chapter 8 growth and characterization of...
TRANSCRIPT
164
CHAPTER 8
GROWTH AND CHARACTERIZATION OF PURE, COBALT,
NICKEL, MANGANESE AND PARANITROPHENOL DOPED
POTASSIUM DIHYDROGEN PHOSPHATE (KDP)
SINGLE CRYSTALS
8.1 INTRODUCTION
Ferroelectrics with hydrogen bonds, due to relatively high nonlinear
efficiency and dielectric permittivity, huge piezoelectric effect and pyroelectric
properties, the possibility of the spontaneous polarization reorientation in a relatively
small field are successfully implemented in a wide class of optoelectronic devices and
sensor technology, nonlinear optical and information optical storage, etc.
Potassium Dihydrogen Phosphate (KH2PO4) single crystals have high laser
damage threshold, large nonlinear optical coefficients, good structural quality and
mechanical properties. KDP crystals have several device applications. The
electro-optic effect in KDP is used to obtain phase and amplitude modulations. The
acousto-optic tunable filters have been developed using KDP [235 - 239]. For the
inertial confinement fusion (ICF) experiments, the performance of large aperture
switches based on KDP have been assessed for high power laser experiments [240].
Rapid growths of large size (40 - 55 cm) KDP crystals as well as rapid growth of
KDP crystals with additives [241] have facilitated to obtain perfect KDP crystals for
device application on large scale.
Huge interest to KDP crystals is caused by their unique physical properties and high manufacturability. In particular, KDP crystals which possess extremely high optical and structural perfection make it possible to produce elements for doubling
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and tripling of laser radiation frequency, electro-optic switches and modulators with an aperture of several tens and hundreds of square centimeters to be used, e.g. in laser fusion facilities. These crystals are distinguished by high efficiency of non-linear conversion and a wide optical transparency range which extends far (up to 176 nm) to the short-wavelength region of the spectrum [243]. The unit cell and morphology representations for KDP are presented in Figure 8.1 and Figure 8.2 respectively.
Figure 8.1 Unit Cell of KDP crystals [151]
Figure 8.2 Morphology of KDP crystal [242]
The effect of dopants of different nature has been studied by different
research groups. The thermal, FTIR and SHG efficiency studies of L-arginine doped
KDP crystals were investigated by Parikh et al [244]. It was observed that the SHG
efficiency increases with increasing concentrations of L-arginine. Parikh et al., [245]
{101} Pyramidal face
{100} Prismatic face
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have grown L-alanine doped KDP crystals. It was found that the dielectric constant
and dielectric loss values of L-alanine doped KDP crystals were lower than pure KDP
crystals. Guohui et al., [241] have grown KCl and EDTA doped KDP crystals by
rapid growth technique. Higher NLO efficiency and laser damage threshold were
observed when the crystals were annealed. Pure and L-Lysine doped KDP crystals
were grown by Kanagathara et al [246]. It was found that the transmittance percentage
is increased for the doped KDP crystals. Rajesh et al., [247] have grown KDP crystals
with EDTA, urea and thiourea as additives. It was observed that the microhardness
increases with organic additive system. Muley et al., [248] have studied the growth of
L-arginine and L-alanine doped KDP crystals. Modifications in the lattice parameters
and improvement in SHG efficiency was observed. Prasanyaa et al., [249] have grown
L-arginine trifluoroacetate (LATF) doped KDP single crystals. The enhancement in
the transmittance of grown KDP with the addition of LATF at different ratios was
determined by UV-Visible spectral analysis.
Suresh Kumar et al., [239] have studied the effect of L-arginine,
L-histidine and glycine on the growth of KDP single crystals. Enhanced SHG
efficiency was observed in the case of doped crystals. Pritula et al., [243] have
investigated the optical, structural and microhardness properties of KDP crystals
grown from urea doped solutions. It was found that the laser damage threshold value
increases by 25% on doping. Kumaresan et al., [250] have grown metal ions and dyes
doped KDP single crystals. It was observed that dye doping improves the nonlinear
optical properties of the grown crystals. Goma et al., [251] have studied the variation
of dielectric parameters when urea is added in KDP. It was determined that the
inclusion of urea leads to the low value of dielectric permittivity.
