CHAPTER V GROWTH AND PROPERTIES OF KAP CRYSTAL

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1 CHAPTER V EFFECT OF TRIVALENT METAL ION (Al 3+, Cr 3+, Fe 3+ ) IMPURITIES ON THE GROWTH AND PROPERTIES OF KAP CRYSTAL 5.1 Introduction The metallic ion dopants (Fe 3+, Cr 3+, Zn 2+, Cu 2+, etc.) in the KAP crystals are reported to induce significant changes in optical, ferroelectric and non linear optical behaviours [55-57]. Kejalakshmy et. al [55] have reported that the metal ion (Cr and Fe) doped KAP crystals serve as good electro-optic modulator. Monica Enculescu [51, 58] has studied the effect of rhodamine 6G, coumarine 6 and polyvinylpyrrolidone (PVP) on KAP crystals using solution growth method. It was reported that the dopants enhance the second harmonic generation (SHG) efficiency of KAP. Parthiban et al. [57] also reported that the Zn 2+ doping into KAP crystals improves its SHG efficiency. Murugakoothan et al. [45] have investigated the effect of impurities like potassium dihydrogen orthophosphate, urea and L-arginine phosphate on KAP crystals and concluded that the urea doping yields high mechanical stability and optical transmission than other dopants. In this background, the goal of present work was set to the growth and characterisation of a series of KAP crystals with different cations (trivalent, divalent and monovalent). In each case, three different ions were chosen as dopants into KAP. This chapter describes the effect of three different trivalent metal ion impurities (Al 3+, Cr 3+ and Fe 3+ ) on the growth, structural, thermal, dielectric, optical, second harmonic generation efficiency and mechanical property of the KAP crystals. The ionic and atomic sizes of the selected metal ions are shown in Table

2 Table 5.1 Ionic and atomic sizes of selected metal ions No. Metal ion Ionic radius (Å) Trivalent Atomic sizes ( Å ) 1. Al Cr Fe Divalent 4. Ba Ca Mg Monovalent 7. Ag Li Na K Effect of trivalent metal ion impurities on KAP crystal Experiment KAP crystals were grown by slow evaporation method at room temperature with double distilled water as solvent. Recrystallised salt of KAP (Merck) was used as source material. Initially saturated KAP solution was prepared at room temperature and filtered using micro filter paper of 0.1µm porosity. The trivalent impurities Al 3+, Cr 3+ and Fe 3+ were selected as additives in the form of Al 2 (SO 4 ) 3.H 2 O, K 2 Cr 2 O 4 and FeCl 3. These impurities were added (0.1 M% each) with pure KAP solution and the final solutions were stirred well, filtered and stored in separate beakers covered with perforated sheets. The ph values of all the solutions were found to be 4. The supersaturated solutions in beakers were placed in room temperature. The crystals were formed by evaporation of solvent. The crystals were allowed grow for favourable size. The grown crystals were harvested and 77

subjected to characterization studies. The ph values of the final solutions after harvesting the crystals were found to be 4.0 as that of initial solution. 5. 3

3 subjected to characterization studies. The ph values of the final solutions after harvesting the crystals were found to be 4.0 as that of initial solution Results and discussion Crystal growth KAP crystals grown with and without dopant metal ions (Al 3+, Cr 3+ and Fe 3+ ) are shown in Fig There has been no significant change in the colour and crystal morphology due to aluminium doping. On the other hand, Cr doping led to light yellow colour and Fe doping resulted in light brown colour for KAP crystals (Figs.5.2c & d). Elemental analyses were made by using Perkin Elmer optima 5300 DV ICP-OES instrument. Inductively coupled plasma-optical emission spectrometric (ICP-OES) analysis confirmed the presence of dopants in the order of 17.15, 15.73, mg/l for Al 3+, Cr 3+ and Fe 3+ ions in as grown KAP crystals respectively. These cations could be placed in between the two adjacent layer of the KAP or substituted only for potassium ion without disturbing the carboxylic acid group. The ph value of the mixed solution remained unchanged throughout the growth run similar to that of pure KAP solution. The ph of the solution depends on the concentration of H + ions present in the solution. So, it is possible that the dopant Al 3+, Cr 3+ and Fe 3+ ions substitute at K + ion site or it get into the interstitial sites without disturbing the carboxylate hydrogen of the compound in the solution. Fig. 5.1 Photograph of as grown KAP crystals (a) pure (b) Al 3 + doped (c) Cr 3 + doped (d) Fe 3+ doped 78

