CHAPTER 2 GROWTH AND CHARACTERISATION OF...
Transcript of CHAPTER 2 GROWTH AND CHARACTERISATION OF...
26
CHAPTER 2
GROWTH AND CHARACTERISATION OF
METAL IONS AND DYES DOPED KDP CRYSTALS
2.1 INTRODUCTION
One of the obvious requirements for a non-linear optical crystal is
that it should have an excellent optical quality. Potassium Dihydrogen
Orthophosphate (KDP) is a model system for non-linear optical device
application (Sangwal, 1998). Optical quality KDP crystals can be grown by
conventional solution growth methods as well as by fast growth techniques
(Guohui Li et al 2005). KDP is an efficient angle tuned dielectric medium for
optical harmonic generation in and near the visible region (Srivastava et al
2004). This material offers high transmission throughout the visible spectrum
and meets the requirement for optical birefringence, large enough to bracket
its refractive index for even extreme wavelength range over which it is
transparent (Robey et al 2000). Among non-linear optical phenomena,
frequency mixing and electro-optic are important in the field of optical image
storage and optical communication (Zaitseva et al 1995).
Studies comprising the crystal growth and characterization of KDP
containing organic molecules particularly sulfonated dyes in the literature are
reviewed (Hirota et al 2002). From the review, it was found that, metal ions
doped KDP was shown to orient a variety of anionic molecules on the [101]
growth sectors due to electrostatic interactions between the molecules and the
crystal surface. The spectroscopic properties of the dye doped KDP crystals
27
were presented to indicate a perturbation in the electronic energy levels of the
dyes in the crystal (Young Shangfeng et al 1999). The sulfonated dyes were
also shown to selectively recognize the various dislocation hillocks on the
pyramidal [101] faces of KDP during growth from aqueous solution (You-Jin
Fu et al 2000).
KDP grown in the presence of Fluorophores (Kumazawa et al
1999), Coumarins and Stilbene provided fluorescent crystals. The Coumarins
in KDP also showed phosphorescence. The fluorescence lifetime of one of the
Coumarins in the crystal showed sharp changes at the ferroelectric phase
transition temperature of KDP. Presence of Stilbene, results the fluorescence
depolarization, anisotropy in the excitation and emission, and orientation in
the KDP lattice.
The study of the effect of dyes on the structural phase transitions of
KDP led to the reinvestigation of the high temperature phase behavior of pure
KDP (Alexandru and Antohe 2003). The tetragonal phase of KDP undergoes
two ill-defined high temperature transitions that have eluded characterization
for more than 30 years. A reexamination of the progression of these
transformations using hot stage optical polarization microscopy and x-ray
diffraction was carried out by Belouet (1976). In that study it was
demonstrated that the progress of the high temperature transformations in
KDP was strikingly heterogeneous with phase transitions and dehydration
taking place concomitantly at different sites in the same crystal, which was
evidenced by the appearance of crystalline and polycrystalline islands upon
heating. Single crystal x-ray diffraction was also performed on the crystalline
domains, which provided three new crystal structures for KDP (Dongli Xu
and Dongfeng Xue 2005). The progressions of changes were also probed
using micro-Raman spectroscopy and confirmed that the signals of the
polycrystalline and crystalline domains are de-convoluted. The results
28
indicate that the pyro- and metaphosphate dehydration products are formed
upon heating KDP and are localized in the polycrystalline material
(Miroslawa Rak et al 2005).
KDP finds widespread use as frequency doublers in laser
applications and was studied in great detail (Eaton 1991). Improvement in the
quality of the KDP crystals and the performance of KDP based devices can be
realized with suitable dopants (Zaitseva et al 1995). To analyze the influence
of metal ions and dyes based dopants on the non-linear optical property of
KDP crystals, efforts were made to dope KDP with dyes (Amaranth,
Rhodamine B and Methyl orange) and metal ions (Na, Al). The effects of
impurity atoms on the quality and performance of the crystals were analyzed.