Dhanraj et al., [252] have investigated the effect of potassium acetate and
potassium citrate on the nucleation studies of KDP crystals. It was found that the
addition of potassium enhances the crystalline quality. Husaini et al., [253] have
performed the high frequency dielectric study of pure and thiourea doped KDP
crystals. It was found that thiourea doped KDP crystals have low dielectric constant
values. Shirsat et al., [240] have studied the influence of lithium ions on the NLO
167
properties of KDP single crystals. Enhancement of SHG efficiency after the addition
of lithium ions was observed.
The present investigation deals with the growth of pure, cobalt, nickel,
manganese and paranitrophenol doped KDP crystals by slow solvent evaporation
technique. The grown crystals have been subjected to powder XRD, FTIR,
UV-Vis-NIR, TGA/DSC, laser damage threshold, second harmonic generation,
dielectric and A.C. Conductivity studies. The results are presented here.
8.2 GROWTH OF PURE AND DOPED KDP SINGLE CRYSTALS
Potassium dihydrogen phosphate (KDP) of analytical grade (AR) was procured from Loba Chemicals. Slow evaporation method was used to grow the KDP crystals. The analytical grade KDP was further purified by repeated recrystallization process using water as solvent.
Generally to grow bulk crystals from solution using isothermal solvent evaporation technique, it is desirable to select a solvent which is moderately soluble. The solubility of KDP crystal was determined by many researchers in many solvents like water, acetone, dimethyl formamide, acetic acid, methanol, ethanol, ethyl acetate. Guohui et al., [241] have studied the solubility curve and metastability limit curve of KCl and EDTA doped KDP crystals. Muley et al., [248] have studied the solubility curve for pure, L-arginine and L-alanine doped KDP crystals. It was observed from these investigations that the solubility increases with increase in temperature.
In the present work, water was used as the solvent for the growth of KDP crystals. The purified salt of KDP was dissolved in water and stirred continuously for 3 hours using a magnetic stirrer to obtain a homogeneous mixture. The solution was filtered twice using ultra micro pore filter paper. The resulting solution is kept in a beaker and covered for controlled evaporation. After a period of 5 days, small crystals were obtained and these crystals were suspended in the mother solution to get good quality crystals as shown in the Figure 8.3. For doped crystals, the percentage of dopants was 1 mole %. The calculated amount of cobaltous chloride hydrate, nickel chloride hexahydrate, manganese sulphate, paranitrophenol were dissolved in 50 ml of water and then mixed with the mother solution. The solutions of the doped KDP crystals
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were filtered and good seed crystals were obtained over a period of 5 days. These seed crystals were suspended in the mother solution to get good quality crystals. The photographs of the pure and the doped KDP crystals are presented in Figure 8.3 (a) to 8.3 (e) respectively.
(a) (b)
(c) (d)
(e)
Figure 8.3 Photograph of (a) pure (b) cobalt (c) nickel (d) manganese and
(e) paranitrophenol doped KDP crystals.
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8.3 RESULTS AND DISCUSSION
8.3.1 Powder X-ray diffraction studies
X-ray powder diffraction (XRD) was performed on the grown crystals, to
study the effect of doping in KDP crystals with cobalt, nickel, manganese,
paranitrophenol. The powder XRD pattern was recorded using powder SEIFERT
X-ray diffractometer with CuK 1 Radiation ( = 1.5406 Å).The powdered samples
were scanned over the range 20o - 80o at a rate of 1o per minute. The powder XRD
data obtained were indexed with the help of UNIT CELL program.
The lattice parameters of the pure and doped crystals are shown in
Table 8.1. The indexed powder data for the pure and doped crystals are presented in
Table 8.2 – Table 8.6. The peaks in the XRD patterns which were obtained in the
present work, are slightly shifted due to the addition of dopants which indicates that
the dopants have entered in to the lattice of the crystal. This is the cause for the
change in the lattice parameters and cell volume on doping. A similar effect was
observed by Prasanyaa et al., [249] by doping KDP crystal with
L-arginine trifluroacetate. Parikh et al., [244, 245] and Kumaresan et al., [250] have
studied the powder X-ray diffraction for the doped KDP crystals. The indexed
powder pattern is shown in Figure 8.4.