4 5.3.2 Powder X-ray diffraction analyses Powder X-ray diffraction pattern of all the grown crystals were recorded on Joel JDX 8030 diffractometer (CuKα1 wavelength Å). The XRD patterns of the grown crystals are shown in Fig The results confirmed that all the crystals grew in orthorhombic structure with space group Pca2 1 according to JCPDS data ( and ). The XRD pattern shows a peak shift for the crystals grown in the presence of cations when compared to the undoped KAP. These shifts in peak positions caused a slight change in lattice parameters when compared to the pure KAP (Table 5.2) which could be attributed to the doping of trivalent impurity ions at potassium site. It should be remembered that ionic radii of K (1.52Å) is higher than that of dopant ions Al (0.53Å), Cr (0.75Å) and Fe (0.63Å) respectively. Thus XRD results confirm the metal ions doping into KAP. There is also noticeable decrease in the peak intensity for the Cr doped KAP when compared to Al 3+ and Fe 3+ doped KAP. Fig. 5.2 Powder X-ray patterns of KAP crystals; (a) pure (b) Al 3+ doped (c) Cr 3+ doped d) Fe 3+ doped 79

5 Table 5.2 Lattice parameters of pure and metal ion doped KAP crystals Lattice parameters Pure KAP Al-KAP Cr-KAP Fe-KAP a (Ǻ) b (Ǻ) c (Ǻ) Volume (Ǻ) Thermal analyses Thermogravimetric analysis was carried out between 30 to 1000 C in nitrogen atmosphere using Universal V4.3A TA instrument (SDT Q600 V8.3 Build 101) with the heating rate of 20 C/min. The TG curves of pure and metal ions doped KAP are shown in Fig Experimentally observed mass loss at various stages of decomposition agrees well with the calculated values according to the following equations. 2KC 8 H 5 O 4 K 2 C 8 H 5 O 4 + C 7 H 5 O 2 + CO (1) K 2 C 8 H 5 O 4 K 2 CO 3 + C 7 H 5 O (2) The weight loss begins at around 250 C and about 40% of the total mass loss has occurred during the initial decomposition of both pure and doped KAP according to equation.1. The onset temperature of the decomposition was found to be 255, 270, 258 and 287 C for pure, Al 3+, Cr 3+ and Fe 3+ doped KAP crystals respectively. Newkirk et al [69] and Belcher et al [70] have carried out an extensive studies on thermal behaviour of KAP in N 2 and air atmospheres. The result obtained in the present work agrees well with these reported results. It can be noticed that the weight loss of Fe-KAP begins ahead at about 50 C and continues to lose weight up to 10 wt. % before decomposing at 287 C. This is 80

