CHAPTER 4 EFFECT OF OXALIC ACID ON THE OPTICAL, THERMAL, DIELECTRIC AND MECHANICAL BEHAVIOUR OF ADP CRYSTALS

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1 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

2 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 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 mm 3 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

3 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 mol% oxalic acid doped ADP single crystals

4 70 Figure mol% oxalic acid doped ADP single crystals Figure mol% oxalic acid doped ADP single crystals 4.4 CHARACTERIZATION 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

5 71 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 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 Transmitance (%) Pure ADP ADP+ 1 mol% of oxalic acid ADP+ 3 mol% of oxalic acid ADP+ 5 mol% of oxalic acid wavelenghth (nm) Figure 4.5 Transmission spectra of the grown crystals

6 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 30 o C to 300 o C. 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 o C (Science lab-material safety data sheet). The presence of oxalic acid appears to decrease the decomposition temperature Weight (%) Pure ADP ADP+1 mol %oxalic acid ADP+3 mol % oxalic acid ADP+5 mol % oxalic acid Temperature ( o C) Figure 4.6 TGA curves of the grown crystals

7 73 40 Microvolt Endodown ( V) Pure ADP ADP+1 mol %oxalic acid ADP+3 mol % oxalic acid ADP+5 mol % oxalic acid Temperature ( o C) Figure 4.7 DTA curves of the grown crystals Table 4.1 Thermal analysis data of the grown crystals Crystal Endothermic peak ( o C) Decomposition Starting temperature ( o C) Pure ADP mol% of oxalic acid added ADP mol% of oxalic acid added ADP mol% of oxalic acid added ADP 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 mm 3 with (1 0 0) face

8 74 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 Pure ADP ADP + 1 mol% of oxalic acid ADP + 3 mol% of oxalic acid ADP + 5 mol% of oxalic acid 60 Hardness (H V ) kg/mm Load (p) g Figure 4.8 Vickers hardness spectra of the grown crystals Vickers microhardness number was determined using H v = p/d 2. 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

9 75 sufficient to form dislocation thus confirming that the crystalline perfection is not good in the crystals 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

10 76 (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 pure ADP ADP + 1 mol% of oxalic acid ADP + 3 mol% of oxalic acid ADP + 5 mol% of oxalic acid Dielectric constant (a) Temperature ( o C) pure ADP ADP + 1 mol% of oxalic acid ADP + 3 mol% of oxalic acid ADP + 5 mol% of oxalic acid Dielectric constant (b) Temperature ( o C) Figure 4.9 Temperature dependence of dielectric constant at (a) 1 khz (b) 100 khz

11 77 Dieelctric loss pure ADP ADP + 1 mol% of oxalic acid ADP + 3 mol% of oxalic acid ADP + 5 mol% of oxalic acid (a) Temperature ( o C) pure ADP ADP + 1 mol% of oxalic acid ADP + 3 mol% of oxalic acid ADP + 5 mol% of oxalic acid Dieelctric loss (b) Temperature ( o C) Figure 4.10 Temperature dependence of dielectric loss at (a) 1 khz (b) 100 khz

12 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 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 mol% of oxalic acid added ADP mol% of oxalic acid added ADP mol% of oxalic acid added ADP The N-H-O bonding in ADP crystals connects the PO 4 tetrahedra with a neighboring NH 4 group. Each oxygen atom is connected to another oxygen atom in the neighbouring PO 4 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

13 79 bonds produce a distorted NH 4 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 (C 2 O 4 ) 2- ions possibly substitute for trivalent phosphate (PO 4 ) 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.

14 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.