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109
CHAPTER 6
GROWTH AND CHARACTERIZATION OF
KDP CRYSTALS WITH POTASSIUM CARBONATE
AS ADDITIVE
6.1 INTRODUCTION
Busch and Scherrer reported the ferroelectric property of
industrially usable potassium dihydrogen phosphate (KH2PO4: KDP) single
crystal in 1935 for the first time (Busch and Scherrer 1935). That was the
beginning of large scale investigations into the properties of KDP family
single crystals and their commercial applications. KDP and its isomorphs are
representative of hydrogen bonded materials which possess important
nonlinear optical, piezoelectric, ferroelectric and electro-optic properties. In
the last seventy years, numerous studies on the growth and properties of these
crystals were reported. They have attracted the interests of many theoretical
and experimental researchers, probably because of their comparatively simple
structure and very fascinating properties associated with hydrogen bond
system involving large isotope effect. The properties of KDP include
transparency in a wide region of the optical spectrum, high resistance to
damage by laser radiation and relatively high nonlinear efficiency, relatively
low production cost in combination with reproducible growth to large size
and easy finishing (Zaitseva et al 2001).
The room temperature structure of KDP was determined by West
(1930) and it was later confirmed by Frazer and Pepinsky (1953)
110
(by X-ray diffraction studies) and Bacon and Pease (1953) (by neutron
diffraction studies). The unit cell dimensions of KDP are a = b = 7.434 (3) Å
and c = 6.945 (2) Å (Ubbelohde and Woodward 1947).
There are 32 atoms in the primitive unit cell of KDP, and each unit
cell is formed by four formula units. The KDP lattice is composed of two sets
of PO4 groups linked to each other by hydrogen bonds. These PO4 groups are
rotated 16° about the c axis, off the a axes. There are also two distinct
potassium ion positions. In particular, each phosphorus ion is surrounded by
four oxygen ions located at the vertices of a nearly regular tetrahedron
(contracted along the c axis by approximately 2%). Each PO4 group is linked
to four other PO4 groups, spaced c/4 apart along the c axis, by hydrogen
bonds. Figure 6.1 represents a projection of the tetragonal KDP lattice onto
the c plane. All atoms contained in one cell are illustrated by the solid lines.
Phosphorus ions lie at the height marked in the figure. The PO4 groups and
the potassium ions are arranged in such a manner that potassium and
phosphorus ions are spaced at a distance of c/2 along the c axis. The PO4
tetrahedra are also connected by potassium ions. Each potassium ion is
surrounded by eight oxygen ions with four of these oxygens belonging to
tetrahedra of neighboring columns. One set of four oxygen ions lies closer to
the potassium ion than the other set of four oxygen ions. The K–O bond
lengths are 2.89 Å and 2.82 Å respectively. A hydrogen bond involves one
upper and one lower oxygen atom of the neighboring PO4 units. As a result,
all hydrogen bonds lie in a plane nearly perpendicular to the c axis of the
crystal.
The heavy solid lines in Figure 6.1 indicate the unit cell. There are
16 oxygen ions and eight hydrogen ions contained within these solid lines.
There is one phosphorus ion contained within the solid lines and three
additional phosphorus ions on the solid lines, which are part of the unit cell.
111
The four potassium ions in the unit cell are not shown. These potassium ions
lie above and below the phosphorus ions. The upper oxygens of one
tetrahedron lie at the same level as the lower oxygens of two neighboring
tetrahedra, and the O–O distances are short, about 2.49 Å. These are obvious
positions for the hydrogen bond. The hydrogen ions link the tetrahedra into a
three-dimensional framework.
Figure 6.1 Room temperature tetragonal structure of KDP
6.2 ROLE OF IMPURITIES IN KDP CRYSTALS
Impurities are present in all crystallization processes. Usually
impurities are adventitious and undesirable but sometimes they are
intentionally added and then they are called additives. The study of the
crystallization behavior of KDP and the factors influencing its structural
perfection is still of great interest (Kuznetsov et al 1998). The growth and
quality of KDP crystals are affected by many factors such as additives,
solution supersaturation and pH value. In this context, the most important
112
factor is additives, which influence the growth kinetics (Rashkovich 1991,
Mullin 1993), the surface morphology of crystal faces (Owczarek et al 1990,
Rashkovich et al 1997) and most of the physical properties of the crystals.
