Growth and Characterization of Novel Nonlinear Optical Potassium Boromalate Monohydrate (KBM) Single Crystal Grown by Modified Sankaranarayanan Ramasamy (SR) Method C. Justin Raj, S. Krishnan, S. Dinakaran, S. Mary Navis Priya, R. Uthrakumar, and S. Jerome Das*
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3956–3958
Department of Physics, Loyola College, Chennai-600 034, India ReceiVed NoVember 23, 2007; ReVised Manuscript ReceiVed August 3, 2008
ABSTRACT: Nonlinear optical active single crystal of potassium boromalate monohydrate (KBM) crystal of 30 mm (1.4 in.) length has been grown with a growth rate of 1 mm per day in a period of 30 days from aqueous solution by the modified Sankaranarayanan-Ramasamy uniaxial crystal growth method. The grown crystal was subjected to single crystal X-ray diffraction analysis, confirming that the crystal belongs to monoclinic structure with space group P21. The presence of functional groups and the protonation of ions for the KBM crystals were confirmed by FTIR analysis. The nonlinear optical property of the grown crystal was confirmed by the Kurtz powder technique and the second harmonic generation efficiency was found to be 1.5 times that of potassium dihydrogen orthophosphate (KDP) crystal. The UV-visible spectrum of the crystal shows that UV cut off wavelength is at 320 nm. TG/DTA studies reveal that the crystal was thermally stable up to 131.5 °C. The frequency-dependent dielectric properties of the crystal were carried out for different temperatures. Introduction Semiorganic materials have gained considerable attention due to their excellent properties for frequency doubling in laser applications such as high laser damage threshold, wide transparency range, less deliquescency and high nonlinear coefficient.1-4 Semiorganic nonlinear optical materials gain importance over the organic counterparts because of their large polarizability, wide transmission window, high mechanical and thermal stability. Extensive investigations resulted in the discovery of a new phase matchable semiorganic nonlinear optical (NLO) crystal, L-arginine phosphate monohydrate (LAP), which has been proposed as an alternate to potassium dihydrogen orthophosphate (KDP).5-7 Borate crystals such as lithium triborate and barium metaborate have been widely used for NLO devices and ultraviolet (UV) laser generation. So efforts to incorporate favorable characteristics of an organic compound to the borate (inorganic) counterpart led to the development of semiorganic NLO materials.8,9 The uniaxial Sankaranarayanan-Ramasamy (SR)10-13 solution growth method has attracted researchers due to the growth of defect-free transparent bulk single crystals along a particular axis and also the entire quantity of the solute was converted into crystal, thus achieving 100% solute-crystal conversion efficiency. In this present work, optically good quality single crystal of potassium boromalate monohydrate (KBM) was grown using the modified uniaxial growth assembly as reported by the authors.14 The crystal was subjected to different characterizations such as single crystal XRD, FTIR, UV-vis absorption analysis, NLO studies, thermal analysis (TG/ DTA) and dielectric studies. Experimental Procedures Synthesis and Growth of Seed Crystals. The seed crystals were synthesized from potassium carbonate, boric acid and DL-malic acid of analar grade (Merck) in the molar ratio 1:1:1 in deionized water. The purity of the synthesized salt was further increased by repeated recrystallization. The supersaturated solution of synthesized potassium * Corresponding author:
[email protected]; jerome@ loyolacollege.edu; tel: 009144 2817 5662; fax: 009144 2817 5566.
Figure 1. Crystal growth assembly. boromalate monohydrate (KBM) was kept undisturbed at room temperature. Transparent seed crystals of KBM were grown within a period of 20 days, among which defect-free seed crystal was chosen for the uniaxial growth along the (101) plane. Experimental Setup and Crystal Growth. An optically good quality seed crystal was mounted on the bottom of the ampule, filled with supersaturated solution of KBM and placed along the axis of the growth assembly. Here, an assembly of alternating 40 W filament lamps serve as a temperature gradient for the growth as shown in Figure 1. The temperature at the top of the ampule was maintained at 45 °C using a temperature controller setup for the evaporation of the saturated solution. To maintain a steady temperature around the ampule, it was rotated 90° per second using a stepper motor along the axis of the growth setup. The temperature gradient makes the concentration gradient maximum at the bottom and minimum at the top of the ampule for avoiding the spurious nucleation along the axis of the ampule. The growth rate of the crystal was found to be 1 mm per day. Crystal of 30 mm length has been grown successfully within a period of 30 days. The morphology of the
10.1021/cg7011556 CCC: $40.75 2008 American Chemical Society Published on Web 09/19/2008
Novel Nonlinear Optical KBM Single Crystal
Crystal Growth & Design, Vol. 8, No. 11, 2008 3957
Figure 4. UV absorbance spectrum of as grown crystal.
Figure 2. Photograph of KBM crystal.
Figure 5. TG/DTG thermogram of KBM crystal.
Figure 3. FTIR spectrum of title crystal. grown crystal is like that of the growth vessel. The photograph of the grown crystal is shown in Figure 2.
