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

An ammonium-containing metal iodate fluoride compound, (NH4)Bi-2(IO3)(2)F-5, featuring a two-dimensional double-layered framework constructed by [BiO2F5](6-) and [BiO4F4](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 (Delta n=0.0690 at 589.3 nm), a wide energy band gap (E-g=3.88 eV), and a high laser damage threshold (LDT; 40.2xAgGaS(2)). 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.