chapter 4 effect of oxalic acid on the optical,...

67

CHAPTER 4

EFFECT OF OXALIC ACID ON THE OPTICAL,

THERMAL, DIELECTRIC AND MECHANICAL

BEHAVIOUR OF ADP CRYSTALS

4.1 INTRODUCTION

Oxalic acid is a hydrogen-bonded material. It is the only possible

compound in which two carboxyl groups are joined directly and for this

reason oxalic acid is one of the strongest acids in organic compounds. It is

expected that the presence of oxalic acid in ADP can enhance the nonlinearity

of the crystal (Chitra et al 2004). It is already found that the addition of

oxalic acid gives significant variations on the properties of the ADP crystals

(Bhagavannarayana et al 2008).

There is no report available in the literature on the effect of oxalic

acid on the optical, thermal, mechanical and dielectric properties of

ammonium dihydrogen phosphate crystals. Keeping this in our mind, in our

laboratory it was proposed to grow ADP crystal added with 1, 3 and 5 mol%

of oxalic acid. The effects of impurity atoms on the quality and performance

of the crystals are analysed. The results of the doped ADP crystals grown by

slow evaporation method are compared with the results of the pure ADP

grown by the same method. Already Bhagavannarayana et al (2008) have

reported the enhanced crystalline perfection and SHG efficiency of the oxalic

acid added ADP crystals. In this present work, we have studied and reported

68

the optical, thermal, dielectric and mechanical behaviours of the oxalic acid

added ADP crystals as against the pure ADP crystals.

4.2 CRYSTAL GROWTH

The commercially available ADP was used for growth, after

repeated recrystallization. Single crystals of pure and oxalic acid added ADP

crystals were grown using deionized water as a solvent by slow evaporation

method. 1000 ml saturated solution of ADP was prepared and filtered at room

temperature and the solution was divided equally into four beakers and it was

named as A, B, C and D. Each beaker contains 250 ml of ADP solution. The

beaker A was kept closed with porously sealed cover. Oxalic acid of 1, 3 and

5 mol% was added into the beakers B, C and D, respectively and the beakers

were closed with the same type of covers. All the beakers were allowed to

evaporate in an identical condition. After four days, tiny crystals were seen in

the beakers A and B, whereas in C and D it was observed one day later only.

All crystals were harvested in 20 days. The colourless transparent crystals

harvested were of size up to 20x10x20 mm3 from the beaker B. All the others

were smaller in size. The grown pure, 1, 3 and 5 mol% oxalic acid added

ADP crystals are respectively shown in the Figures 4.1 to 4.4. Ammonium

dihydrogen phosphate and oxalic acid used in the present study were bought

from Merck, India and the deionized water got from Millipore water

pre-filtration unit. The resistivity of the used deionized water is 18.2 M cm.

4.3 GROWTH RATE

The growth rate of crystals was high in 1 mol% of oxalic acid

added ADP and it was decreased in the higher concentrations. Quite likely, at

high concentrations of addition, the adsorption film blocked the growth

surface and inhibited the growth process. Bulk single crystals were grown

using optimized growth parameters. High-quality transparent crystals were

69

harvested from the growth medium in the presence of low concentration of

oxalic acid (1 mol%). Less transparency and also occlusions were observed

in the crystals harvested at higher concentrations of oxalic acid (3 and

5 mol%).

Figure 4.1 Pure ADP single crystals

Figure 4.2 1 mol% oxalic acid doped ADP single crystals

70



4.4 CHARACTERIZATION

4.4.1 UV-Vis-NIR Spectroscopy

Crystal plates of pure, 1, 3 and 5 mol% of oxalic acid added ADP

crystals of thickness 2 mm were cut and polished at the face (1 0 0) without

any coating for optical measurements. The transmission spectra of the crystals

71

200 400 600 800 1000 1200

10

15

20

25

30

35

40

45

50

55

wavelenghth (nm)

Tra

nsm

ita

nce

(%

)