According to Chernov and Rashkovich (1986), additives may have a
significant effect on crystal growth even at concentrations of 0.3 ppb by
weight. For this reason, the mechanisms of impurity trapping by KDP crystals
and the structure of the resulting defect centers have been intensely studied
from both theoretical and experimental part.
An impurity can suppress, enhance or stop the growth of crystal
completely. Impurities usually act on certain crystallographic faces. The
impurity effect depends on the impurity concentration, supersaturation,
temperature and pH of the solution. The effect of impurities on the growth
rate and habit of the crystals growing in the solution has been the subject of
many experimental and theoretical studies (Sangwal 1996, Kuznetsov et al
1998). Certain impurities cause inhibitions in crystal growth and this effect
was explained by the adsorption processes at different sites on the growing
surface. The extreme aspect of impurity adsorption film is that it blocks the
growing surface and makes the crystal growth impossible. Such impurities are
called tailor-made impurities.
Some research papers report an increase in the growth rate of
crystal faces in the presence of low concentrations of additives. Such growth
promoting effect of additives is called the catalytic effect of additives. The
growth promoting effect of KDP crystals is observed in the presence of
organic additives (Hottenhuis et al 1988, Rajesh et al 2002) as well as
inorganic additives (Seif et al 2001, Podder 2002). This contribution can be
attributed to the adsorption of additives on the surface of the nuclei resulting
in change of the nuclei surface free energy and nucleation mechanism. One of
113
the major growth inhibitors in the KDP system is the transition metal ions like
Fe and Cr which are inherently present. For the great majority of elements,
segregation coefficient k has a tendency to decrease with increasing impurity
concentration in the solution. The decrease is particularly strong in the second
group of cations: increasing the initial Co2+, Mn2+, and Ni2+ concentrations by
one to two orders of magnitude reduces k by one to three orders of magnitude.
However, this effect, typical of metal (M2+) cations, is much weaker in the
case of trivalent cations, which occupy the same interstitial position in the
structure of KDP (Efremova et al 2004). Kannan et al (2006) reported that
optimal addition of trivalent La3+ ions considerably prevents these bivalent
ions from entering into the crystal lattice and results in reduced defects and
dislocations.
The capture of an impurity in a crystal during its growth from a
solution is the combined effect of various factors: the solubility of the host
and the impurity phase, character of the mother phase, interaction between the
host and the impurity molecules, relative size of impurity and host ions,
similarity in the crystallographic structure of the two phases, relative size of
the impurity and the host ions and other crystallization conditions (Kirkova
et al 1996).
New technical tasks like high power laser systems for nuclear
fusion have a great demand for very large size crystals. For obtaining large
size KDP plates, increasing the growth rate of the crystals is a vital factor. So
it is necessary to study the dynamics of the medium’s effect on the growth,
and to understand the relation of the growth conditions, the solution stability,
growth mechanism and properties of the crystals. The use of special additives
is an effective way to accelerate the growth rate. The beneficial effect of
additives on the growth process and properties of crystals has been applied
114
(Srinivasan et al 2000, Li et al 2005, Jayaprakasan et al 2007). The most
efficient additives are reagents with metal ions that have the same properties
as that of bulk solutions which can change the properties of solution, such as
viscosity, surface tension etc., without deteriorating the optical qualities of
crystals.
Podder (2002) reported that the presence of KCl in the growth
medium of KDP crystals is found to suppress the metal ion impurities to a
large extent and increases the growth rate. The increase in the quality of the
KDP crystal in presence of KCl is due to the complexation of trace metal ion
impurities in solution by Cl ion. These complex metal impurities cannot get
into the crystal lattice. The doped crystal shows better nonlinear optical
properties than pure KDP. Li et al (2005, 2005 a) applied this new technology
in the rapid growth of KDP crystals. They had grown doped KDP crystal of
size of 54 × 54 × 42 mm3 with growth rate more than 20 mm/day. The X-ray
curves recorded for the crystals grown from 5 mol% KCl added solutions
proved that this KCl incorporation does not affect the crystalline perfection
and its quality.