In addition, the growth aspects of KDP and doped KDP were studied in detail.
Bulk crystals of KDP and doped KDP were grown by solution growth
techniques. The structural, chemical, optical, mechanical and non-linear
optical properties of the doped crystals were analyzed with the
characterization studies such as powder XRD, FT-IR, UV-Visible, Micro
hardness and SHG measurements respectively. The results for doped KDP are
compared with the results of the pure KDP crystals.
2.2 GROWTH OF METAL IONS AND DYES DOPED KDP
CRYSTALS
2.2.1 Solubility studies for doped solution
Solubility studies were carried out by using recrystallized salts of
KDP in water with suitable dopants. About 100 ml of water was taken in an
airtight container and the recrystallized salt was added. The experiment was
carried out in a constant temperature bath with a cryostat facility. The bath
was set at 20º C and the solution was stirred continuously for six hours using
motorized stirrer by ensuring homogeneous temperature and concentration
29
through out the volume of the solution (Zaitseva et al 1997). Once the
saturation is reached, the solution was further stirred for six hours and the
equilibrium concentration of the solute was analyzed gravimetrically.
Similarly several trials were made to get the concurrent values. The
experiment was carried out at various temperatures from 20 to 50ºC in steps
of 5ºC and the solubility of the solute was obtained. The solubility of doped
KDP was measured for each dopant and was found to be 32.5 g/100 ml at
40oC for Sodium, 31.5 g/100 ml at 40oC for Aluminium, 32.75 g/100 ml at
40oC for Amaranth, 31.5 g/100 ml at 40oC for Rhodamine B and 32 g/100ml
at 40oC for Methyl Orange.
2.2.2 Crystal Growth
Pure KDP crystals were grown from aqueous solution by slow
evaporation and also by slow cooling method (0.5ο C /day). The same method
was followed for doped KDP crystals (0.1 mol % of Na2CO3 or AlPO4 or
Amaranth or Rhodamine B or Methyl orange). The seed crystals were
prepared at low temperature by spontaneous nucleation. Seed crystals with
perfect shape and free from macro defects were used for growth experiments.
Large single crystal of KDP and doped KDP (Na, Al) were grown
using a Constant Temperature Bath (CTB), controlled by the Indtherm
temperature programmer/controller. The mother solution was saturated using
the initial pH values, 4.5 and 4.6 for Sodium and Aluminium dopants
respectively. The growth was carried out for more than 20 days by keeping
the saturated solution in the bath at a temperature of 38 οC.
The supersaturated solution of KDP was first prepared at 313 K in
1 litre beaker. And then, Amaranth or Rhodamine B or Methyl orange in
0.1 M aqueous solution with initial pH values 4.8, 4.5 and 4.25 respectively
and was added into the supersaturated solution of KDP at 313 K. The solution
30
was kept for few weeks at 313 K. A few nuclei of doped KDP had appeared at
the bottom of the beaker and grew for few days. The change of
supersaturation was greater than 15% of the critical super saturation during
the growth of doped KDP crystals. Constant temperature bath was used for
the bulk growth of doped KDP crystals. Dye concentrations in the prismatic
and pyramidal sections of the crystals were measured. Transparent crystals
were obtained after 20 days (Figures 2.1(a), 2.1(b), 2.1(c)).
2.3 POWDER X-RAY DIFFRACTION ANALYSIS
Powder X-ray diffraction studies were carried out for the grown
crystals using a Rich Seifert X-ray diffractometer with CuKα (λ = 1.5405 Å)
radiation. Powder X-ray diffraction spectra of the crystals grown from pure
and doped (dyes) KDP are presented in Figure 2.2. Powder XRD spectra for
the pure and dyes doped KDP reveal that the structures of the doped crystals
are slightly distorted compared to the pure KDP crystals. This may be
attributed to strain on the lattice by the absorption or substitution of dyes. It
was observed that the reflection lines of the doped KDP crystal correlate well
with those observed for the individual parent compound with a slight shift in
the Bragg angle.