Table 8.1 Lattice parameters of pure and doped KDP crystals by powder
X-ray diffraction
Crystal a (Å) b (Å) c (Å) V (Å)3
Pure KDP 7.436 7.436 6.979 385.92
Cobalt doped 7.427 7.427 6.936 382.65
Nickel doped 7.408 7.408 6.992 383.70
Manganese doped 7.431 7.431 6.955 384.10
Paranitrophenol doped 7.449 7.449 6.936 384.91
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20 30 40 50 60 70 80-200
0200400600800
1000
( 4 0
4)
( 5 1
2)
( 2 2
4)
( 3 2
3)
( 4 2
0)
( 3 1
2)
( 1 0
3)
( 3 0
1)
( 1 1
2)
( 2 1
1)
( 2 0
0 )
Two Thetha(Degrees)
Inte
nsity
(cps
)
-200020040060080010001200140016001800
( 4 0
4)
( 5 1
2)
( 2 2
4)
( 3 2
3)
( 2 0
4)
( 3 1
2)
( 1 0
3)
( 3 0
1)
( 2 0
2)
( 2 2
0)
( 1 1
2)
( 2 0
0 )
0500
100015002000
( 4 0
4)
( 2 3
4)
( 4 0
3)
( 3 3
2)
( 3 1
2)
( 1 0
3)
( 3 0
1)
( 2 2
0)
( 1 1
2)
( 2 0
0 )
-1000
100200300400500
( 4 0
4)
( 5 1
2)
( 3 1
4)
( 3 3
2)
( 0 0
4)
( 3 1
2)
( 1 0
3)
( 3 0
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2)
-2000
200400600800
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( 4 0
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( 2 0
0 )
(1 1
5)
( 2 2
4)
( 2 3
3)
( 2 0
4)
( 4 2
0)
( 3 1
2)
( 3 2
1)
( 1 0
3)
( 3 0
1)
( 2 2
0)
( 1 1
2)
( 2 1
1) (a)
(b)
(c)
(d)
(e)
( 2 0
0 )
Figure 8.4 Powder X-ray diffraction pattern of (a) pure (b) cobalt (c) nickel
(d) manganese and (e) paranitrophenol doped KDP crystals
In
tens
ity (c
ps)
Two Theta (degrees)
171
Table 8.2 Indexed Powder XRD data of Pure KDP crystal
S. No. h k l d(obs)
(Å) d(calc)
(Å) Diff (d)
(Å) 2 (obs)
(deg) 2 (calc)
(deg) Diff (2 )
(deg) 1 2 0 0 3.71079 3.71804 -000725 23.960 23.913 0.047 2 2 1 1 2.99553 3.00213 -0.00660 29.800 29.733 0.067 3 1 1 2 2.91159 2.90755 0.00404 30.680 30.724 -0.044 4 2 2 0 2.61805 2.62905 -0.0110 34.220 34.072 0.148 5 3 0 1 2.33803 2.33576 0.00228 38.470 38.509 -0.039 6 1 0 3 2.21597 2.22028 -0.00431 40.680 40.598 0.082 7 3 2 1 1.97904 1.97785 0.00119 45.810 45.839 -0.029 8 3 1 2 1.95285 1.95007 0.00279 46.460 46.530 -0.070 9 4 2 0 1.66365 1.66276 0.00090 55.160 55.192 -0.032 10 2 0 4 1.58056 1.57952 0.00104 58.330 58.372 -0.042 11 2 3 3 1.54307 1.54327 -0.00020 59.890 59.881 0.009 12 2 2 4 1.45474 1.45378 0.00097 63.940 63.988 -0.048 13 1 1 5 1.34827 1.34911 -.00084 69.680 69.630 0.050 14 4 0 4 1.27194 1.27222 -0.00029 74.540 74.520 0.020
Table 8.3 Indexed Powder XRD data of Cobalt doped KDP crystal
S.No h k l d(obs) (Å)
d(calc) (Å)
Diff (d) (Å)
2 (obs) (deg)
2 (calc) (deg)
Diff (2 ) (deg)
