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, Bi-3(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. Bi-3(SeO3)(3)(Se2O5)F with the polar monoclinic space group, P2(1), exhibits a three-dimensional structure composed of BiO7, BiO6F, SeO3, and Se2O5 polyhedra. The infrared (IR) spectral data of Bi-3(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. Bi-3(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 Bi-3(SeO3)(3)(Se2O5)F is a net moment toward the [010] direction arising from the polyhedra of highly distortive lone pair cations.