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.