Bi 3 (SeO 3 ) 3 (Se 2 O 5 )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 (SeO 3 ) 3 (Se 2 O 5 )F with the polar monoclinic space group, P2 1 , exhibits a three-dimensional structure composed of BiO 7 , BiO 6 F, SeO 3 , and Se 2 O 5 polyhedra. The infrared (IR) spectral data of Bi 3 (SeO 3 ) 3 (Se 2 O 5 )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 Bi 3+ and Se 4+ by presenting the stereoactive lone pairs on each cation. Bi 3 (SeO 3 ) 3 (Se 2 O 5 )F exhibits a very large second-harmonic generation (SHG) response of 8 times that of KH 2 PO 4 (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 (SeO 3 ) 3 (Se 2 O 5 )F is a net moment toward the [010] direction arising from the polyhedra of highly distortive lone pair cations.

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