Pure ADP

ADP+ 1 mol% of oxalic acid



were recorded using the Perkin-Elmer Lambda-35 UV-Visible spectrometer

in the wavelength region from 200 to 1100 nm. The recorded optical spectra

are shown in the Figure 4.5. It is observed from the figure that the

transmittance is good in the entire visible region for the pure ADP crystals. It

has approximately 50 % of transmittance. The 1, 3 and 5 mol% of oxalic acid

added crystals have transmittance less than the pure crystal. The addition of

oxalic acid gradually decreases the transparency of the crystals. In order to

confirm the reproducibility, the transmittance studies were repeated several

times for the crystal plates cut from the different parts of the grown crystals

and the same results were observed, thus confirming also the uniformity of

the dopant in a given crystal. There are steps in the transmission curves

between the region 300-400 nm. This is due to change of optics in the

photometer. The above results indicate that the increasing oxalic acid

concentrations decreased the transmittance gradually.

Figure 4.5 Transmission spectra of the grown crystals

72

4.4.2 TG-DTA Analysis

The thermo gravimetric analysis and differential thermal analysis

were performed using Perkin-Elmer Diamond TG/DTA thermal analyzer at

heating rate 10 °C/min. in inert nitrogen atmosphere ranging from 30oC to

300oC.

Figures 4.6 and 4.7 show the TGA and DTA curves for pure and

doped ADP crystals respectively. The DTA curve shows an endothermic peak

at 215 °C for the pure ADP at 207 °C for 1 mol% of oxalic acid added ADP.

This endothermic peak corresponds to the decomposition temperature of the

crystal. The detailed data for all the crystals are shown in the Table 4.1. It is

seen from the table that an increase in oxalic acid concentration decreases the

decomposition temperature. The measurement was repeated several times and

same results were observed. The melting point of the oxalic acid is 101.5oC

(Science lab-material safety data sheet). The presence of oxalic acid appears

to decrease the decomposition temperature.

Figure 4.6 TGA curves of the grown crystals

50 100 150 200 250 300

75

80

85

90

95

100

______Pure ADP

______ADP+1 mol %oxalic acid

______ADP+3 mol % oxalic acid


We

igh

t (

%)

Temperature (oC)

73

Figure 4.7 DTA curves of the grown crystals

Table 4.1 Thermal analysis data of the grown crystals

Crystal

Endothermic

peak

(oC)

Decomposition

Starting temperature

(oC)

Pure ADP 215 197

1 mol% of oxalic acid added ADP 209 196



4.4.3 Vickers Hardness Measurements

Vickers hardness studies have been carried out using the instrument

Shimadzu microhardness tester – HMV –2T. The indentation hardness was

measured as the ratio of applied load to the surface area of the indentation.

Hardness is one of the important mechanical properties of the materials. It is used

to measure the plastic properties and strength of a material. Hardness is defined

as resistance against lattice destruction (Kishan Rao and Srideshmukh 1983).

The pure and oxalic acid added crystals of size 5x5x3 mm3 with (1 0 0) face

50 100 150 200 250 300

-60

-40

-20

0

20

40

______Pure ADP

______ADP+1 mol %oxalic acid



Mic

rovo

lt E

ndo

do

wn

(V

)

Temperature (oC)

74

0 50 100 150 200 250 300

35

40

45

50

55

60

65

70 Pure ADP

ADP + 1 mol% of oxalic acid



Hard

ne

ss (H

V)

kg

/mm

2

Load (p) g

were selected for microhardness studies. Indentations were carried out using

Vickers indenter for varying loads. For each load (p), several indentations

were made and the average value of the diagonal length (d) was used to

calculate the microhardness of the crystals.

Figure 4.8 Vickers hardness spectra of the grown crystals

Vickers microhardness number was determined using Hv =1.8544 p/d2.