1 2 0 0 3.62730 3.71380 -0.08650 24.520 23.940 0.580 2 1 1 2 2.83676 2.89404 -0.05728 31.510 30.871 0.639 3 3 0 1 2.31203 2.33176 -0.01973 38.920 38.578 0.342 4 1 0 3 2.19429 2.20755 -0.01326 41.100 40.842 0.258 5 3 1 2 1.94063 1.94476 -0.00413 46.770 46.665 0.105 6 0 0 4 1.73690 1.73402 0.00288 52.650 52.744 -0.094 7 3 3 2 1.56977 1.56286 0.00692 58.770 59.056 -0.286 8 3 1 4 1.39462 1.39504 -0.00042 67.050 67.027 0.023 9 5 1 2 1.34473 1.34301 0.00172 69.890 69.993 -0.103
10 4 0 4 1.27296 1.26735 0.00561 74.470 74.856 -0.386
172
Table 8.4 Indexed Powder XRD data of Nickel doped KDP crystal
S. No. h k l d(obs)
(Å) d(calc)
(Å) Diff (d)
(Å) 2 (obs)
(deg) 2 (calc)
(deg) Diff (2 )
(deg) 1 2 0 0 3.65964 3.72377 -0.06413 24.300 23.875 0.425 2 1 1 2 2.87592 2.89759 -0.02167 31.070 30.832 0.238 3 2 2 0 2.59380 2.63310 -0.03930 34.550 34.018 0.532 4 3 0 1 2.33745 2.33747 -0.00002 38.480 38.480 0.000 5 1 0 3 2.21649 2.20927 0.00722 40.670 40.809 -0.139 6 3 1 2 1.94063 1.94870 -0.00807 46.770 46.565 0.205 7 4 0 0 1.88267 1.86188 0.02078 48.300 48.874 -0.574 8 3 3 2 1.56977 1.56639 0.00339 58.770 58.910 -0.140 9 2 2 4 1.44928 1.44880 0.00048 64.210 64.234 -0.024
Table 8.5 Indexed Powder XRD data of Manganese doped KDP crystal
S.
No. h k l d(obs) (Å)
d(calc) (Å)
Diff (d) (Å)
2 (obs) (deg)
2 (calc) (deg)
Diff (2 ) (deg)
1 2 0 0 3.72612 3.71570 0.01042 23.860 23.928 -0.068 2 1 1 2 2.89593 2.90004 -0.00411 30.850 30.805 0.045 3 2 2 0 2.62624 2.62739 -0.00115 34.110 34.095 0.015 5 3 0 1 2.32467 2.33355 -0.00888 38.700 38.547 0.153 6 1 0 3 2.21649 2.21319 0.00330 40.670 40.733 -0.063 7 3 1 2 1.94929 1.94712 0.00217 46.550 46.605 -0.055 9 3 3 2 1.56977 1.56215 0.00762 58.770 59.085 -0.315 10 4 0 3 1.44857 1.44952 -0.00105 64.230 64.178 0.052 11 2 3 4 1.34189 1.33123 0.01066 70.060 70.704 -0.644 12 4 0 4 1.26932 1.27109 -0.00177 74.720 74.598 0.122
173
Table 8.6 Indexed Powder XRD data of paranitrophenol doped KDP crystal
S. No h k l d(obs)
(Å) d(calc)
(Å) Diff (d)
(Å) 2 (obs)
(deg) 2 (calc)
(deg) Diff (2 )
(deg) 1 2 0 0 3.65964 3.72453 -0.06489 24.300 23.870 0.430 2 2 1 1 2.97796 3.00299 -0.02503 29.980 29.724 0.256 3 1 1 2 2.87592 2.89682 -0.02090 31.070 30.840 0.230 4 3 0 1 2.32467 2.33777 -0.01310 38.700 38.475 0.225 5 1 0 3 2.19429 2.20836 -0.01407 41.100 40.826 0.274 6 3 1 2 1.94063 1.94868 -0.00805 46.770 46.565 0.205 7 4 2 0 1.66060 1.66566 -0.00505 55.270 55.088 0.182 8 3 2 3 1.55915 1.54063 0.01852 59.210 59.994 -0.784 9 2 2 4 1.44928 1.44841 0.00086 64.210 64.253 -0.043 10 5 1 2 1.35200 1.34633 0.00567 69.460 69.795 -0.335 11 4 0 4 1.26672 1.26914 -0.00242 74.900 74.732 0.168
8.3.2 FTIR spectral analyses
The FTIR spectral analysis were carried out to identify the chemical
bonding and molecular structure of the material. The FTIR were carried out using
Bruker IFS-66V spectrophotometer in the region 450 - 4000 cm-1 using KBR pellet
technique.
