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

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 in-house 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 α ≥ 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 (α < 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.