Structural, Electronic, and Optical Properties of Bulk Boric Acid2Aand3TPolymorphs: Experiment and Density Functional Theory Calculations

Abstract

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.

Vibrational Modes and Phonon and Thermodynamic Properties of the Metaboric Acid Polymorphs α-, β-, and γ-(BOH)3O3 within a Density Functional Theory Framework

“…With this approach, unit cell deviations Δ a , Δ b , and Δ c (in comparison with X-ray data ,, ) as small as −0.08, −0.09, and −0.06 Å for α-(BOH) 3 O 3 ; 0.00, 0.06, and −0.09 Å for β-(BOH) 3 O 3 ; and 0.01, 0.01, and 0.01 Å for γ-(BOH) 3 O 3 were obtained. The layered α-MA polymorph was predicted to have a direct gap of 6.26 eV, which is close to the values found recently for the boric acid triclinic (H 3 BO 3 - 2A , 6.25 eV) and trigonal (H 3 BO 3 - 3T , 6.28 eV) indirect gap systems . van der Waals interactions between the metaboric acid α-(BOH) 3 O 3 planes (distance of about 3.1 ± 0.01 Å) were a relevant aspect in determining its energy gap, while its direct or indirect character was shown to depend on its crystal structure and molecular components.…”

“…The layered α-MA polymorph was predicted to have a direct gap of 6.26 eV, which is close to the values found recently for the boric acid triclinic (H 3 BO 3 -2A, 6.25 eV) and trigonal (H 3 BO 3 -3T, 6.28 eV) indirect gap systems. 13 van der Waals interactions between the metaboric acid α-(BOH) 3 O 3 planes (distance of about 3.1 ± 0.01 Å) were a relevant aspect in determining its energy gap, while its direct or indirect character was shown to depend on its crystal structure and molecular components. On the other hand, for the polymeric monoclinic β-(BOH) 3 O 3 an indirect gap of 6.56 eV was obtained, while for the covalent cubic γ-(BOH) 3 O 3 a direct gap of 7.28 eV was obtained.…”

“…The metaboric acid [(BOH) 3 O 3 or HBO 2 ] family of inorganic crystals is formed by three polymorphs, α, β, and γ, all of them of white color in the solid state and obtained through a controlled heating process of an aqueous solution of boric (named also orthoboric) acid B­(OH) 3 in ampules kept at different temperatures and pressure conditions for a certain time (see refs − ). Structural analysis reveals that the α metaboric acid polymorph is formed by cyclotrimers of sheetlike structures with Pbnm symmetry, as the boric acid polymorphs 2A and 3T , while the β phase is formed by polymeric zigzag chains in a crystal structure with P 2 1 /c space group (see refs and and references therein). Lastly, the γ phase has a covalent structure in which each boron atom is bonded tetrahedrally to four oxygen atoms and each oxygen atom to two boron atoms.…”

Vibrational Properties of Bulk Boric Acid 2A and 3T Polymorphs and Their Two-Dimensional Layers: Measurements and Density Functional Theory Calculations

“…The structural results indicate that a plane-wave energy cutoff of 830 eV is enough to ensure good convergence of the unit cell (see Table 1), as our research group has obtained for many other molecular crystals whose structural, optoelectronic, and vibrational properties were studied within the DFT scope. 12,21,48 The quality of this basis set was kept fixed even as the unit cell volume varied during the geometry optimization procedure. After finding the minimum-energy structures, we obtained the infrared and Raman spectra as well as the phonon dispersion curves and the partial phonon density of states for the GGA+TS structure only, as well as the heat capacity at constant pressure, Debye temperature, and other thermodynamic properties.…”

“…Therefore, the two crystalline layered boric acid polymorphs were labeled as H 3 BO 3 - 2A (double layer) and H 3 BO 3 - 3T (triple layer). Notwithstanding these experimental structural studies, a theoretical investigation of the structural, electronic, and optical properties of these compounds within the density functional theory (DFT) framework was performed only recently, demonstrating that the properties of both boric acid 2A- and 3T -polymorphs are ruled by intra- and interplane hydrogen bonds and weak van der Waals dispersion interactions between the B­(OH) 3 units, where the relatively close interplanar distances between parallel boric acid single layers lead to subtle differences in the electronic and optical features of these systems. Although hydrogen bonds are much weaker than covalent bonds, they have important consequences for the optical, electronic, and vibrational properties of layered molecular crystals. , …”