Growth of Benzimidazole Single Crystal by Sankaranarayanan-Ramasamy Method and Its Characterization by High-Resolution X-ray Diffraction, Thermogravimetric/ Differential Thermal Analysis, and Birefringence Studies
CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 2 445-448
N. Vijayan,† K. Nagarajan,‡ Alex M. Z. Slawin,§ C. K. Shashidharan Nair,| and G. Bhagavannarayana*,† Materials Characterization DiVision, National Physical Laboratory, New Delhi 110 012, India, CGRC, Karunya Institute of Technology and Science, Coimbatore 641 114, India, School of Chemistry, UniVersity of St. Andrews, Fife, KY16 9ST, United Kingdom, and Department of Physics, P.S.G. College of Technology, Coimbatore 641 004, India ReceiVed August 2, 2006; ReVised Manuscript ReceiVed NoVember 24, 2006
ABSTRACT: In the present study, single crystals of benzimidazole (BMZ) possessing excellent nonlinear optical properties were grown for the first time by the recently invented Sankaranarayanan-Ramasamy (SR) method at 25 °C using a constant temperature bath. High-resolution X-ray diffraction (HRXRD) analysis indicates that the crystalline perfection is excellent without having any low/very low angle internal structural grain boundaries. Single-crystal XRD analysis confirms that the grown ingot belongs to the orthorhombic crystal system with a space group of Pna21. Thermogravimetric/differential thermal analysis (TG/DTA) analysis indicates a single-stage weight loss observed at 165 °C. Also, it has been found that the birefringence measured over a wide wavelength range between 5200 and 6860 Å is nearly constant showing that the BMZ crystals grown are suitable for efficient second harmonic generation and polarization devices. The present study indicates that the SR method is superior to slow evaporation solution and vertical Bridgman techniques for the growth of BMZ single crystals. Introduction Organic nonlinear optical crystals play an important role in second-harmonic generation (SHG), frequency mixing, electrooptic modulation, and optical parametric oscillation, etc.1 By considering the above-said importance, the job for crystal growers is to search for cost-effective and efficient nonlinear optical crystals to satisfy the day-to-day technological requirements. Benzimidazole (BMZ) is one of the organic materials with a good nonlinear optical efficiency; it has the molecular formula C7H6N2 with a molecular weight of 118.3. It belongs to the orthorhombic crystal system and is noncentrosymmetric in nature. Its SHG relative efficiency is 4.5 times greater than that of an inorganic potassium dihydrogen phosphate (KDP) single crystal.2 BMZ is a fused, two-ring, conjugated system with six carbon atoms in one ring and five atoms in the other. The smaller ring has nitrogen atoms in the first and third positions. Single crystals of the title compound have already been grown by using the slow evaporation solution (SEST) and vertical Bridgman (VBT) technique, and the results have been reported elsewhere.3,4 The SEST growth yields small-size single crystals with different crystallographic faces. For the phase-matching applications where the specimen should have more size along a particular direction, the crystals grown by this method are not economical. In the case of VBT growth, although the size of the crystals is good with the desired orientation, the quality of the crystal is generally not up to mark, as these crystals grow with structural grain boundaries due to unavoidable thermal gradients, which affect the device performance. The recently * To whom correspondence should be addressed. Tel: +91-11-25742610 ext. 2261/2263. Fax: +91-11-25726938. E-mail: bhagavan@ mail.nplindia.ernet.in. † National Physical Laboratory. ‡ Karunya Institute of Technology and Science. § University of St. Andrews. | P.S.G. College of Technology.