FTIR spectra of pure and doped KDP crystals are presented in the
Figure 8.5(a)-(e) respectively. From the literature it is found that the main cause of
absorption in matter in the broad spectral range between 3400 cm-1 and 1700 cm-1 is
the OH vibrations [254]. There are strong absorptions in the wave number region
between 3900 cm-1 to 2700 cm-1. These strong peaks are assigned to OH asymmetric
vibrations. The corresponding OH symmetric vibrations are observed in the range
from 2700 cm-1 to 2300 cm-1 [244]. The strong bands at around 2430 cm-1 and
1700 cm-1 are attributed to O=P-OH stretch of the phosphate ions in the KDP crystal.
The very strong absorption band at around 1300 cm-1 corresponds to
OPO-asymmetric stretching vibrations. There is a strong absorption peak at 1093 cm-1
which is assigned to OPO-asymmetric stretch and CN stretching vibrations.
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4000 3500 3000 2500 2000 1500 1000 5000.00.20.40.60.81.0
0.00.20.40.60.81.0
0.00.20.40.60.81.0
0.00.20.40.60.81.0
0.00.20.40.60.81.0
Wave Number(cm-1)
Tran
smitt
ance
( %
)
(a)
(b)
(c)
(d)
(e)
Figure 8.5 FTIR spectra of (a) pure (b) cobalt (c) nickel (d) manganese and
(e) paranitrophenol doped KDP crystals
The strong peak in the region 530 cm-1 is attributed to OPO-bending
vibrations. In the case of doping KDP crystals with metal ions and paranitrophenol,
the bands are almost similar indicating that the transformation produced in the FTIR
spectrum is minimal. There are some additional peaks formed due to the process of
doping. Some absorption peaks due to OH vibrations are changed because of the
incorporation of dopants in the lattice. In the case of manganese and paranitrophenol
T
rans
mitt
ance
(%)
Wave number (cm-1)
175
doped KDP crystals, additional peaks are found at 1648 cm-1 and 1518 cm-1
which are attributed to the P=O-stretch / NH3+-symmetric deformation and
NH3+-symmetric deformation. The detailed vibrational band assignments are
presented in the Table 8.7.
Table 8.7 Vibrational Band Assignments for pure and doped KDP crystals
Wave number (cm-1)
Vibrational Band Assignments Pure
KDP Cobalt
doped
Nickel
doped
Manganese doped KDP
Paranitrophenol
doped KDP
3898(vw) 3888(vw) 3889(vw) 3891(vw) 3890(vw) OH-asymmetric stretch
- - 3869(vw) 3858(vw) 3858(vw) OH-asymmetric stretch
3822(vw) 3836(vw) 3826(vw) 3826(vw) 3825(vw) OH-asymmetric stretch
3741(w) 3739(w) 3747(w) 3746(w) 3737(w) OH-asymmetric stretch
2770(s) 2774(s) 2776(s) 2762(s) 2770(s) OH-asymmetric stretch
2430(s) 2441(s) 2434(s) 2445(s) 2438(s) O=P-OH stretch
1705(vs) 1696(vs) 1694(vs) 1695(vs) 1699(vs) O=P-OH stretch
1305(vs) 1299(vs) 1299(vs) 1297(vs) 1297(vs) OPO-asymmetric stretch
1093(vs) 1094(vs) 1099(vs) 1103(vs) 1099(vs) OPO-asymmetric stretch / CN-stretch
892(vs) 886(vs) 897(vs) 908(vs) 907(vs) CCN-stretch / C=C-symmetric stretch
529(s) 534(s) 540(s) 536(s) 536(s) OPO-bend
176
8.3.3 Optical studies
The UV-Vis spectral analysis were performed to the grown crystals. The recorded absorption spectra are shown in Figure 8.6. From the spectrum, it is found that the cut off wavelength for the pure and doped crystals are around 240 - 380 nm and the maximum transmission levels are in the wavelength range 230 - 800 nm which are most desirable characteristic of a NLO material for applications. As there is no absorption in the visible region, these crystals are suitable for device
applications.
The cut off wavelength for pure KDP crystal is 310 nm whereas for the
doped crystals there is a change in the absorption percentage and cut off wavelength.
The cut off wavelength is in agreement with the literature [248]. The cobalt doped
KDP crystals have cut off wavelength of 290 nm. The nickel doped KDP crystals
show a cut off wavelength of 240 nm whereas the manganese and paranitrophenol
doped KDP crystals have, 380 nm and 270 nm respectively. The changes in the cut
off wavelength is due to the addition of dopants which are of different nature. The
increase in the cut off wavelength in the case of metal doped KDP crystals are
attributed to the free metal ions present in the crystal. In the case of nitrophenol doped
KDP crystal, the increase in the cut off wavelength is due to the phenolic group. The
transmission levels are in good agreement with the reported values for the KDP
crystals [239].
