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.