6 contrary to the results of Al 3+ and Cr 3+ doped KAP and pristine KAP. It may be due to the uptake of water during crystallisation of KAP in the presence of Fe ion. The total weight loss up to 1000 C was found to be 81%, 91%, 79% and 93% for pure, Al 3+, Cr 3+ and Fe 3+ doped KAP crystals respectively. The results show the highest weight loss for the Fe doped KAP at 1000 C. Fig. 5.3 Thermogravimetric curves of pure and metal ion doped KAP crystals: (a) pure (b) Al 3+ doped, (c) Cr 3+ doped, (d) Fe 3+ doped Optical transmittance studies UV-NIR spectrum was recorded on a Perkin Elmer Lamda 25 spectrometer in transmission mode. The optical properties of a material are important, as they provide information on the electronic band structures, localized states and types of optical transitions. For optical applications, especially for second harmonic generation, the crystal should be highly transparent in the considerable region of wavelength. The recorded transmittance spectra of pure, and Al 3+, Cr 3+ and Fe 3+ doped KAP crystals in the wavelength range nm are shown in Fig It can be seen that the crystals have sufficient transmission in the entire visible and near infrared region. The cut off wavelength was almost the same (~ 300 nm) for pure and doped KAP crystals (Fig.5.4), 81

7 which is due to the n-π transition of the carbonyl group of the carboxyl functions [71]. The Al 3+ and Cr 3+ doped KAP crystals have lower transmittance values than that of pure and Fe 3+ doped KAP crystals. These results revealed that Al 3+ and Cr 3+ doping into KAP suppress its optical transparency. Fig.5.4 UV-NIR spectra of pure and trivalent metal ion doped KAP crystals FTIR analyses FTIR transmittance spectra of pristine and doped KAP crystals were recorded in the range of 450 to 4000 cm -1 using Lamda 35 make Perkin Elmer (Spectrum RX1) spectrometer. Fourier transform infrared (FTIR) spectra of pure and trivalent cation doped KAP are shown in Fig The observed vibrational frequencies and their assignments are listed in Table 5.3. The peaks in the region below 900 cm -1 appear due to C-H bending vibrations. The C=C mode of vibration is present between 1476 cm -1 to 1478 cm -1. The asymmetric stretching modes of vibrations of C-O have appeared around 1380 and 1565 cm -1 for both pristine and metal ion doped KAP respectively [57]. The aromatic ring group appears in the frequency range cm -1. C-H stretching vibration appears near 2480 cm -1 in all the investigated samples. In general there is no considerable change in peak positions corresponding to carboxylic O-H stretching in the frequency range between 82

8 2200 and 3400 cm -1. However the characteristic peak around 3445 cm -1 corresponding to O-H stretching hydrogen bond was varying significantly in terms of intensity. Table 5.3 Vibrational assignments of pure and metal ion doped KAP crystals Pure KAP Al-KAP Cr-KAP Fe-KAP Assignments (cm -1 ) (cm -1 ) (cm -1 ) (cm -1 ) O-H stretching hydrogen bond C-H aromatic stretching C=C asymmetric stretching C=C stretching C=O Carboxylate ion =O asymmetric stretching C=C ring stretching C=O Carboxylate ion =O symmetric stretching C- O stretching C-C stretching C- C=O stretching =C-H out of plane bending C-H out of plane bending C-C sretching C-O wagging C=C-C out of plane ring deformation C=C-C out of plane ring deformation C=C out of plane ring bending 83

9 Fig. 5.5 FTIR spectra of pure and trivalent metal ion doped KAP crystals In particular the peak intensity of 3445 cm -1 is very strong for Fe 3+ doped KAP when compared to the peaks corresponding to Cr 3+ (3425cm -1 ) and Al 3+ (3430cm -1 ) doped samples. The difference in sharpness of multiple bands in this region could be taken as evidence for doping of trivalent metal ions Dielectric studies Dielectric studies were carried out on (010) faces of the grown crystals using Hioki LCR Hitester. A sample with graphite coating on two opposite faces was placed between two copper electrodes and thus a parallel plate capacitor was formed. The capacitance of the sample was measured at various frequencies ranging from 100 Hz to 5 MHz. The dielectric constant is calculated using the relation ε r = Cd/ε 0 A, where C is the capacitance, d is the thickness, A is the area of the crystal and ε 0 is the absolute permittivity of the free space (8.854 x F/m). The dielectric constant and dielectric loss for pure and doped crystals measured at 40 C are shown in Figs. 5.6 and 5.7 respectively. The dielectric constant has high values in the low frequency region for all the investigated samples. From the graphs, it is clear that dielectric constant (ε r ) decreases as the frequency increases in all cases. The higher values of dielectric constant at low 84