8.3.4 Thermal analyses
The TGA indicates the change in the mass of a substance, continuously monitored as a function of temperature when it is heated. The DSC shows the variation of heat flow with temperature. The TGA/DSC were carried out in nitrogen atmosphere at a heating rate of 20 oC / min in the temperature of 50 °C to 800 °C. Alumina was used as the reference material. The TGA/DSC curves for pure and doped KDP crystals are presented in Figure 8.7 (a) - (e) respectively.
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200 400 600 800 1000-0.020.000.020.040.060.080.100.120.140.16
Abso
rban
ce (
au )
Wavelength ( nm )
(e)
200 400 600 800 1000
0.000.050.100.150.200.250.300.35
(d)
200 400 600 800 1000
-0.010.000.010.020.030.040.050.060.07
(c)
200 400 600 800 10000.000.050.100.150.200.25
(b)
200 400 600 800 1000-0.010.000.010.020.030.040.050.06
(a)
Figure 8.6 UV-Visible spectra of (a) pure (b) cobalt (c) nickel
(d) manganese and (e) paranitrophenol doped KDP crystals.
A
bsor
banc
e (A
U)
Wavelength (nm)
178
Figure 8.7 TGA/DSC trace for (a) pure and (b) cobalt doped KDP crystal
Hea
t flo
w (m
W/m
g)
Wei
ght (
%)
Temperature (oC)
Hea
t flo
w (m
W/m
g)
Wei
ght (
%)
Temperature (oC)
(a)
(b)
179
Figure 8.7 TGA/DSC trace for (c) nickel and (d) manganese doped KDP crystal
Hea
t flo
w (m
W/m
g)
Wei
ght (
%)
Temperature (oC)
Hea
t flo
w (m
W/m
g)
Wei
ght (
%)
Temperature (oC)
(c)
(d)
180
Figure 8.7 TGA/DSC trace for (e) paranitrophenol KDP crystal
Lee et al., [255] reported that the high temperature phenomenon is not a
structural phase transition but an effect of thermal decomposition at the surface in
KDP. This result was reported by them based on the dielectric constant, thermal
analysis and thermal microscopic data Oritz et al., [256] and Jurado et al., [257]
supported the asseration that the high temperature phenomena exhibited by KDP are
the effects of thermal decomposition on the basis of their X-ray, thermo gravimetric
analysis and differential scanning calorimetric analysis. They have concluded that the
high temperature phenomena exhibited by KDP type crystals near TP are not related to
a physical change like a structural phase transition but related to a chemical change
caused by thermal decomposition such as:
nKH2PO4(s) KnH2Pn O3n+1(s) + (n - 1) H2O (v)
where TP is the characteristic temperature, n is the number of molecules participating
in the thermal decomposition, s and v denotes solid and vapour state respectively.
Hea
t flo
w (m
W/m
g)
Wei
ght (
%)
Temperature (oC)
(e)
181
From the thermogram, it is observed that the starting decomposition temperature for pure KDP crystal is 210 °C. It is found that the loss of mass is minimal even at an high temperature of 800 °C indicating that the crystal is extremely stable. The residual mass present in the crucible at 800 °C is 87%. The decomposition temperature shifts to 182 °C, 188 °C, 180 °C and 185 °C when cobalt, nickel, manganese and paranitrophenol are used as dopants. In all the crystals, there is a residual mass of more than 85 °C at a temperature as high as 795 °C.
From the DSC, there are sharp endothermic peaks are obtained at temperatures around 285 °C. The pure KDP crystal has an sharp endothermic peak at 285 °C with two peaks at 217 °C and 450 °C which are less significant. The area under the major peak at 285 °C is 225.6 J/g. The doped KDP crystals have sharp endothermic peaks at 283 °C, 275 °C, 274 °C and 278 °C respectively. The area under the sharp endothermic peaks are in the same range for doped KDP crystals. There is a slight shift in the endothermic peaks due to the addition of dopants in to the KDP crystal. It found that the addition of dopants basically alters the thermal stability of the KDP crystals. The results are in good agreement with the already reported values [244].