In order to identify other useful additives, we have chosen
potassium carbonate as an additive in the present investigation. Potassium
carbonate (K2CO3) added KDP crystals were grown from the aqueous solution
with a simple apparatus that can be applied in certain forced convection
configurations to maintain a higher homogeneity of the solution. With the aim
of improving the quality of KDP crystals with better nonlinear optical
properties for both academic and industrial uses, an attempt has been made in
this present work to grow the KDP crystals by doping it with divalent anionic
soluble impurity potassium carbonate in different molar ratios. The effect of
seed rotation on the crystalline perfection is also studied.
115
6.3 DETERMINATION OF SOLUBILITY AND METASTABLE
ZONE WIDTH
KDP, K2CO3 (GR grade) from Merck and Millipore water of
resistivity 18.2 M cm were used for the studies. No further purification was
done. Solubility of pure and 5 mol% doped KDP were determined by
gravimetric analysis for different temperatures (30–50 C) with the interval of
5 oC. Metastable zone width is an essential parameter for the growth of large
size crystals from a solution, since it is the direct measure of the stability of
the solution in its supersaturated region. Metastable zone width is an
experimentally measurable quantity which depends on number of factors,
such as stirring rate, cooling rate of the solution and presence of additional
impurities (Nyvlt et al 1970, Sangwal 1989, Zaitseva et al 1995). The
metastable zone width studies of pure KDP and K2CO3 added KDP solutions
were measured by the polythermal method (Nyvlt et al 1970). The KDP
solution (500 ml) saturated at 30 C was prepared according to the solubility
diagram and filtered. Two similar beakers with 250 ml solution each were
used, one containing pure KDP solution and the other 5 mol% K2CO3 was
added. Then pure and K2CO3 added KDP solutions were kept in a CTB with
cooling facility and the solutions were stirred continuously for a period of 6 h
for stabilization. The metastable zone width was determined for temperatures
35, 40, 45 and 50 C. Several nucleation runs (7–9 times) were carried out
under controlled conditions and reproducible results with the accuracy of
±0.25% were obtained. The metastability limit of K2CO3 added solution is
shown in Figure 6.2 in comparison with the pure system.
116
26
28
30
32
34
36
38
40
42
44
20 25 30 35 40 45 50 55 60
Temperature (oC)
Co
nce
ntr
atio
n (
g/1
00
CC
)
Saturation curve (Pure KDP)
Metastability limit (Pure KDP)
Metastability limit (KDP+5 mol% K2CO
3)
Figure 6.2 Solubility and metastability limit curves of pure and K2CO3
added KDP solutions
It is obvious from the figure that the zone widths for both the
solutions decrease as the temperature increases. At the same time, the addition
of K2CO3 enhances the metastable zone width of KDP solutions for all the
temperatures, and makes the KDP solution more stable. Also, it was observed
that during the experiment the number of tiny crystals formed by spontaneous
nucleation was appreciably reduced in the case of the K2CO3 added solution
compared with the pure one. The change in pH of the KDP solution by the
addition of dopant K2CO3 has a major role in the suppression of spontaneous
nucleation. The addition of K2CO3 can make KDP solution more stable and
increase the growth rate of the KDP crystal under higher supercooling. At the
same time, the addition of K2CO3 enhances the zone width of the KDP
solution for all the temperatures.
6.4 CRYSTAL GROWTH
KDP crystal doped with K2CO3 was grown from aqueous solution
with a simple apparatus that can be applied in certain forced convection
117
configurations to maintain a higher homogeneity of the solution. This
apparatus consists of seed rotation controller coupled with a stepper motor,
which is controlled by using a microcontroller based drive. This controller
rotates the seed holder in the crystallizer. The seed crystal is mounted on the
center of the platform made up of acrylic material and is fixed into the
crystallizer. The seed mount platform stirs the solution very well and makes
the solution more stable, which resulted in better crystal quality. The
schematic diagram of the seed rotation controller designed for low
temperature solution growth method is shown in Figure 6.3. The uniform
rotation of the seed is required to avoid stagnant regions or re-circulating
flows, otherwise inclusions in the crystals will be formed due to
inhomogeneous supersaturation in the solution (Fu et al 2000).
Figure 6.3 Schematic diagram of the seed rotation controller
The crystal growth is carried out in a 5000 ml standard crystallizer
used for conventional crystal growth by the method of temperature reduction.
