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Structural, Electronic, and Optical Properties of Bulk Boric Acid 2A and 3T Polymorphs: Experiment and Density Functional Theory Calculations

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journal contribution
posted on 12.10.2016, 16:22 by M. Bezerra da Silva, R. C. R. dos Santos, A. M. da Cunha, A. Valentini, O. D. L. Pessoa, E. W. S. Caetano, V. N. Freire
Boric acid (H3BO3) is being used effectively nowadays in traps/baits for the management of Aedes aegypti L. and Aedes albopictus Skuse species of mosquitos, which are the main spreading vectors for diseases like malaria, dengue, and zika worldwide. Due to its renewed importance, we have studied in this work the structural, electronic, and optical properties of its molecular triclinic H3BO3-2A and trigonal H3BO3-3T polymorphs within the framework of density functional theory (DFT) at the local density and generalized gradient levels of calculations, LDA and GGA, respectively, improving and extending previously published theoretical results on triclinic boric acid structural properties. In addition, the optical absorption of the 2A polymorph was measured in this work for the sake of comparison with our DFT estimate. In comparison to published X-ray diffraction data, unit cell deviations as small as Δa ∼ −0.13 Å (−0.12 Å), Δb ∼ −0.13 Å (−0.12 Å), Δc ∼ 0.18 Å (−0.31 Å), and interplanar distance deviation Δd ∼ −0.11 Å (−0.10 Å) for H3BO3-2A (H3BO3-3T) were obtained using a Tkatchenko and Scheffler dispersion corrected GGA functional. The properties of the polymorphs are shown to be ruled by an interplay between in-plane hydrogen bonds and interplanar van der Waals interactions. However, the molecular stacking pattern, AB for H3BO3-2A and ABC for H3BO3-3T, does not lead to significantly distinct electronic and optical properties. Both polymorphs are suggested to be insulators with indirect bang gaps of about 6.26 ± 0.01 eV, which is in close agreement with the 5.98 eV indirect band gap we have measured for triclinic boric acid by optical absorption. Notably the usual DFT gap underestimation is not observed due to the extra shrinking of the unit cell caused by the inclusion of van der Waals forces in the geometry optimization. The complex dielectric function and optical absorption of both boric acid polymorphs were characterized as well.

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