A plot drawn between the hardness value and corresponding loads is shown in

Figure 4.8. It is observed from the figure that the hardness increases with

increase in load for all the crystals and up to 300 g no cracks have been

observed for the pure and 1 mol% of oxalic acid added crystals, whereas in

3 and 5 mol% of oxalic acid added ADP cracks were observed at 200 g itself

and the hardness started to decrease. Similarly addition of higher

concentration of oxalic acid decreases the hardness value of the crystal.

Hardness is the resistance offered by a solid to the movement of dislocation.

Due to the application of mechanical stress by the indenter, dislocations are

generated locally at the region of the indentation. Lower hardness value of

3 and 5 mol% oxalic acid added ADP crystals indicates that lesser stress is

75

sufficient to form dislocation thus confirming that the crystalline perfection is

not good in the crystals.

4.4.4 Dielectric Studies

The temperature dependence of dielectric constant and the

dielectric loss of the pure, 1, 3, 5 mol% of oxalic acid added ADP crystals

were measured at the frequencies 1 kHz and 100 kHz using Agilent 4284-A

LCR meter. Cylindrical crystal plate of 9 mm diameter and 2 mm thickness

was placed between the two electrodes, which acts as a parallel plate

capacitor. Silver paint was coated on the surface of the sample in order to

make firm electrical contact. The capacitances of the sample were measured

for the applied frequency at different temperatures.

Figure 4.9 (a) and (b) show the variation of dielectric constants

with temperature at the frequencies 1 kHz and 100 kHz. It is observed from

the figures that the dielectric constant increases with increase of temperature

for the frequencies. This is normal dielectric behaviour of an antiferroelectric

ADP crystal. The dielectric constant of materials is due to the contribution of

electronic, ionic, dipolar and space charge polarizations, which depend on the

frequencies. At low frequencies, all these polarizations are active. The space

charge polarization is generally active at low frequencies and high

temperature. It is seen from the figures that the dielectric constant decreases

while increasing the concentration of oxalic acid.

The variation of dielectric loss with frequency is shown in

Figures 4.10 (a) and (b) at the frequencies 1 kHz and 100 kHz, respectively.

It is observed from the figures that the dielectric loss increases with increase

in temperature for the frequencies. It is also observed from the figures that

dielectric loss is high for oxalic acid added crystals. The low values of

dielectric loss indicate that the grown crystal contains minimum defects. In

accordance with Miller rule, the lower value of dielectric constant at higher

frequencies is a suitable parameter for the enhancement of SHG coefficient

76

40 60 80 100 120 140 160

6.0

6.5

7.0

7.5

8.0

8.5 pure ADP




Temperature (oC)

Die

lectr

ic c

on

sta

nt

40 60 80 100 120 140 160

6

7

8

9

10

11

12

13

14

15

16

17

Die

lectr

ic c

on

sta

nt

Temperature (oC)

pure ADP




(a)

(b)

(Hundelshause 1971, Balarew and Duhlev 1984). In the present case addition

of more than 1 mol% oxalic acid increases dielectric loss of the crystals. This

indicates that 3 and 5 mol% of oxalic acid doped crystals have defects more

than the pure ADP.

Figure 4.9 Temperature dependence of dielectric constant at (a) 1 kHz

(b) 100 kHz

77

Figure 4.10 Temperature dependence of dielectric loss at (a) 1 kHz

(b) 100 kHz

40 60 80 100 120 140 160

0.20

0.25

0.30

0.35

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

1.05

1.10

1.15 pure ADP ADP + 1 mol% of oxalic acid



Die

elc

tric

loss

Temperature (oC)

40 60 80 100 120 140 160

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22 pure ADP




Temperature (oC)

Die

elc

tric

lo

ss

(a)

(b)