8.3.5 Dielectric studies
The dielectric constant and the dielectric loss were determined as a function of temperature and the A.C. Conductivity were also done to the pure and doped KDP crystals. The dielectric measurements were carried out with an LCR meter (Agilant 4284A) for different frequencies of 100 Hz, 1 kHz, 10 kHz, 100 kHz and 1 MHz at temperature ranges.
The variation of dielectric constant with temperature and dielectric loss with temperature are presented in the Figure 8.8 and Figure 8.9 respectively. It is found that the dielectric constant and the dielectric loss generally increases with increase in temperature. The high value of dielectric constant in the low frequency region may be due to the contributions from all four polarizations, namely, electronic, ionic, orientational and space charge polarizations. The electronic exchange of the number of ions in the crystals gives local displacement of electron in the direction of the applied field, which in turn give rise to polarization. As the frequency increases, a point will be reached where the space charge cannot sustain and comply with the external
182
305 310 315 320 325 330 335 340
6
9
Die
lect
ric c
onst
ant (
r )
Temperature ( K)
(100 Hz)(1 kHz)(10 kHz)(100 kHz)(1 MHz)
305 310 315 320 325 330 335 340
1
2
3
4
5
6
7
8
Die
lect
ric c
onst
ant (
r )
Temperature ( K)
(100 Hz)(1 kHz)(10 kHz)(100 kHz)(1 MHz)
305 310 315 320 325 330 335 3402.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
Die
lect
ric c
onst
ant (
r )
Temperature ( K)
(100 Hz)(1 kHz)(10 kHz)(100 kHz)(1 MHz)
305 310 315 320 325 330 335 3403.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
8.0D
iele
ctric
con
stan
t ( r )
Temperature ( K)
(100 Hz)(1 kHz)(10 kHz)(100 kHz)(1 MHz)
305 310 315 320 325 330 335 3403
4
5
6
7
8
9
10
11
Die
lect
ric c
onst
ant (
r )
Temperature ( K)
(100 Hz)(1 kHz)(10 kHz)(100 kHz)(1 MHz)
Figure 8.8 Variation of dielectric constant with temperature for (a) pure (b) cobalt (c) nickel (d) manganese and (e) paranitrophenol doped KDP crystal
(a) (b)
(c) (d)
(e)
183
305 310 315 320 325 330 335 340
0.03
0.06 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz
Die
lect
ric lo
ss
Temperature ( K ) 305 310 315 320 325 330 335 340
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz
Die
lect
ric lo
ss
Temperature ( K )
305 310 315 320 325 330 335 340
0.05
0.10
0.15
0.20
0.25 100 Hz 1 kHz 10 kHz 100 kHz 1 MHz
Die
lect
ric lo
ss
Temperature ( K )
305 310 315 320 325 330 335 3400.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6100 Hz 1 kHz 10 kHz 100 kHz (1 MHz
Die
lect
ric lo
ss
Temperature ( K )
305 310 315 320 325 330 335 340
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6100 Hz 1 kHz 10 kHz 100 kHz 1 MHz
Die
lect
ric lo
ss
Temperature ( K ) Figure 8.9 Variation of dielectric loss with temperature for (a) pure (b)
cobalt (c) nickel (d) manganese and (e) paranitrophenol doped KDP crystal
(a) (b)
(c) (d)
(e)
184
300 305 310 315 320 325 330 335 3400.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
1.2x10-5
100 Hz 1 KHz 10 KHz 100 KHz 1 MHz
Temperature (K)
ac(m
ho c
m-1K
)
300 305 310 315 320 325 330 335 340
0.0
5.0x10-7
1.0x10-6
1.5x10-6
2.0x10-6
2.5x10-6
100 Hz 1 KHz 10 KHz 100 KHz 1 MHz
Temperature (K)
ac(m
ho c
m-1K
)
300 305 310 315 320 325 330 335 3400.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
6.0x10-6
100 Hz 1 KHz 10 KHz 100 KHz 1 MHz
Temperature (K)
ac(m
ho c
m-1K
)
300 305 310 315 320 325 330 335 340
0.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
100 Hz 1 KHz 10 KHz 100 KHz 1 MHz
Temperature (K)
ac(m
ho c
m-1K
)
300 305 310 315 320 325 330 335 3400.0
2.0x10-6
4.0x10-6
6.0x10-6
8.0x10-6
1.0x10-5
100 Hz 1 KHz 10 KHz 100 KHz 1 MHz
Temperature (K)
ac(m
ho c
m-1K
)
Figure 8.10 Variation of A.C. conductivity with temperature for (a) pure (b) cobalt (c) nickel (d) manganese and (e) paranitrophenol doped KDP crystal
(a) (b)
(c) (d)
(e)
185
varying field and hence the decrease by exhibiting the diminishing values of dielectric
constant. Continuous and gradual decrease in dielectric constant suggests that pure
KDP and doped KDP crystals like any normal dielectric may possess domains of
different size and varying relaxation times. The variation of A.C. Conductivity with
temperature for pure and doped KDP crystals are presented in Figure 8.10. It is found
that the A.C. Conductivity also increases with increase in temperature. For pure KDP
crystal, the increase in A.C. Conductivity is more compared to doped crystals. The
decrease in the value of A.C. Conductivity on doping is due to positive and negative
ion orientation, defect vacant hydrogen bonds and doubly occupied hydrogen bonds
[258].