Results and Discussion Single Crystal X-ray Analysis. Carefully selected crystal of dimensions 1 × 0.5 × 0.5 mm3 was indexed for single crystal data collection using an ENRAF NONIUS CAD-4 X-ray diffractometer. The X-ray diffraction study reveals that the crystal belongs to the noncentro symmetric monoclinic system with space group P21 and the lattice parameters are a ) 5.540 Å, b ) 11.980 Å, c ) 10.740 Å, γ ) R ) 90°, β ) 113.20° and V) 655.436 Å3, which are in good agreement with that of reported values in the literature.15 FTIR Spectral Study. Fourier transform infrared spectra (FTIR) of KBM crystal was carried out in the middle IR region between the wavenumber 400 to 4000 cm-1 by KBr pellet technique and the resultant spectrum is shown in Figure 3. The sharp peak at 3500 cm-1 is due to the O-H stretching vibration. The peaks at 2971, 2753, 2639 cm-1 are assigned to the stretching vibrations of CH2 and C-H groups. The CdO, C-H, CO-OH stretching vibrations of COOH carboxylate groups were found at 1731, 1687, 1053 cm-1 respectively. The wagging of CH2 and bending of COO- groups are assigned at 1328 and
670 cm-1. These peaks confirm the presence of carboxylic acid group in the grown crystal. The borate groups are found at 926 cm-1 for B-O symmetrical stretching vibrations and 1053 cm-1, 1251 cm-1 are assigned B-O asymmetrical stretching vibrations. The peak at 512 cm-1 indicates the presence of hydrates of potassium ion in the crystal lattice.16 Optical Absorption Spectral Studies. The optical absorption spectrum was measured in the range 200-1500 nm using a VARIAN CARY 5E UV-vis-NIR spectrophotometer and the obtained spectrum is shown in Figure 4. When the absorbance is monitored from longer to shorter wavelength, the absorbance is found to be low throughout the entire visible and IR region due to the delocalization of electronic cloud through charge transfer axis. This is the most desirable property of the materials possessing NLO activity. The UV cutoff of the crystal in which the transmittance falls to zero is found to be at 320 nm. Thermal Analysis. The TG/DTA thermogram of the KBM crystal was obtained using a NETZSCH STA 409 C/CD thermal analyzer and the resultant thermogram is shown in Figure 5. A ceramic crucible was used for heating the sample of weight 5.62 mg and the analysis was carried out in nitrogen atmosphere at a heating rate of 10 °C per minute in the temperature range 30-1400 °C. The TG thermogram reveals that KBM decomposes in four stages. The weight loss at 131.5 °C is due to the loss of water of hydration present in the crystal lattice and this is assigned as the melting point of the crystal. In the second stage, 4.6% was lost at 171.2 °C and at 243.0 °C, a sharp endothermic peak of DTA coincides with TG. In the final step, there is almost complete volatilization of the KBM leaving only
3958 Crystal Growth & Design, Vol. 8, No. 11, 2008
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with lesser defects and this parameter is of vital importance for nonlinear optical applications.19 Conclusions
Figure 6. Variation of dielectric constant with log frequency.
Figure 7. Variation of dielectric loss with log frequency.
8.8% of the residue at 1200 °C. From the thermal study, it can be concluded that the crystal was stable up to 131.5 °C. NLO Studies. The NLO property of KBM crystal was studied by the Kurtz and Perry powder technique.17 The sample was illuminated using Q-Switched Nd: YAG laser with the first harmonic output of 1064 nm and pulse width of 8 ns. The emission of green radiation from the crystal confirmed the second harmonic signal generation (SHG). KDP sample was used as the reference material and the output power intensity of KBM was found to be 1.5 times than that of KDP crystal. Dielectric Studies. The dielectric constant and the dielectric loss of the KBM crystals were studied at three different temperatures (311 K, 328 and 358 K) using a HIOKI 3532 LCR HITESTER in the frequency region 50 Hz to 5 MHz. Figure 6 shows the plot of dielectric constant (εr) versus log frequency. The dielectric constant has high values in the lower frequency region and then it decreases with the increase in frequency. The very high value of εr at low frequencies may be due to the presence of all the four polarizations, namely, space charge, orientational, electronic and ionic polarization and its low value at higher frequencies may be due to the loss of significance of these polarizations gradually. From the plot, it is also observed that dielectric constant increases with an increase in temperature, and this is attributed due to the presence of space charge polarization near the grain boundary interfaces, which depends on the purity and perfection of the sample.18 The variation of dielectric loss with frequency is shown in Figure 7. The characteristics of low dielectric loss with high frequency for the sample suggest that it possesses enhanced optical quality
Single crystals of KBM were grown by the unidirectional solution growth method. The crystal shows morphology of the growth vessel with maximum growth rate of 1 mm per day and crystal of length 30 mm have been successfully grown along the (101) plane within a period of 30 days. From the growth, it was found that high viscosity of the solution suppresses the growth rate of the crystal. Single crystal X-ray diffraction analysis confirms that the crystal belongs to monoclinic system with space group P21. The FTIR spectral analysis confirms the presence of functional groups of the crystals. The optical absorption study reveals the high transparency of the crystal with a UV cut off at 320 nm. The TG and DTA analysis shows that the crystal is stable up to 131.5 °C. The NLO efficiency of the crystal is found to be 1.5 times greater than that of KDP. The high nonlinearity, transparency, thermal stabilities and low dielectric loss make this crystal suitable for SHG and optoelectronic device applications. Further, investigations on structural and transport properties are in progress. Acknowledgment. The authors thank Fr. Albert Muthumalai, Principal, Loyola College, Chennai and Rev. Dr. S. Peter Xavier, Secretary, Loyola College, for their encouragements through out the work. The encouragements and support rendered by Dr. G. Shekaran, Dr. L. John Kennedy (CLRI, Chennai) and Prof. P. Ramasamy, Dean of Research, SSN College of engineering and technology are gratefully acknowledged.
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