78

4.4.5 SHG Measurements

It is seen from the above that the addition of oxalic acid plays an

important role in optical, thermal, mechanical, and dielectric behaviours of

ADP crystals. It was already found that the addition of 1 mol% of oxalic acid

enhanced the SHG efficiency of the ADP crystal. Similarly, in the present

case also, it is found that the addition of 1 mol% of oxalic acid has enhanced

SHG efficiencies of the crystal. The intensity of SHG gives an indication of

the nonlinear optical efficiency of the material. The doubling of frequency is

confirmed by green radiation of 532 nm. SHG output for pure and oxalic acid

added crystals are given in Table 4. 2. As seen in the table, SHG efficiency is

enhanced in 1 and 3 mol% of oxalic acid added ADP crystals where as in the

case of 5 mol% of oxalic acid, the SHG efficiency is decreased in comparison

to that of undoped material. These results are in good agreement with the

reported results (Bhagavannarayana et al 2008).

Table 4.2 SHG output signal values of the grown crystals

Crystal Output energy (mV)

Pure ADP 410 - 440

1 mol% of oxalic acid added ADP 480 - 500



The N-H-O bonding in ADP crystals connects the PO4 tetrahedra

with a neighboring NH4 group. Each oxygen atom is connected to another

oxygen atom in the neighbouring PO4 ion by two kinds of bonds: O-H-O and

N-H-O. When oxygen is connected with shorter N-H-O bond it tends to keep

other proton off in the O-H-O bond and when it is connected with the longer

N-H-O bond it tends to keep the acid proton nearby. Thus the extra hydrogen

79

bonds produce a distorted NH4 ion lattice at low temperature and cooperate

with the acid protons in causing proton configurations (Shunmugam et al

1987). The electrical conduction in the dielectric is mainly a defect-controlled

process in the low temperature region. The defect concentration will increase

exponentially with temperature and consequently the electrical conduction

also increases and the dielectric constant decreases. Hydrogen atoms present

in the oxalic acid combine with the ADP and the divalent (C2O4)2-

ions

possibly substitute for trivalent phosphate (PO4)3-

ions in the ADP crystal

lattice and the charge compensation vacancies are created (Shunmugam et al

1982). These vacancies produce the defects in the crystal lattice. The increase

in the oxalic acid concentration increases the defects. Due to this defect the

dipole moment of the crystal is decreased. Decrease in the dipole moment

gradually decreases the dielectric constant of the crystal (Shunmugam et al

1982). Similarly, the more number of defects due to oxalic acid impurity

gradually decreases the transmittance of the crystal.

The initial concentration of oxalic acid in the solution was chosen

to be 1, 3 and 5 mol%. The concentration of oxalic acid in a crystal is

expected to be nearly uniform, irrespective of the distribution coefficient, as

large quantity of solution is involved in growing a crystal which is relatively

very small in weight compared to the ingredient in the mother solution. The

oxalic acid molecule is much large in size compared to ADP molecule. At

very low concentration the oxalic acid molecules may get distributed within

the crystal more or less uniformly. When the concentration is higher, the

oxalic acid molecules can occupy neither the interstitial positions nor the

substitutional sites of the crystalline matrix, but most of them seem to be

segregated at the boundaries. As the oxalic acid could not be distributed

statistically in the crystal, one cannot expect full enhancement of the physical

properties for which the dopants are meant. This has been seen clearly in the

SHG measurements wherein a decrease of SHG efficiency was found at

higher concentrations.

80

4.5 CONCLUSIONS

Single crystals of oxalic acid added ADP were grown successfully

and it was found that 1 mol% oxalic acid added ADP crystal has growth rate

higher than the pure ADP. Addition of higher concentrations of oxalic acid

suppresses the growth rate and other properties. Increasing oxalic acid

concentration gradually decreases the optical transparency, hardness,

dielectric constant, and the decomposition temperature. With increase in

oxalic acid concentration, lowering of the decomposition temperature as well

as development of cracks at lower loads seem to be in conformity with each

other. It is concluded that, it is possible to harvest ADP crystals with higher

SHG efficiency in the presence of 1 mol% of oxalic acid.

chapter 4 effect of oxalic acid on the optical,...

Documents