A Comparative Study on Solution- and Bridgman-Grown Single Crystals of Benzimidazole by High-Resolution X-ray Diffractometry, Fourier Transform Infrared, Microhardness, Laser Damage Threshold, and Second-Harmonic Generation Measurements
CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 6 1542-1546
N. Vijayan,† G. Bhagavannarayana,*,† R. Ramesh Babu,‡,§ R. Gopalakrishnan,‡ K. K. Maurya,† and P. Ramasamy§ Materials Characterization DiVision, National Physical Laboratory, New Delhi 110 012, India, Department of Physics, Anna UniVersity, Chennai 600 025, India, and SSNCE, KalaVakkam, Anna UniVersity, Chennai 603 110, India ReceiVed January 4, 2006
ABSTRACT: Nowadays more attention has been paid to the growth of organic nonlinear optical single crystals due to their high nonlinear optical efficiency and fairly good optical damage threshold comparable to that of inorganic counterparts. The organic nonlinear optical single crystals of benzimidazole (BMZ) grown by the slow evaporation solution growth technique (SEST) and the vertical Bridgman technique (VBT) were characterized, and their results have been compared. Characterization has been made by high-resolution X-ray diffractometry (HRXRD), Fourier transform infrared (FTIR), laser damage threshold, microhardness, and second-harmonic generation (SHG) measurement studies. The high-resolution X-ray diffraction curves (DCs) recorded by an inhouse developed multicrystal X-ray diffractometer (MCD) revealed that the crystals grown by both methods contain internal structural grain boundaries. However, VBT crystals normally contain multiple low angle (tilt angle R g 1 arc min) boundaries due to thermal stress caused during the cooling cycle by the difference in the lattice expansion coefficients of the glass tube and the crystal, whereas SEST crystals were found to contain only one very low angle (R < 1 arc min) boundary probably due to entrapment of solvent in the crystal during growth. From FTIR studies, it was found that the packing of molecules is more dense in the case of VBT-grown crystals than the case of SEST-grown crystals. From the Vicker’s microhardness measurements made along the [100] direction, superior mechanical behavior is observed in VBT crystals than in SEST crystals. The mechanical behavior is correlated with their laser damage threshold values. 1. Introduction The search for new nonlinear optical (NLO) materials has been increased in the last few decades because of their potential applications in optical modulation, second-harmonic generation (SHG),1 optical signal processing, optical switching,7 optical data storage devices, etc.12 It is also used in phase conjugation and image reconstruction.8 Organic molecules containing conjugate systems have some advantages over inorganic materials because of the possibility of highly enhanced electronic nonlinear optical polarization responses. The basic structure of organic NLO materials is based on the π bond system;13 due to the overlap of π orbitals, delocalization of electronic charge distribution leads to a high mobility of the electron density. Functionalization of both ends of the π bond system with appropriate electron donor and acceptor groups can enhance the asymmetric electronic distribution in either or both ground and excited states, leading to an increased optical nonlinearity.16 Some of the other advantages of organic materials over inorganic materials are the scope for altering the properties by functional substitutions, high degree of nonlinearity, and high damage resistance. Currently there is much interest in organic NLO materials because of their large second- or third-order hyperpolarizabilities compared to inorganic NLO materials.16 Because of the large nonlinearities and optical threshold of organic materials, a wide range of such materials have been found by many researchers. Benzimidazole crystals have high * Corresponding author: E-mail:
[email protected]. Phone: +91-11-25742610, ext. 2261/2263. † National Physical Laboratory. ‡ Department of Physics, Anna University. § SSNCE, Kalavakkam.
structural perfection and performance stability. Benzimidazole single crystals have been successfully grown by the slow evaporation solution growth technique (SEST) using methanol as the solvent.14 By analysis of the thermal behavior of benzimidazole, because there is no decomposition before melting, it was decided to grow these crystals by the vertical Bridgman technique (VBT)15 in view of growing bulk crystals. Since SEST and VBT methods are quite different, the crystals grown by these methods may have different crystalline perfection, which may lead to some differences in their physical properties. So, it is interesting to study their crystalline perfection and other physical properties. In the present paper, we have reported the comparative analysis of the specimens grown by both these techniques by employing high-resolution X-ray diffractometry (HRXRD), Fourier transform infrared (FTIR), microhardness, laser damage threshold, and SHG efficiency measurements. 2. Crystal Growth 2.1. Slow Evaporation Solution Growth Technique (SEST). The commercially available benzimidazole (∼98% purity, molecular weight 118.3, and melting point 172 °C) powder was purified by repeated crystallization processes using methanol as the solvent, and the recrystallized material was used for the growth of bulk crystals. The solubility was tested in three different solvents, and it was found that methanol is a good solvent for the growth. The detailed growth procedure and solubility graph is reported elsewhere.14 The bulk single crystal was harvested from the mother solution after 15 days. Figure 1a shows the solution-grown single crystals of benzimidazole using methanol as a solvent.