The temperature of solution in the crystallizer was controlled using a CTB
and the temperature fluctuations are less than 0.01 oC. The saturation
118
temperature was 50 oC. The solution was filtered by filtration pump and
Whatman filter paper of pore size 11 µm to remove extraneous solid and
colloidal particles, which may act as the centers of spontaneous nucleation
during growth. After filtration, the solution was overheated at 70 oC for 24 h.
This duration of overheating was found to be effective to destroy the molecule
clusters existing in the solution and to make the solution stable against
spontaneous nucleation under a high supersaturation (Zaitseva et al 1995,
Nakatsuka et al 1997). After overheating, the temperature of the solution was
reduced slightly above the saturation point and seed crystal was mounted on
the platform. The rotation rate of the platform with the crystal was 40 rpm.
From the saturation point, the temperature was decreased at 0.1 oC/day at the
beginning of the growth. As the growth progressed the temperature lowering
rate was increased. After reaching the room temperature, an optically
transparent KDP single crystal of size 45 × 25 × 15 mm3 was obtained. The
as–grown K2CO3 doped (5 mol%) KDP crystal is shown in Figure 6.4.
Figure 6.4 Photograph of K2CO3 doped KDP crystal
119
For various characterization techniques, pure and K2CO3 doped
(in different concentrations viz., 1, 5 and 10 mol%) KDP crystals were grown
by slow cooling method with stirring under identical conditions.
6.5 POWDER X-RAY DIFFRACTION STUDIES
The X-ray powder diffraction analysis was used to confirm the
physical phase of the product. Grown crystals were ground using an agate
mortar and pestle in order to determine the crystal phases by XRD. The XRD
analysis (SAIFERT, 2002 DLX model) was performed using a tube voltage
and current of 40 kV and 30 mA respectively. Figure 6.5 shows X-ray powder
diffraction pattern of 5 mol% K2CO3 doped KDP compared with pure KDP.
As seen in the figure, X-ray powder diffraction patterns of pure and doped
KDP crystal are identical.
10 20 30 40 50 60 70
Inte
nsity (
cp
s)
Diffraction angle 2
(200
)(2
00)
(21
1)
(211
)
(11
2)
(112
)(2
20)
(220
)
(301)
(30
1)
(321
)(3
21)
Pure KDP
KDP+5 mol% K2CO
3
Figure 6.5 Powder XRD patterns of pure and doped KDP
120
6.6 HRXRD ANALYSIS
It is known that in the presence of suitable additives or dopants
which can make complexes with the impurities present in the solution during
growth, complex or block structures form on the surface of the crystal, which
in turn helps to improve the crystalline perfection (Bhagavannarayana et al
2006). To know the effect of seed rotation on the crystalline perfection and in
particular to see whether there is any effect of seed rotation on such complex
structure due to doping of K2CO3, two typical specimens grown under the
same conditions (with slow cooling, etc., as mentioned in crystal growth
section) and nearly the same size but with and without seed rotation have
been chosen and subjected to high-resolution X-ray diffractometry.
Figure 6.6 shows the high-resolution diffraction curve (DC)
recorded for the K2CO3 doped (5 mol%) KDP crystal grown without seed
rotation. As seen in the figure, the curve does not contain a single diffraction
peak. The solid line, which follows well with the experimental points (filled
circles), is the convoluted curve of four peaks using the Lorentzian fit. At the
first instance, the coexistence of small additional peaks along with the main
peak indicates the possibility of the presence of very low-angle structural
grain boundaries (Bhagavannarayana et al 2005) in the crystal. However, in
case of KDP crystals this type of complex structure that resembles the
existence of a mixture of very low-angle boundaries is seldom found.
Therefore, it is more appropriate to interpret their origin due to impurity
complexes formed by the impurities present in the solution with the dopant
ions, which are confined to the surface of the sample as observed in the
studies on zinc tris thiourea sulfate (ZTS) crystals grown in the presence of
ethylene diamine tetra acetic acid (EDTA) (Bhagavannarayana et al 2006).
The lesser intensity of the satellite peaks of the DC confirms the confinement
of the defect structure to the surface.