invented Sankaranarayanan-Ramasamy (SR) method5,6 has given the solution, and it is possible to grow bulk-size single crystals along a desired orientation needed for device fabrication. In the present investigation, we have grown for the first time BMZ single crystals by using the SR method and characterized the crystals by carrying out high-resolution X-ray diffraction (HRXRD), single-crystal X-ray diffraction (XRD), TG/DTA, and birefringence measurements. The results obtained are reported herein and discussed. Experimental Procedures Growth of Single Crystals. To grow the BMZ single crystal by the SR method, a seed crystal was grown by the SEST method and a well-faceted crystal was chosen. Then, the seed crystal was carefully prepared along the [100] direction. The ampule or container used in this method was made up of an ordinary hollow glass tube with a tapered V-shaped bottom to mount the seed crystal and a U-shaped top portion to fill a good amount of supersaturated solution to grow a relatively good size crystal. The middle portion was cylindrical in shape with lesser diameter than that of the U-shaped portion, wherein one can get a cylindrical ingot/boule. For controlled evaporation, the top portion was closed with some opening at the center. The schematic diagram of the presently used SR method set up (ampule) is shown in Figure 1. The ampule was well-degreased with acetone and then dried in the oven. The seed was carefully mounted in the bottom of the glass tube, by choosing the [100] direction of the seed crystal parallel to the length of the glass tube. Before the saturated solution was prepared, the material was purified by repeated recrystallization processes. The recrystallized salt was used to prepare the supersaturated solution using N,N-dimethylformamide (DMF) as the solvent. Then, the solution was filtered using Whatman filter paper (no. 1001 125). The filtered solution was taken in a beaker and then carefully decanted into the glass tube without disturbing the specimen seed. The top portion of the tube was covered by a plastic sheet with a hole at the center to limit the evaporation. Then, the whole tube was housed in a constant temperature bath with a setting temperature of 25 °C, which is the optimized growth temperature. The experimental conditions were closely monitored, and we found that the seed crystal started to grow after 10 days. In a time span of 40-45 days, a good quality single crystal of BMZ was harvested
10.1021/cg0605180 CCC: $37.00 © 2007 American Chemical Society Published on Web 01/10/2007
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Figure 1. Schematic diagram of growth setup used for the present study.
from the glass tube. The grown crystal was separated by cutting the walls of the ampule carefully. Disc-shaped specimens required for various measurements were cut by the crystal cutter perpendicular to the crystal boule. Care was taken to minimize mechanical stress/damage, which affects the crystalline perfection and also influences other physical properties of the specimens. The specimens needed for HRXRD were lapped and polished by nonpreferential chemical etchent using acetone and water in a 1:1 ratio. The as-grown crystal and the polished specimen of BMZ are shown in Figure 2a,b, respectively. XRD and HRXRD Measurements. The lattice parameters of the grown single crystal have been determined by adopting a Rigaku 007 generator with a Mercury detector (Mo radiation with an extremely low-temperature apparatus). The crystalline perfection of the as-grown single crystal of BMZ was characterized by HRXRD by employing a multicrystal X-ray diffractometer developed at NPL.7 The wellcollimated and monochromated Mo KR1 beam obtained from the three monochromator Si crystals set in dispersive (+, -, -) configuration was used as the exploring X-ray beam. The specimen crystal was aligned in the (+, -, -, +) configuration. Because of dispersive configuration, although the lattice constant of the monochromator crystal(s) and the specimen are different, the unwanted dispersion broadening in the diffraction curve of the specimen crystal was insignificant. The specimen can be rotated about a vertical axis, which is perpendicular to the plane of diffraction, with a minimum angular interval of 0.5 arc s. The diffracted intensity was measured by using a scintillation counter. TG/DTA Measurements. Thermal analysis was used to find out the weight (TGA) and energy change (DTA) in the sample with respect to the temperature. In the present study, thermal analysis was carried out on the crushed specimen of BMZ by employing a Mettler Toledo Star simultaneous DTA/TGA analyzer at 10 °C/min heating rate in the nitrogen atmosphere. Birefringence Measurement. The optically polished single crystal of BMZ was subjected to birefringence measurements using the channeled spectrum method with a halogen lamp as a source. The experimental set up used for the present study is shown in Figure 3. The source (S) energized with 500 W was collimated by the collimator (C) and optically polarized by a polarizer (P). The polarizer and analyzer (A) were placed in crossed positions, and the crystal (Cr) was placed in between them in such way that its optic axis was perpendicular to the incident ray. The transmitted light components from the analyzer interfere and an interference pattern were observed through the constant deviation spectrometer (CDS). For each dark band, the corresponding wavelength was read out directly from the drum of the CDS.