31
Figure 2.1 Seed crystals of (a) Amaranth doped KDP (b) Rhodamine B
doped KDP and (c) Methyl orange doped KDP
(a)
(b)
(c)
32
Figure 2.2 XRD spectra of (a) Pure, (b) Amaranth, (c) Rhodamine-B and
(d) Methyl Orange doped KDP crystals
33
2.4 SHG MEASUREMENTS
SHG Measurements were made, using the Kurtz and Perry powder
method. Schematic representation of the SHG setup is as shown in the
Figure 2.3.
Figure 2.3 SHG set up
The sample cell was formed between two Pyrex 25 75 1 mm3
microscope slides (Can lab). Two strips of masking tape with a 2 cm diameter
hole cut through the centre were placed 3 cm from the end of one of the
slides. The circular window was filled with a thin layer of sample (0.3 mm
thick). Approximately 50 mg of material can be loaded into the sample cell.
The second microscope slide was clipped on to the first, then sandwiched the
sample between the slides. The fundamental output (1064 nm) from a
Q-switched Nd-YAG laser (Quanta Ray) was divided by a beam splitter,
where one portion of the light was directed on to a reference cell (pure KDP).
The resultant second – harmonic signal (532 nm) was passed through a sharp-
cut filter (Corning 527) designed to eliminate any stray visible light from the
34
laser flash pump, and several neutral density filters designed to attenuate the
second-harmonic signal, before being focused on to a photo multiplier tube
(RCA IP28).
The second portion of the fundamental beam was directed on to a
second cell containing the sample to be measured. Any second harmonic
produced was passed through a series of neutral density filters, and then
focused into a monochromator (Beckmann DU spectrometer, model 2400) in
order to discriminate between SHG light and any stray visible light and IR
light. The SHG efficiency of dyes doped KDP were measured and tabulated
in Table 2.1.
Table 2.1 SHG of pure and Dyes doped KDP Crystals
S.No. Compound NLO efficiency with
respect to pure KDP
1. KDP 1.00
2. Amaranth doped KDP Crystal 1.47
3. Rhodamine B doped KDP Crystal 1.59
4. Methyl Orange doped KDP Crystal 1.69
35
2.5 MICROHARDNESS ANALYSIS
Impurities present in KDP crystals influence the mechanical
properties. The presence of impurity in a crystal changes its elastic constants
and hardening characteristics. The Metallurgical Microscope fitted with a
Vickers’s Pyramidal Diamond Indenter is shown in Figure.2.4.
Figure 2.4 Metallurgical Microscope fitted with a Vickers’s
Pyramidal Diamond Indenter
The hardness measurements were made using the Vickers pyramidal
indenter for various loads. The hardness of the metal ions doped KDP was
high compared with pure crystals. Micro cracks form around the impressions
apart from cracks at the corners of impressions for dyes doped KDP crystals
for higher loads. The applied load was 5,10 and 25 g .The doped materials are
hard and brittle in nature. It is observed that the Vickers’s hardness number
increases with the addition of dyes with KDP. It was observed that the
hardness values decrease with increase of load. The hardness values obtained
for the pure and doped KDP crystals are tabulated in Table 2.2.
36
Table 2.2 Micro hardness values of pure and doped KDP
S.No Crystal Microhardness (kg/mm2)
1. KDP 165.79
2. Na doped KDP 168.67
3. Al doped KDP 169.24
4. Amaranth doped KDP
171.54
5. Rhodamine B doped KDP
169.67
6. Methyl orange doped KDP
172.50
The maximum value of hardness was obtained for cleavage plane of
Methyl orange doped KDP crystal as 172.5 kg/mm2. Addition of impurities in
KDP extensively modifies the hardness values and the doped KDP crystals
were much harder than the pure crystal.
2.6 OPTICAL STUDIES ON METAL IONS AND DYES DOPED
KDP CRYSTALS
2.6.1 UV-Visible transmission studies on doped crystals
Since single crystals of KDP family are mainly used in optical
applications, the optical transmission range was determined for these crystals.