121
-300 -200 -100 0 100 200 3000
200
400
600
800
KDP+K2CO
3
(200) Planes
MoK1
(+,-,-,+)
56"
40" 39"15"
45"
18"93"
Gla
ncin
g a
ngle
[arc
s]
Glancing angle [arc s]
Diffr
acte
d X
-ra
y inte
nsity
[c/s
]
Figure 6.6 Diffraction curve recorded for a typical K2CO3 doped KDP
single crystal for (200) diffracting planes
Figure 6.7 shows the DC recorded for a typical specimen grown
under the same conditions as that of Figure 6.6 but with an additional
experimental condition of seed rotation during growth by the optimum
rotation (40 rpm) of the seed crystal. The curve is extremely sharp having the
full-wave at half-maximum (FWHM) of 2.7 arc s as expected for a perfect
crystal according to the plane wave dynamical theory of XRD (Batterman et
al 1964). Absence of additional peaks and the very sharp DC shows that the
crystalline perfection of the specimen crystal is extremely good. This clearly
shows that the specimen does not contain any complex structure, which is
otherwise observed when there is no seed rotation (Figure 6.6). The high
reflectivity ( 80%) and the very small value of FWHM indicate that even the
unavoidable point defects like interstitials and vacancy defects (Lal et al 1989)
are also extremely low. This comparative study of K2CO3 doped KDP crystals
with and without seed rotation reveals that the seed rotation helps to a
significant extent in improving the crystalline perfection. It seems that due to
seed rotation, the complex layers containing impurities and dopants are not
122
allowed to stay on the surface of the growing crystal due to centripetal force
and also helps to keep a homogeneous saturated solution at the liquid–solid
interface and there by the crystal grown under seed rotation condition has got
an excellent crystalline perfection as evident from the very sharp and single
DC of Figure 6.7.
-50 0 500
1000
2000
3000
4000
5000 KDP+K2CO
3
(200) Planes
MoK1
(+,-,-,+)
2.7"
Diffr
acte
d X
-ra
y in
ten
sity [
c/s
]
Glancing angle [arc s]
Figure 6.7 Diffraction curve recorded for a typical K2CO3 doped KDP
single crystal grown by optimum seed rotation for (200)
diffracting planes
6.7 FTIR SPECTRAL STUDIES
The structural change during crystallization has been studied
by FTIR spectroscopy. The FTIR spectra were recorded in the region
400–4000 cm-1 using a Perkin-Elmer FTIR Spectrum RXI spectrometer by
KBr pellet technique. Figure 6.8 shows the FTIR spectra of the pure KDP and
KDP doped with K2CO3 in different concentrations.
123
4000 3500 3000 2500 2000 1500 1000 500
Tra
nsm
itta
nce
(%
)
Wavenumber (cm-1)
Pure KDP
KDP+1 mol% K2CO
3
KDP+5 mol% K2CO
3
KDP+10 mol% K2CO
3
Figure 6.8 FTIR spectra of pure KDP and KDP doped with K2CO3
In the spectrum of KDP, there is a broadband in the higher energy
region due to O–H stretching vibration of KDP and water. Hydrogen bonding
with in the crystal is suggested to be the cause for broadening. Presence of
water is supported by its bending vibrations occurring at the band 1630 cm-1
(Banwell et al 1994). The bands below 1300 cm-1 are due to PO4 vibrations.
The FTIR spectrum of 1 mol% K2CO3 added KDP shows slight decrease in
intensity for all the peaks. It is attributed to neutralization of acidic OH group
of KDP by K2CO3. In the spectra of 5 and 10 mol% of K2CO3 added KDP,
there is drastic change for the band appears just below 1000 cm-1 in the
spectrum of KDP. Hence the OH groups might be much more neutralized
than in 1 mol% doped KDP. Neutralization of OH groups by K2CO3 might be
the cause for enhanced NLO property, as electron delocalization to be much
more enhanced than pure KDP. Hence the added K2CO3, taken as dopant,
enhance the NLO property by neutralization.
124
6.8 DIELECTRIC ANALYSIS
Crystals with high transparency and large surface defect-free
(i.e. without any pit or crack or scratch on the surface, tested with a traveling
microscope) grown by slow cooling method were used for the dielectric
measurements. The size of the crystals was 6 × 6 × 2 mm3. Samples were
coated with good quality graphite to obtain a good conductive surface layer.
