(NH4 )Bi2 (IO3 )2 F5 : An Unusual Ammonium-Containing Metal Iodate Fluoride Showing Strong Second Harmonic Generation Response and Thermochromic Behavior.

An ammonium-containing metal iodate fluoride compound, (NH4 )Bi2 (IO3 )2 F5 , featuring a two-dimensional double-layered framework constructed by [BiO2 F5 ]6- and [BiO4 F4 ]9- polyhedra, as well as [IO3 ]- groups, was successfully synthesized. The well-ordered alignment of these SHG-active units leads to an extraordinary strong SHG response of 9.2 times that of KDP. Moreover, this compound possesses a large birefringence (Δn=0.0690 at 589.3 nm), a wide energy band gap (Eg =3.88 eV), and a high laser damage threshold (LDT; 40.2×AgGaS2 ). In particular, thermochromic behavior was observed for the first time in this type of compound. Such multifunctional crystals will expand the application of nonlinear optical materials.