Figure 2. Single crystal of BMZ grown by the SR method: (a) as grown and (b) cut and polished specimens.
Figure 3. Block diagram of the birefringence experimental set up.
Results and Discussion Crystalline Perfection. The crystallographic orientation of the surfaces of the disc-shaped specimens whose preparation is mentioned above (Growth of Single Crystals section) was verified by an accurate method developed at NPL, which is based on high-resolution X-ray diffractometry.8 The nominal orientation of the surfaces was found to be along [100]. Figure 4 shows the high-resolution diffraction curve (DC) recorded for (200) diffracting planes of a typical specimen using Mo KR1 radiation in symmetrical Bragg geometry. As seen in the figure, the DC contains only a single peak and is quite sharp with a full width at half-maximum (fwhm) of 15 arc s. The full width at half maximum (FWHM) value of 15 arc s is very close to that expected from the plane wave theory of dynamical X-ray diffraction.9 The single sharp diffraction curve with very low FWHM indicates that the crystalline perfection is extremely good. The specimen is a nearly perfect single crystal without having any internal structural grain boundaries and dislocations (or very low density of dislocations, which could not be detected by high-resolution X-ray topography), which were otherwise observed in BMZ crystals grown by SEST and VBT methods.2,10
Growth of Benzimidazole Single Crystal
Crystal Growth & Design, Vol. 7, No. 2, 2007 447
Figure 6. Birefringence vs wavelength for BMZ crystal.
Figure 4. High-resolution diffraction curve recorded for (200) diffracting planes using Mo KR1 radiation in symmetrical Bragg geometry.
characteristic behavior may be used in birefringent crystal polarization devices.13 The birefringence was calculated using the formula 3n ) kλ/t14 where λ is the wavelength, t is the thickness of the crystal, and k is the fringe order. In the present experimental study, the thickness of the crystal was around 0.088 mm. The graph drawn between birefringence and wavelength is depicted in Figure 6. From this measurement, we found that the value of birefringence lies between 0.0962 and 0.0879 in the wavelength region of 520-686 nm. Such a nominal variation of birefringence over a wide range of wavelength shows the suitability of BMZ crystals for efficient second harmonic generation (SHG) and polarization devices. The detailed study of the SHG efficiency was reported in our recent literature.2 The SHG efficiency was found to be 4.5 times greater than that of KDP single crystal. Conclusions
Figure 5. TG/DTA spectrum of BMZ.
The FWHM of these crystals was in the range of 20 arc s to a few min of arc. Therefore, one can say that the crystalline perfection of the present specimens grown by SR method is much better than that of SEST and VBT methods. Lattice Parameters. From the single-crystal XRD measurement, we found that the grown single crystal belongs to the orthorhombic system and has a noncentrosymmetric nature with the space group of Pna21. The determined cell dimensions are a ) 6.7922 (29) Å, b ) 6.8937 (40) Å, c ) 13.3878 (56) Å, and R ) β ) γ ) 90°. The cell volume is V ) 626.8622 Å3. The observed values are consistent with the reported literature values.3,11 Thermal Behavior. The TG/DTA spectrum recorded for the present study is shown in Figure 5. The measurement indicates that the material exhibits single-stage weight loss starting at 165 °C, which may be due to the decomposition of BMZ, and below this temperature, no significant weight loss is observed. The DTA analysis of BMZ was also performed between 25 and 400 °C in the nitrogen atmosphere. The resulting spectrum is shown in