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.