10 frequencies may be due to the presence of all the four polarizations namely space charge, orientation, electronic and ionic polarization and its low values at high frequencies might be due to the loss of importance of these polarizations [57]. In the low frequency region, dielectric constant of Al 3+ and Fe 3+ doped crystals was found to be higher than that of pristine and Cr 3+ doped crystals. These higher values of dielectric constant in the low frequency region are due to the fact that all the four polarizations are present initially. The corresponding dielectric losses for all the investigated crystals are shown in Fig The characteristic of low dielectric loss with high frequency for a given sample suggests that the sample possesses good quality with lesser defects. This parameter is of vital importance for nonlinear optical applications. Fig. 5.6 Dielectric constant versus log frequency measured at 40 C of trivalent metal ion doped KAP crystals 85

11 Fig. 5.7 Dielectric loss vs. log frequency measured at 40 C of trivalent metal ion doped KAP crystals Vickers microhardness studies Microhardness measurements were carried out using Shimadzu tester on (010) face of all the crystals. Indentations were made with different loads. The hardness values were calculated from the formula Hυ = P/d 2 kg/mm 2, where Hυ is the Vickers microhardness number, P is the applied load (in kg) and d is average diagonal length of the indentation (mm). The hardness values as a function of load are shown in Fig The result reveals that the initial microhardness values are very high for Cr doped KAP crystal when compared to undoped, Al and Fe doped KAP. The hardness values increased with increase in load for pure, Al and Fe doped KAP crystals. Contrarily, Hυ decreased with increase in load for Cr doped crystal. Beyond 100g, significant cracking occurs, which may be due to release of internal stress generated with indentation. 86

12 Fig. 5.8 Vickers hardness vs. load of pure and trivalent metal ion doped KAP crystals SHG measurements The Kurtz and Perry powder technique remains an extremely valuable tool for initial screening of materials for second harmonic generation [76]. A Q switched Nd:YAG laser beam of wavelength 1064 nm was used within an input beam energy of 5.02 mj/pulse and pulse width of 8 ns, the repetition rate being 10Hz. The SHG radiations of 532 nm (green light) emitted were collected by a photo multiplier tube (PMT-Philips Photonics model 8563) and the optical signal incident on the PMT was converted into voltage output at the CRO (Tektronix-TDS 3052). The results obtained by this method show that the SHG efficiencies for pure, Al 3+, Cr 3+ and Fe 3+ doped KAP crystals are about 34%, 34%, 30% and 28% as that of KDP. 5.4 Conclusion Single crystals of pure, Al 3+, Cr 3+ and Fe 3+ doped KAP have been grown from aqueous solutions by slow evaporation method. The powder XRD analysis confirmed that all the doped materials are crystallized in the orthorhombic structure. Elemental analysis by ICP-OES confirmed the metal ions doping into the grown KAP crystals. The FTIR 87

13 spectrum confirmed the presence of functional groups in the pure and doped compounds. Thermal decomposition begins at 255, 270, 258 and 287 C for pure, Al 3+, Cr 3+ and Fe 3+ doped KAP crystals respectively. These variations have been attributed due to the doping of metal ion into KAP. Optical transmission analysis revealed that the doped KAP crystals have very high percentage of transmission in the entire visible region, which is very essential for NLO crystals. The variation of dielectric constant and dielectric loss were studied as a function of frequency. Al 3+ and Fe 3+ doped crystals have positive influence on dielectric, thermal stability and hardness values. The hardness values increased with increase in load for pure, Al and Fe doped KAP crystals. The SHG conversion efficiency of the doped samples were nearly equal to that of undoped KAP and all of them are suitable for NLO applications. 88

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