The UV-Visible transmission spectra were recorded for the samples of pure
and doped KDP crystals (Figure 2.5). The spectra were recorded in the
wavelength region from 190 to 1800 nm using Varian Cary 2300
spectrophotometer. The transmission was improved for the doped KDP
crystals.
37
Figure 2.5 UV-Visible spectra of pure and dyes doped KDP crystal
All the crystals irrespective of the dopants are transparent in the
entire visible region. This transparency in the visible region is a desired
property of materials for NLO applications. The UV-visible spectrum for dyes
(Amaranth, Rhodamine B, Methyl orange) doped KDP indicates that the
Methyl orange enhances the transmission of KDP crystal. Absorption edge
was shifted to blue region for dyes doped KDP. The blue shift increases in
accordance of mole fractions of dopants. The Rhodamine B doped crystal has
higher transmission compared to pure as well as other dyes doped KDP
crystal.
2.6.2 FT-IR studies on pure, metal ions and dyes doped KDP
crystals
The FT-IR spectra of pure, metal ion and dyes doped KDP samples
were recorded on a Bruker IFS 66V model spectrophotometer using 1064 nm
4
2
3
1
38
output of a cw diode pumped Nd:YAG laser as a source of excitation in the
region 4000 - 400 cm-1 operating at 200 mW power. Assignments were made
on the basis of relative intensities, magnitudes of the frequencies and
comparing the literature data (Raskovich 1997 and Silverstein et al 1981). The
values of bond length and bond angles were taken from Sutton’s table.
Internal co-ordinates for the out-of- plane torsional vibrations were defined as
recommended by IUPAC. The general quadratic valence force was adopted
for both in-plane and out of plane vibrations. The normal co-ordinate
calculations were performed using the program given by Thomas
(Thomas et al 2004 a&b). The initial sets of force constants were taken from
the literature for the derivatives of allied molecules. The calculated
frequencies agree favorably with the observed frequencies.
2.6.2.1 FT-IR studies on pure and metal ions doped KDP crystals
The observed FT-IR spectra of pure and metal ions doped KDP are
shown in Figure 2.6.
From the FT-IR spectra, the weak absorption band appearing at
3600 cm-1 in pure KDP and the band appearing at 3500 cm-1 in doped KDP
was assigned to free O-H stretching. It confirms that atleast one of the –OH
group in KDP should be freed after they were doped with metal ions. The
broad band appearing at 3200 cm-1 was due to intermolecular H-bonded O-H
stretching with -C=O group occurred only by the doping of KDP with
Na2CO3. Intermolecular H-bonding increases where as the concentration of
the solution increases. The broad absorption band appearing at 3100 cm-1 was
assigned to intramolecular hydrogen bonded O-H stretching frequency, which
was only in KDP. The absence of this peak in Na2CO3, AlPO4 doped KDP,
indicates the strong interaction of that dopants with -OH groups of KDP and
the possible entry of the dopants in the lattice site of KDP crystal.
39
Figure 2.6 FT-IR Spectra of pure and doped (Na, Al) KDP Crystal
The broad bands at 2650 cm-1 and 1650 cm-1 were due to asymmetric and symmetric O=P-OH stretching frequencies of KDP and it has appeared at 1600 cm-1 in the metal ions doped KDP, the corresponding bending vibration occurred at around 940 - 950 cm-1. It indicates that all the vibrations were involved in a dipole moment changes. The bands appeared at 2250 cm-1 in pure KDP and metal ions doped KDP, which indicated clearly after the interaction of dopants with P-OH groups of KDP, does not weakening the strength of bond between P-O-H groups.
350
0
32
00
27
00
2250
600
940
1
105
1295
3500
600
950
1100
13
00
2650
2360
1650
2250
3100
1295
3600
2250
1600
16
00
110
5
945
600
2650
40
The sharp and strong intense bands appearing at around 1300 cm-1
and 1295 cm-1 were due to P=O symmetric stretching in the aliphatic nature.