The dielectric constant ( r) and dielectric loss factor (tan ) of pure and doped
KDP crystals were measured using the conventional parallel plate capacitor
method with a fixed frequency (f) of 1 kHz using Agilent 4284A LCR meter
at various temperatures ranging from 313 to 423 K. The measurements were
done on a–b directions of the crystals. The samples were annealed up to
423 K to remove water molecules if present. The AC conductivity ( ac) was
calculated using the relation:
ac = o r tan (6.1)
where o is the permittivity of free space (8.85 x 10–12 F/m) and ( = 2 f) is
the angular frequency.
The experiments were repeated for several times (5–7) under
controlled conditions and the standard deviation was determined.
Reproducible results with the accuracy of ± 2% were obtained. The values of
r, tan and ac obtained in the present study are shown in Figures 6.9, 6.10
and 6.11. The r values obtained in the present study are of the same order
with those obtained by the previous authors for the pure and certain other
impurity added KDP crystals (Varma et al 1983). It is obvious from the
figures that the values of r, tan and ac increase with the increase in
temperature for all impurity concentrations considered in the present study.
The dielectric constant, dielectric loss and AC conductivity depends strongly
on temperature (Askeland et al 2003, Salman 2004).
125
313 323 333 343 353 363 373 383 393 403 413 423
6
8
10
12
14
16
18
20
22
24
Die
lectr
ic c
onsta
nt
Temperature (K)
Pure KDP
KDP+1 mol% K2CO
3
KDP+5 mol% K2CO
3
KDP+10 mol% K2CO
3
Figure 6.9 Plot of dielectric constant versus temperature
313 323 333 343 353 363 373 383 393 403 413 4230.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
Die
lectr
ic lo
ss
Temperature (K)
Pure KDP
KDP+1 mol% K2CO
3
KDP+5 mol% K2CO
3
KDP+10 mol% K2CO
3
Figure 6.10 Plot of dielectric loss versus temperature
126
313 323 333 343 353 363 373 383 393 403 413 4231
2
3
4
5
6
7
8
9
10
acx 1
0-7 m
ho
/m
Temperature (K)
Pure KDP
KDP+1 mol% K2CO
3
KDP+5 mol% K2CO
3
KDP+10 mol% K2CO
3
Figure 6.11 Plot of conductivities versus temperature
From Figure 6.9, it is observed that the r values do not vary
systematically with impurity concentration. However, it can be seen that these
values are minimum for the K2CO3 added KDP with impurity concentration
of 5 mol%. The values increase when the impurity concentration increases
further. As the samples were annealed before making measurements this may
not be due to adsorbed water. Increase in K2CO3 concentration may lead to
high density of induced bulk defect states due to competition in getting the
interstitial sites for the K2CO3 molecules to occupy. Decrease in K2CO3
concentration may lead to high density of induced bulk defect states due to
availability of unoccupied interstitial sites. A 5 mol% may be the proper
concentration for the K2CO3 molecules to occupy the available interstitial
sites in the KDP crystal structure. This may be the reason for the complex
situation observed with the above dielectric parameters in the present study.
However, it is interesting to note that 5 mol% K2CO3 addition to KDP leads to
a significant reduction of r value and consequently leads to low r value
dielectrics, a knowledge gaining importance of late.
127
6.9 OPTICAL TRANSMISSION STUDIES
Optical transmission spectra were recorded for the samples
obtained from pure as well as doped crystals grown by slow cooling method.
The spectra were recorded in the wavelength region from 200 to 1000 nm
using Lambda 35 spectrophotometer. Crystal plates with 2 mm thickness were
used for the study. The reported value of the optical transparency for KDP is
from 190 to 1500 nm (Dmitriev et al 1991). The UV–vis–NIR spectra
recorded for pure and different molar ratios of K2CO3 added KDP crystals are
shown in Figure 6.12. It is clear from the figure that the crystal has sufficient
transmission in the entire visible and IR region. The optical transparency of
the KDP crystal is increased by the addition of 5 mol% K2CO3 additive. The
addition of the dopant K2CO3 in the optimum conditions to the solution is
found to suppress the inclusions and improve the quality of crystal with
higher transparency.