the same figure (Figure 5). The heating rate was maintained at 10 °C min-1. In DTA, there is a sharp endotherm at 172 °C, which is assigned to the melting point of the specimen. This is in good agreement with the reported DSC values.3 Below this endotherm, no exothermic or endothermic peak is observed. The sharpness of the endothermic peak observed in DTA shows good degree of crystallinity12 of the specimen, which is in tune with the HRXRD results. Birefringence. Birefringence is a characteristic of an optical material and is defined as the difference between the two refractive indices of ordinary and extraordinary rays. This
Good quality single crystals of BMZ were successfully grown by the unidirectional solution growth (SR) method by optimizing the growth conditions. The lattice dimensions were determined from the single-crystal XRD, and we found that it belongs to the orthorhombic crystal system with a space group of Pna21. The crystalline perfection was found to be excellent as compared to SEST- and VBT-grown single crystals of BMZ. The low value of FWHM of the high-resolution X-ray diffraction curve and the sharp endothermic peak observed in DTA analysis confirm excellent quality of the crystal grown in the present investigation. From the TG analysis, a single-stage weight loss was observed at 165 °C. The very low variation in the value of birefringence over a wide wavelength range indicates that the BMZ single crystals are suitable for efficient SHG and polarization applications. Also, it is understood that the SR method is highly suitable for growing high-quality, large-size single crystals of BMZ. Acknowledgment. N.V. and G.B. are thankful to Prof. Vikram Kumar, Director, NPL, and Dr. S. K. Gupta, Head, Materials Characterization Division, NPL, for their kind support and encouragement. They are also thankful to Dr. S. K. Dhawan, NPL, for extending DTA/TGA facilities for thermal analyses and we also thank Prof. P. Ramasamy, Dean-Research, SSN College of Engineering, Kalavakkam, for useful discussions for growth of the title compound by the SR method. References (1) Badan, J., Hierle, R., Perigaud, A., Zyss, J., Eds. NLO Properties of Organic Molecules and Polymeric Materials; American Chemical Society Symposium Series 233; American Chemical Society: Washington, DC, 1993. (2) Vijayan, N.; Bhagavannarayana, G.; Kanagasekaran, T.; Ramesh Babu, R.; Gopalakrishnan, R.; Ramasamy, P. Cryst. Res. Technol. 2006, 41, 784-789.
448 Crystal Growth & Design, Vol. 7, No. 2, 2007 (3) Vijayan, N.; Ramesh Babu, R.; Gopalakrishnan, R.; Ramasamy, P.; Harrison, W. T. A. J. Cryst. Growth 2004, 262, 490-498. (4) Vijayan, N.; Balamurugan, N.; Ramesh Babu, R.; Gopalakrishnan, R.; Ramasamy, P.; Harrison, W. T. A. J. Cryst. Growth 2004, 267, 218-222. (5) Sankaranarayanan, K.; Ramasamy, P. J. Cryst. Growth 2005, 280, 467-473. (6) Sankaranarayanan, K. J. Cryst. Growth 2005, 284, 203-208. (7) Lal, K.; Bhagavannarayana, G. J. Appl. Crystallogr. 1989, 22, 209215. (8) Lal, K.; Bhagavannarayana, G.; Vijay, K.; Halder, S. K. Meas. Sci. Technol. 1990, 1, 793-800.
Vijayan et al. (9) Batterman, B. W.; Cole, H. ReV. Mod. Phys. 1964, 36, 681-717. (10) Vijayan, N.; Bhagavannarayana, G.; Ramesh Babu, R.; Gopalakrishnan, R.; Maurya, K. K.; Ramasamy, P. Cryst. Growth Des. 2006, 6, 1542-1546. (11) Escande, A.; Galigne, J. L. Acta Crystallogr. 1974, B30, 1647-1648. (12) Hameed, A. S. H.; Ravi, G.; Dhanasekaran, R.; Ramasamy, P. J. Cryst. Growth 2000, 212, 227-232. (13) Buse, K.; Luennemann, M. Phys. ReV. Lett. 2000, 85, 3385-3387. (14) Fischer, D. W.; Ohmer, M. C.; Schunemann, P. G.; Pollak, T. M. J. Appl. Phys. 1995, 77, 5943-5945.
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