10.1021/cg060002g CCC: $33.50 © 2006 American Chemical Society Published on Web 05/13/2006
Solution- and Bridgman-Grown Single Crystals of BMZ
Crystal Growth & Design, Vol. 6, No. 6, 2006 1543
Figure 1. Single crystals of benzimidazole grown by (a) SEST and (b) VBT growth methods.
2.2. Vertical Bridgman Technique (VBT). The benzimidazole powder was purified by the zone refining method and was taken for growth. The bulk single crystals of benzimidazole have been successfully grown by the Bridgman technique using a glass ampule under vacuum. In the present study, the quartz heating oven is enclosed coaxially by two quartz cylinders to prevent heat fluctuations due to variation in ambient temperature. The average growth rate of 1 mm/h was adopted for most of the growth experiments. The growth experiments were performed with different ampule configurations, and the experiment was successful when the wall thickness was around 1.2 mm and the ampule was burst when the ampule wall thickness is less than the above thickness. The ampule was well degreased with ethyl alcohol and then dried in the oven for about 3 days. The zone-purified material was charged into the ampule and then evacuated to ∼10-6 Torr and sealed carefully. The twozone resistive heating quartz furnace was used for growth so that in-situ growth observation was possible during growth. The ampule translation speed was 1 mm/h. The details of the growth process were published in our recent article.15 A transparent single crystal was harvested after 10 days (Figure 1b). 3. Characterization In the present investigations, the grown specimens by both SEST and VBT techniques have been characterized by HRXRD, FTIR, microhardness, laser damage threshold, and SHG efficiency measurements, and their results have been compared as described in the following sections. 3.1. High-Resolution X-ray Diffractometry (HRXRD). A multicrystal X-ray diffractometer (MCD) developed at NPL10 was used to evaluate the crystalline perfection of the grown specimen. In this system, a well-collimated and monochromated Mo KR1 beam obtained from a set of three plane (111) Si monochromator crystals set in dispersive (+,-,-) configuration has been used as the exploring X-ray beam. The specimen crystal is aligned in the (+,-,-,+) configuration. Due to dispersive configuration, though 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 is considerably less. Figure 2a shows a typical diffraction curve (DC) recorded for a SEST-grown specimen for (200) diffracting planes using Mo KR1 radiation in symmetrical Bragg geometry using MCD for the VBT-grown single crystal. As seen in the figure, the diffraction curve does not contain multiple peaks as observed in Figure 2b for the VBT specimen except one additional peak at an angular separation of 46 arc sec. The additional peak shows that the specimen contains one Very low angle boundary. When we compare the intensity of the main peaks in Figure 2a,b, the intensity of the main peak in Figure 2a is much higher. The solid line in the figure is the convoluted curve obtained by the
Figure 2. High-resolution X-ray diffraction curve recorded in symmetrical Bragg geometry for (200) planes using Mo KR1 radiation for typical benzimidazole crystals grown by (a) SEST and (b) VBT growth methods.
Gaussian fit of the two curves (dotted lines), whose half widths are 19 and 23 arc sec. Figure 2b shows the DC recorded for (200) diffracting planes under identical conditions as that of the DC recorded for the SEST specimen (Figure 2a). As seen in the figure, the diffraction curve contains multiple peaks in an angular range of ∼40 min. There are four main peaks whose half widths are in the range of a few minutes having angular separations 10, 9, and 4.4 arc min. These multipeaks indicate that the specimen contains many structural internal low angle grain boundaries whose tilt angles range from 4.4 to 10 arc min. On careful observation, the individual peaks due to low angle boundaries further contain subpeaks whose angular tilts are less than 1 arc min. The lower tilt angles (
