The 1H-Benzimidazole Oxalic acid (BO) single crystal was synthesised and grown by the slow evaporation technique. Single crystal X-ray diffraction (XRD) confirmed that the crystal structure of 1H-Benzimidazole Oxalic acid (BO) single crystal belongs to the monoclinic system with the (P21/c) space group. Powder XRD was utilised to verify the phase purity and crystallinity of the grown crystal. Hirshfeld surface analysis and fingerprint analysis show that it governs the maximum of the intermolecular bonding O–-H/H–-O, followed by N–-H/H–-N and C–-H/H–-C, which is 45.9%, 21.4% and 16.2%, respectively. FTIR spectroscopy confirmed the formation of the proton-transfer salt through the characteristic N⁺–H stretching band at 3130 cm⁻1 and dual carbonyl absorptions at 1742 and 1697 cm⁻1. Thermogravimetric and differential thermal analysis revealed thermal stability up to 180 °C, with a single sharp decomposition event between 180–258 °C involving 99% mass loss, confirming the phase purity and entirely organic composition of the salt. UV–Vis spectrophotometric analysis revealed a wide optical transparency window of 380–1400 nm, a direct optical bandgap of 3.28 eV from Tauc plot analysis, and an exceptionally low Urbach energy of 0.1109 m eV, indicative of high crystalline quality and minimal defect density. The refractive index was estimated as n = 2.268–2.32 using semi-empirical dispersion models. Vicker’s microhardness study shows that the crystal falls under soft material, and the mechanical properties, like elastic stiffness constant (C11), brittleness index (Bi)<InlineEquation ID="IEq1"> <EquationSource Format="TEX">$,$</EquationSource> <EquationSource Format="MATHML"><math> <mo>,</mo> </math></EquationSource> </InlineEquation> and the yield strength was calculated for the grown crystal, which would help us to identify the material suitability for device fabrication. The etching study shows the rectangular etch patterns and one-layer growth with low dislocation density. The third-order nonlinear optical properties of the material were investigated, specifically the nonlinear refractive index (n2), nonlinear absorption coefficient (β), and linear absorption coefficient (χ3). The negative n₂ and positive β confirm self-defocusing and reverse saturable absorption (RSA) behavior, attributed to the donor–acceptor intramolecular charge transfer (ICT) framework of the benzimidazolium–oxalate ion pair. These results establish the BO crystal as a promising material for optical limiting, UV photonic, and optoelectronic device applications. The results indicate a significant third-order nonlinearity of the BO, highlighting the potential of the BO crystal for application in photonic and optoelectronic devices.

Structural, optical, mechanical and third order NLO characterization of a newly synthesized 1H-Benzimidazole oxalic acid single crystal

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2026年7月16日

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Priyanka devi,
M. Rajalakshmi,
R. Indirajith,
M. Muthumeenal

摘要

The 1H-Benzimidazole Oxalic acid (BO) single crystal was synthesised and grown by the slow evaporation technique. Single crystal X-ray diffraction (XRD) confirmed that the crystal structure of 1H-Benzimidazole Oxalic acid (BO) single crystal belongs to the monoclinic system with the (P2₁/c) space group. Powder XRD was utilised to verify the phase purity and crystallinity of the grown crystal. Hirshfeld surface analysis and fingerprint analysis show that it governs the maximum of the intermolecular bonding O–-H/H–-O, followed by N–-H/H–-N and C–-H/H–-C, which is 45.9%, 21.4% and 16.2%, respectively. FTIR spectroscopy confirmed the formation of the proton-transfer salt through the characteristic N⁺–H stretching band at 3130 cm⁻¹ and dual carbonyl absorptions at 1742 and 1697 cm⁻¹. Thermogravimetric and differential thermal analysis revealed thermal stability up to 180 °C, with a single sharp decomposition event between 180–258 °C involving 99% mass loss, confirming the phase purity and entirely organic composition of the salt. UV–Vis spectrophotometric analysis revealed a wide optical transparency window of 380–1400 nm, a direct optical bandgap of 3.28 eV from Tauc plot analysis, and an exceptionally low Urbach energy of 0.1109 m eV, indicative of high crystalline quality and minimal defect density. The refractive index was estimated as n = 2.268–2.32 using semi-empirical dispersion models. Vicker’s microhardness study shows that the crystal falls under soft material, and the mechanical properties, like elastic stiffness constant (C₁₁), brittleness index (B_i) $,$ $,$ and the yield strength was calculated for the grown crystal, which would help us to identify the material suitability for device fabrication. The etching study shows the rectangular etch patterns and one-layer growth with low dislocation density. The third-order nonlinear optical properties of the material were investigated, specifically the nonlinear refractive index (n₂), nonlinear absorption coefficient (β), and linear absorption coefficient (χ³). The negative n₂ and positive β confirm self-defocusing and reverse saturable absorption (RSA) behavior, attributed to the donor–acceptor intramolecular charge transfer (ICT) framework of the benzimidazolium–oxalate ion pair. These results establish the BO crystal as a promising material for optical limiting, UV photonic, and optoelectronic device applications. The results indicate a significant third-order nonlinearity of the BO, highlighting the potential of the BO crystal for application in photonic and optoelectronic devices.