The sharp bands at around 1100 cm-1 was due to symmetric P=O aliphatic
stretching in KDP and appeared at 1105 cm-1 in the doped KDP. Another
sharp band at 600 cm-1 was due to HO-P-OH bending. The calculated IR
frequencies were in close agreement with the experimentally obtained
frequencies. The frequencies with their relative intensities obtained in FT-IR
spectra of pure and metal ions doped KDP most probable assignments are
presented in Table 2.3.
Table 2.3 Observed and calculated IR frequencies (cm-1) of Pure KDP
and KDP doped with Na2CO3, AlPO4
Calculated Frequencies
cm-1
Observed IR frequencies (cm-1) and intensities
Assignments
Pure KDP
KDP Doped with
Na2CO3
KDP doped with
AlPO4 3615 3600(w) 3500(w) 3500(vw) Free –OH stretching 3200 - 3200(br) - Intermolecular H-bonded –OH
stretching with –C=O group 3100 3100(br) - - Intramolecular H-bonded –OH
stretching 2650 2650(br) 2650(vw) 2700(w) O=P-OH asymmetric stretching
2350
-
2360(sh)
-
ring stretching vibration of O -O-C-O- group
2250 2250(w) 2250(sh) 2250(br) P-O-H asymmetric stretching 1600 1650(br) 1600(br) 1600(br) O=P-OH symmetric stretching 1350 1300(sh) 1295(sh) 1295(sh) P=O symmetric stretching
(aliphatic) 1110 1100(sh) 1105(sh) 1105(sh) P-O-H symmetric stretching 975 950(s) 940(s) 945(s) O=P-OH bending 625 600(sh) 600(sh) 600(sh) HO-P-OH bending
s- strong w-weak vw-very weak sh-sharp br-broad
41
2.6.2.2 FT-IR studies on pure and dyes doped KDP crystals
The FT-IR spectral studies on pure and dye doped KDP clearly
indicates the effects of dopants on the crystal structure of pure KDP, which
leads to the change in the absorption of IR frequencies. The observed FT-IR
spectra of pure and dye doped KDP are shown in Figure 2.7.
Figure 2.7 FT-IR spectra of pure and dyes doped (Amaranth,
Rhodamine B, Methyl Orange) KDP single crystal
3600
3100
2650
2250
1650
1300
1100
950
600
3650
3250
2900
2250
1700
1400
1120
3650
3250
2900
2150
600
1650
1410
1150
3200
2950
2450
1650
1350
1280
950 60
0
3700
42
From the FT-IR spectra, the weak absorption band appears around
3600 – 3700 cm-1 in pure KDP, dye doped KDP was assigned to free O-H
stretching. Slight deviation from pure KDP to higher frequencies at 3650,
3650 and 3700 cm-1 in KDP doped with Amaranth, Rhodamine B and Methyl
orange respectively indicates clearly that the interaction of dopants with free –
OH groups of the KDP, which weakening the strength of the bond between
oxygen and hydrogen. This is also reflected in the intramolecular H-bonded
O-H stretching.
The broad bands at 2650 cm-1 and 2450 cm-1 were due to O=P-OH
asymmetric stretching, which was occurred only in pure KDP and Methyl
orange doped KDP spectra. The absence of this peak in the Amaranth and
Rhodamine B doped KDP spectra, indicates no dipole moment changes
occurred in O=P-OH groups after they incorporation into KDP. So, they
became IR inactive and which does not appear in the IR spectra. The
sharp and very strong absorption bands at 1100 cm-1, 1120 cm-1 and
1150 cm-1 were due to P-O-H symmetric stretching in KDP, Amaranth doped
KDP and Rhodamine B doped KDP respectively. It indicates that no much
interaction between P-OH groups of KDP with dyes (Amaranth and
Rhodamine B). But in Methyl orange doped KDP that peak was completely
vanished, because after the doping there was no P-OH bonds in the KDP.