8.3.6 SHG and laser damage threshold studies
The pure and doped KDP crystals were made into fine powders of the size
of 10 µm. The microparticles were exposed to 1064 nm laser beam from a pulsed
Nd: YAG laser to test the second harmonic generation efficiency. An input pulse of
5.8 mJ/pulse was supplied. Signal amplitude in millivolts on the oscilloscope
indicates the efficiency of the sample. The pure KDP crystal gave an output 24 mV
whereas the doped KDP crystal showed an increase in the SHG efficiency. The
cobalt, nickel, manganese and paranitrophenol doped KDP crystal gave an output of
27 mV, 26 mV, 26 mV and 29 mV respectively. Thus, the SHG efficiencies of the
doped crystals are 1.12, 1.08, 1.08, 1.20 times greater than the standard KDP crystals
respectively.
The laser damage threshold were determined for the pure and doped KDP
crystals using single shot laser mode. A Q-switched Nd-YAG laser (pulse duration
47 ns) at 1064 nm was focused on the sample using a 10 cm focal length lens. The
incident fluence was varied using neutral density filters. The spot radius (1/e2 points)
was estimated by measuring the transmission of the laser beam through a known
aperture (Diameter 100 m) and is 126 m. Reflection losses (2.9 % at 20o for KDP)
were taken into account for estimating the incident fluence. The input energy given
186
(a) (b)
(c) (d)
(e)
Figure 8.11 Laser damage photograph of (a) pure KDP (b) cobalt (c) nickel
(d) manganese and (e) Paranitrophenol doped KDP crystals.
187
was 20.5 millijoules. The beam diameter used was 8 mm. The energy required to
damage the crystals is given in Table 8.8. The laser damage photographs are shown in
Figure 8.11. It is found that the laser damage threshold of pure and doped KDP
crystals are in the range 0.25 GW/cm2 to 0.30 GW/cm2 which is consistent with the
already available literature for pure KDP [180]. It is also observed that doping the
KDP crystals increases the laser damage threshold as reported by Pritula et al [243].
Table 8.8 Laser Damage Energy values of Pure and doped KDP crystals
S. No Crystal Laser Damage Threshold (GW/cm2)
1. Pure KDP 0.25 2. Cobalt doped KDP 0.27 3. Nickel doped KDP 0.26 4. Manganese doped KDP 0.26 5. Paranitrophenol doped KDP 0.27
8.4 CONCLUSION
Fine crystals of pure, cobalt, nickel, manganese, paranitrophenol doped
KDP have been grown by slow solvent evaporation technique. The various planes
were indexed using powder XRD. TGA and DSC studies reveal that the presence of
dopants decreases the thermal stability of the crystals. The optical behaviour is
assessed by UV-Visible studies and it indicates the change in the cut off wavelengths.
It is also found that these crystals have transmission in the visible region. The FTIR
studies reveals the presence of different functional groups in the crystals. The SHG
studies and laser damage threshold studies indicate that doped KDP crystals have
enhanced NLO efficiency and laser damage threshold values respectively. The
dielectric studies reveal the low dielectric constant and low dielectric loss of the
crystals at high frequency range, which is ideal for NLO materials. Thus, the good
laser damage threshold values, SHG efficiency, excellent transmission in the visible
region and low dielectric constants makes the pure, doped KDP crystals, a promising
candidate for NLO applications.