200 400 600 800 1000 12000
20
40
60
80
100
Tra
nsm
issio
n (
%)
Wavelength (nm)
Pure KDPKDP+1 mol% K
2CO
3
KDP+5 mol% K2CO
3
KDP+10 mol% K2CO
3
Figure 6.12 UV–vis–NIR transmission spectra of pure and K2CO3 added
KDP crystals
128
6.10 NLO PROPERTY
Kurtz and Perry powder technique remains an extremely valuable
tool for initial screening of materials for SHG. The fundamental beam
1064 nm from Q-switched Nd: YAG laser (Pro Lab 170 Quanta ray) is used
to test the SHG property of the KDP crystal by using Kurtz and Perry
technique. Pulse energy of 4 mJ/pulse and pulse width of 10 ns and repetition
rate of 10 Hz is used. Geometry of 90o was employed. The fundamental beam
was filtered by using IR filter. The Photo multiplier tube (Philips Photonics)
was used as the detector. It was observed that the measured SHG efficiency of
5 mol% K2CO3 added KDP crystal was 1.32 that of pure KDP crystal. In the
K2CO3 added KDP crystal, the additive neutralizes the OH group of KDP as
stated in the FTIR analysis. Neutralization of OH groups by K2CO3 might be
the cause for enhanced NLO property, as electron delocalization to be much
more enhanced than pure KDP.
6.11 LASER INDUCED DAMAGE THRESHOLD STUDIES
One of the most important considerations in the choice of a material
for nonlinear optical applications is its optical damage tolerance. Because of
the high optical intensities involved in nonlinear processes, the nonlinear
materials must be able to withstand high power intensities. In the present
study, an actively Q-switched diode array side pumped Nd: YAG laser is used
for the laser induced damage threshold studies. Active Q-switching is done by
an acousto-optic Q-switch. The pulse width and the repetition rate of the laser
pulses are 65 ns and 10 KHz respectively, at 1064 nm radiation. For this
measurement 1.64 mm diameter beam is focused onto the sample with a
10 cm focal length lens. The beam spot size on the sample is 0.51 mm. Well-
polished samples with clean surface were chosen for the present study. The
calculated laser induced damage threshold of pure KDP crystal is 6.84 J/cm2
while that of 5 mol% K2CO3 added KDP is 7.26 J/cm2. Optimal addition of
129
dopant slightly increases the damage threshold, which could be attributed to
the better crystallinity in the bulk of the crystal.
6.12 PIEZOELECTRIC MEASUREMENTS
The piezoelectric property of a crystal is related to the polarity of
the material (Ge et al 2008). The piezoelectric studies were made using
piezometer system. A precision force generator applied a calibrated force
(0.25 N) which generated a charge on the piezoelectric material under test.
The output was measured directly from oscilloscope which gives the
d33 coefficient in units of pC/N. Without poling the crystal, the piezoelectric
measurements were carried out for the grown crystals. The obtained
piezoelectric coefficient (d33) value for pure KDP crystal is 0.33 pC/N and for
5 mol% K2CO3 doped KDP crystal is 0.53 pC/N. Thus d33 value of 5 mol%
K2CO3 doped KDP crystal was found to be 1.6 times higher than that of pure
KDP crystal. Higher crystalline perfection may be the reason for the same.
6.13 CONCLUSIONS
A new additive potassium carbonate was added to KDP in different
molar concentrations and crystals were grown by slow cooling method. The
nucleation studies show that the addition of K2CO3 enhances the zone width
of KDP solution for all temperatures. The powder X-ray diffraction curves
recorded for pure and doped crystals are identical. The HRXRD results reveal
that the 5 mol% K2CO3 added KDP single crystals with optimum seed
rotation give good crystallinity. Dielectric studies indicate that 5 mol% K2CO3
addition to KDP leads to low r value dielectrics, which is gaining more
importance in microelectronics industry. The transmission spectrum reveals
that the crystal has sufficient transmission in the entire visible and IR region.
A positive effect is observed in the nonlinear optical properties like powder
SHG and laser damage threshold. The piezoelectric coefficient value for
doped crystal is higher than the pure one. This study will help the growth of
high quality large size KDP single crystals.
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