This confirms the absence of P-O-H stretching in the Methyl orange doped
KDP.
The FT-IR spectral studies confirmed that the dopants had entered
the lattice sites of tetragonal KDP and also confirmed that the optical property
of KDP was much altered by the doping of methyl orange compared to
Amaranth and Rhodamine B dyes. The frequencies with their relative
intensities obtained in FT-IR spectra of pure and metal ions doped KDP most
probable assignments are presented in Table 2.4.
43
Table 2.4 Observed and calculated IR frequencies (cm-1) of Pure KDP
and KDP doped with Amaranth, Rhodamine B and Methyl
orange
Calculated Frequencies
cm-1
Observed IR frequencies (cm-1) and intensities
Assignments
Pure KDP
Amaranth Rhodamine B
Methyl orange
3615 3600(w) 3650(s) 3650 (vs) 3700(vw) Free –OH stretching 3200 - 3250(w) 3250(w) 3200(sh) Intermolecular H-
bonded –OH stretching with –NH2
3100 3100(br) - - - Intramolecular H-bonded –OH stretching
2870 - 2900(br) 2900(br) 2950(w) -N=N- stretching 2650 2650(br) - - 2450(br) O=P-OH asymmetric
stretching 2250 2250(br) 2250(br) 2150(br) - P-O-H asymmetric
stretching 1600 1650(br) 1700(sh) 1650(sh) 1650(vw) O=P-OH symmetric
stretching 1415 - 1400(sh) 1410(vs) 1350(w) C=C stretching
(skeletal) vibrations 1350 1300(sh) - - - P=O symmetric
stretching (aliphatic) 1260 - - - 1280(sh) SO3 asymmetric
stretching 1110 1100(sh) 1120(vs) 1150(vs) - P-O-H symmetric
stretching 975 950(s) - - 950(vw) O=P-OH bending 625 600(sh) 600(sh) 600(sh) 600(sh) HO-P-OH bending
s- strong vs-very strong w-weak vw-very weak sh-sharp br-broad
44
2.6.3 Raman studies on doped KDP crystals
The FT Raman spectra of the dyes (Amaranth, Rhodamine B and
Methyl orange) doped KDP was also recorded in the region 2000-200 cm-1
with FRA Raman module equipped with Nd:YAG laser source operating at
1.06 m line, with scanning speed of 30 cm-1 min-1 of spectral width 20 cm-1.
The frequencies for all sharp bands were accurate to 2 cm-1.
The Raman spectrum of the crystal with different scattering
geometry was recorded at room temperature. The observed spectra of
(a) Pure KDP, (b) Amaranth, (c) Rhodamine B and (d) Methyl orange doped
KDP are shown in Figure 2.8.
Figure 2.8 Raman spectra of (a) Pure KDP, (b) Amaranth,
(c) Rhodamine B and (d) Methyl orange doped KDP
45
The Raman spectra can be subdivided into two frequency
regions (100-300 cm-1 lattice modes, 300-1200 cm-1 - PO4* internal modes).
Compared with the Raman spectrum of an aqueous solution of KH2PO4, it is
easy to determine the four internal vibrational modes of the H2PO4 – in
KH2PO4 as 1495 cm-1 (1), 1275 cm-1 (2), 1350 cm-1 (3) and 1600 cm-1(4).
The distortion of the H2PO4 will result in line broadening or even splitting.
It is very interesting to note that the fundamental vibrations assigned
in the Raman spectrum agree favorably well with the infrared frequencies of
dye doped KDP (Gui-Wu Lu et al 2001). Further study on Raman spectrum
shows that the decrease in the O-H hydrogen bonded stretching frequency by
275 cm-1 in the case of KDP doped with dyes compared to free O-H of pure
KDP revealed the extend of hydrogen bonding between H+ of KDP with ring
nitrogen of dyes and in weakening the O-H bond strength of pure KDP
(Yoshioka et al 1998).
