Bi₃(SeO₃)₃(Se₂O₅)F: A Polar Bismuth Selenite Fluoride with Polyhedra of Highly Distortive Lone Pair Cations and Strong Second-Harmonic Generation Response

A polar bismuth selenite fluoride, Bi₃(SeO₃)₃(Se₂O₅)­F, consisting of extremely distortive lone pair cations as well as a very electronegative fluoride anion has been synthesized in high yield via a unique hydrothermal condition using the starting oxides and a small amount of a highly concentrated aqueous HF solution. Bi₃(SeO₃)₃(Se₂O₅)F with the polar monoclinic space group, P2₁, exhibits a three-dimensional structure composed of BiO₇, BiO₆F, SeO₃, and Se₂O₅ polyhedra. The infrared (IR) spectral data of Bi₃(SeO₃)₃(Se₂O₅)F do not just confirm the existence of all the constituting bonds but also indicate a wide transparent IR region over 1000 cm^–1 for the compound. The reported selenite fluoride also reveals a large bandgap of ca. 3.8 eV attributed to the distortions arising from the constituting asymmetric units as well as the highly electronegative F^– anion. Electron localized function (ELF) calculations clearly visualize unsymmetrical polyhedra of Bi³⁺ and Se⁴⁺ by presenting the stereoactive lone pairs on each cation. Bi₃(SeO₃)₃(Se₂O₅)F exhibits a very large second-harmonic generation (SHG) response of 8 times that of KH₂PO₄ (KDP) and type-I phase-matching behavior. A closer structural analysis as well as dipole moment calculations consistently suggest that the origin of the very large SHG response of Bi₃(SeO₃)₃(Se₂O₅)F is a net moment toward the [010] direction arising from the polyhedra of highly distortive lone pair cations.