materials with non-centro
symmetric crystal structure, it is also used as a screening
technique to identify the materials with the capacity for
phase matching. The SHG efficiency from the material is
measured as a function of particle size. The continuous
increase of SHG efficiency with increase of particle size
and remaining essentially constant at particle sizes great-
er than the coherence length confirms the phase matching
behavior of the material [10-12].
The powder second harmonic generation (SHG) test
was carried out for LKS using Kurtz and Perry technique.
Powdered sample of LKS was tightly packed in the mi-
cro capillary tubes of uniform diameter (1.5 mm) and
irradiated by an incident laser radiation 1064 nm of pulse
width 8 ns and pulse energy of 11.4 mJ from a Q-switch-
ed quanta ray of Nd:YAG laser. KDP was used for cali-
brating the SHG efficiency. The second harmonic non-
linearity of LKS was confirmed by the emission of green

G. PASUPATHI, P. PHILOMINATHAN
906
radiation (532 nm) by the crystal. The powder SHG effi-
ciency of LKS was found to be 1.6 times that of the
standard KDP. The measurements of SHG output at
various particle size show increasing SHG intensities
with increasing particle sizes (Figure 4). From this
measurement, it was observed that the grown crystal
proving the phase matching property.
3.4. Microhardness Measurement
Regarding mechanical properties, hardness testing pro-
vides useful information on the strength and deformation
characteristics of the material [13] and yield stress [14].
The hardness of a material is defined as the resistance it
offers to the motion of dislocations, deformations or da-
mage under an applied stress [15]. The chemical forces
in a crystal resist the motion of dislocations as it involves
the displacement of atoms. This resistance is the intrinsic
hardness of a crystal. As hardness properties are basically
related to the crystal structure of the material, hardness
studies are carried to understand the plasticity of the
crystal [16].
For the static indentation test, loads varying from 25 to
200 g were applied on the selected faces over a fixed
interval of 10 s. The indented impressions were ap-
proximately square. For each load (P), an average of di-
agonal lengths (d) of the indentation mark after unload-
ing was obtained using a calibrated micrometer attached
to the eyepiece of the microscope. The Vicker’s hardness
number (Hv) were calculated using the formula
2
2
1.8544P
Hvkg mm
d
 (3)
where, Hv is the Vicker’s microhardness number in
kg/mm2, P is the applied load in gm and d is the average
diagonal length of the indentation in mm2. The plot
drawn between the corresponding loads and hardness
values of LKS is shown in Figure 5. Maximum inden-
terload applied for grown crystal was 100 g, above this
load microcracks were observed around the impression
and hence readings were not taken for higher loads.
From this figure, it was observed that the hardness of the
title compound decreases with increase in load.
The relation between load and the size of the indenta-
tion is given by Meyer’s law [14] as
n
Pad (4)
where a is the arbitrary constant, n is the Meyer index (or
work-hardening coefficient). Using Equations (3) and (4),
we have

n2n
HvbP   (5)
where, b is a constant. The above relation indicates that
Hv should increases with P if n > 2 and decrease with P
when n < 2. The plot log P versus log d is a straight line
Figure 4. Phase matching curve of LKS.
Figure 5. Hardness behaviour of LKS.
and the work hardening coefficient “n” was found to be
3 - 3.6 which is greater than 2. On the basis of careful
investigation on various substances, Onitsch [17] and
Hanneman [18] had shown that the value of n comes out
to be 1 - 1.6 for hard materials and more than 1.6 for soft
ones. Thus the grown LKS crystal belongs to the soft
material category.
3.5. TGA/DTA Studies
Single crystal of LKS was subjected to TGA/DTA stud-
ies. A heating rate of 20˚C per minute was employed to
melt the mixture in the ceramic (Al2O3) crucible. The
initial mass of the material subjected to analysis was
44.600 mg. The final mass of the residue after the analy-
sis was 8% of the initial mass. The results of the thermal
analyses are represented by the curves in Figure 6. DTA
curve indicates that the grown crystal was stable upto at
260.1˚C (melting point). The TGA curve shows the loss
of weight of 61.87% at 260˚C, is may be due to liberation
of volatile substances like sulfur in the compound.
4. Conclusion
Single crystals of lithium potassium sulphate, an inorganic
nonlinear optical material, were grown by slow evapora-
tion method at room temperature. The single crystal

G. PASUPATHI, P. PHILOMINATHAN
907
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Figure 6. TG/DTA spectrum of LKS crystal.
X-ray diffraction analysis confirms the grown crystal
belongs to hexagonal system. The optical transmittance
study shows the crystal has good transmittance in the
entire visible-NIR region and wide band gap. The Kurtz
and Perry powder SHG method confirm the SHG effi-
ciency of LKS is 1.6 times that of KDP and phase-
matching property. The mechanical property of the mate-
rial was studied by Vicker’s hardness measurement.
[8] A. A. El-Fadl, M. A. Gaffar and M. H. Omar, “Absorp-
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5. Acknowledgements
One of the authors Mr. G. Pasupathi, thankful to the Uni-
versity Grant Commission, New Delhi for providing the
funding agency through Minor Research Project.
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