2.7 SCANNING ELECTRON MICROSCOPE ANALYSIS
Surface morphology of the cut and polished wafers were observed
through Scanning Electron Microscope (Model-LEO Stereoscan 440). Surface
micrograph observed for Amaranth and Rhodamine B doped crystals are
shown in Figures 2.9 and 2.10. The step grown surface morphology was
observed on the as grown Amaranth and Rhodamine B doped KDP crystals
(Kumaresan et al 2005 c). By improved growth procedures, defect free
smooth surface morphology was obtained on both Amaranth and Rhodamine
B doped KDP crystals.
46
Figure 2.9 SEM picture of as grown surface morphology of Amaranth
of doped KDP crystal
Figure 2.10 SEM picture of as grown surface morphology of
Rhodamine B doped KDP crystal
The compositional homogeneity of the crystal was analyzed using
energy-dispersive X-ray analysis (EDAX). EDAX was performed at various
positions on each surface along the radial direction. The analysis was done
using a Horiba EMAX 5770 spectrometer coupled with a Hitachi S-5000
47
Scanning Electron Microscope. The compositional homogeneity determined
for Al doped KDP was as shown in Figure 2.11.
Figure 2.11 EDAX spectrum for Al doped KDP crystal
2.8 ATOMIC FORCE MICROSCOPE STUDIES
The surface morphology of pure and Amaranth doped KDP crystal
was investigated using atomic force microscopy (AFM). The micrographs
were recorded for pure and Amaranth doped KDP crystals (Figure 2.12). As
grown pure KDP crystals appears to be smooth, in an ordered arrangement
with least dislocations. The corresponding root mean square (rms) surface
roughness value was 0.214 nm. Amaranth doped KDP has a surface
roughness value of 0.35 nm.
48
a ba b
Figure 2.12 AFM image of (a) Pure KDP crystal (b) Amaranth doped KDP crystal
2.9 RESULTS AND DISCUSSION
Earlier studies have reported that selective adsorption of metallic
cation suppresses the growth of surfaces like the prismatic section (100) or
pyramidal section (101) of KDP crystals. The suppression is explained due to
the pinning effect of impurities on the step growth of the crystal and the
adsorption model of impurity on the crystal. It is well known that metallic
cations (Al3+, Na+) influence the growth of the prismatic section of KDP
crystals and change the habit from the needle like towards pyramidal
(Kumaresan et al 2007 a). H2PO42- anions appear on the prismatic surface of
the crystals. Cations play a significant role in suppressing the crystal growth
of the prismatic section. In this connection, the dopants play an important role
in the habit modification of the doped crystals.
The dopants sodium and aluminium are expected to substitute for the
potassium ions in the KDP lattice due to their valency as well as their
49
similarity of ionic radius. The partial substitution of potassium ions may be
explained as the consequence of the following chemical reaction (Kumaresan
et al 2007e).
2 KH2PO4 + Na2CO3 → 2 NaH2PO4 + K2 CO3
However, the aluminium ions can occupy the interstitials instead of
the Potassium sites. Powder XRD spectra for the pure and dyes doped KDP
revealed that the structures of the doped crystals are slightly distorted
compared to the pure KDP crystal. This may be attributed to strain on the
lattice by the absorption or substitution of dyes. It was observed that the
reflection lines of the doped KDP crystal correlate well with those observed in
the individual parent compound with a slight shift in the Bragg angle.
The UV-visible spectra show that the crystals irrespective of the
dopants are transparent in the entire visible region. The transparency in the
visible region is a desired property of materials for NLO applications. The
UV-visible spectra for dyes (Amaranth, Rhodamine B, Methyl orange) doped
KDP indicates that the methyl orange enhances the transperency property of
KDP crystal. The Rhodamine B doped crystal is invariably has higher
transmission percentage compared to pure KDP crystal.
The hardness measurements were made using the Vickers pyramidal
indenter for various loads. The hardness values decreased rapidly when
indenter load was increased in lower ranges and it remained almost unaffected
above this load. The maximum value of hardness was observed for cleavage
plane of Methyl orange doped KDP crystal (172.50 kg/mm2) with Methyl
orange.
50
The FT-IR spectra show that pure and doped KDP and their most
probable assignments were made on the basis of relative intensities,
magnitudes of the frequencies comparing the literature data. The general
quadratic valence force was adopted for both in-plane and out of plane
vibrations. The calculated frequencies agree favorably with the observed
frequencies. The very weak bands indicate the presence of low concentration
of Na and Al in KDP. The absence of this peak in KDP doped with AlPO4
supported again the strong interaction of Al3+ with O–H groups and the
possible entry of these ions in the lattice site of KDP crystal.
KDP doped with Na2CO3 and in AlPO4 gave a multiplet at
2924 cm–1, which indicated clearly the interaction of dopants with P–O–H
group of KDP and in weakening the strength of the bond between oxygen and
hydrogen. This leads to the decrease in the frequency of O–H stretching and
confirmed the non-linear optical property of pure and doped KDP crystals at
these sites in the crystal lattice.
Further study on Raman spectrum shows that the decrease in the
O-H hydrogen bonded stretching frequency by 275 cm-1 in the case of KDP
doped with dyes compared to free O-H of pure KDP revealed the extend of
hydrogen bonding between H+ of KDP with ring nitrogen of dyes and in
weakening the O-H bond strength of pure KDP.
The SEM studies reveal that the Amaranth present in the solution
causes to form a surface layer that prevents the entry of impurities and
thereby it helps to grow the crystal with high crystalline quality. The SEM
picture confirms the formation of a layer on the surface of the crystal due to
impurities. There was only a slight variation of atomic percentages along the
length of the crystals indicating the high homogeneity of stoichiometry
throughout the boule.
51
2.10 CONCLUSION
Optically clear KDP and doped KDP (Al, Na,
Amaranth, Rhodamine B and Methyl orange) crystals with dimension up to
25 22 10 mm3 were grown by a slow evaporation technique and also by a
slow cooling technique. In the FT-IR spectrum, the characteristic peaks due to
C-O-H in-plane and out-of-plane bands clearly demonstrate protonation of
COO– group. The functional groups present in the grown crystals have been
confirmed by FT-IR spectral analysis. The observed frequencies were
assigned on the basis of symmetry operation on the molecule and normal
coordinate analysis. The study not only confirmed the strong interaction of
Al3+ and Na+ ions of the dopants with the –OH group of KDP but also the
entry of these ions into the crystal lattice of the tetragonal KDP crystal. It was
found that the optical property of pure and doped KDP were changed not only
due to the weakening of the bond between O–H and C=O and also due to
hydrogen bonding formed by the substitution of metal ions in the crystal
lattice of tetragonal KDP crystals which increases the bond strengths. Since
H+ ion having the radii of only 0.3 Å was being replaced by Al3+ ion
(0.535 Å) and Na+ ion (1.02 Å), there must be a strain in the accommodation
of these ions instead of H+ ion. Hence, only a limited number of ions can
diffuse into the lattice sites of tetragonal KDP crystals.
The doped crystals of KDP undergo two–stage thermal
decomposition similar to that of pure crystals. The micro hardness studies of
doped KDP were analyzed. Doped crystals have relatively higher hardness
values than the pure crystals. Powder XRD was taken to analyze the
structures of the doped crystals. It confirms the structure and change in lattice
parameter values for the doped crystals. The presence of dopants in the crystal
lattice has been qualitatively confirmed by FTIR analysis. Dyes doped with
KDP changes the optical properties. The second harmonic signal, generated in
52
the crystal was confirmed from the emission of green radiation by the crystals
on Laser irradiation. The SEM picture confirms the formation of a layer on
the surface of the crystal due to impurities. The SEM studies reveal that the
Amaranth present in the solution causes to form a surface layer that prevents
the entry of impurities and thereby it helps to grow the crystal with high
crystalline quality. The NLO studies analyzed with Nd – YAG laser confirm
that the grown crystals have better NLO property.