A₂Bi₂(SeO₃)₃F₂ (A = K and Rb): Excellent Mid-Infrared Nonlinear Optical Materials with Both Strong SHG Responses and Large Band Gaps

A family of noncentrosymmetric alkali-metal bismuth selenite fluorides, A 2 Bi 2 (SeO 3 ) 3 F 2 [A = K( 1 ) and Rb( 2 )], has been synthesized through a mild hydrothermal method. These isostructural compounds all had one-dimensional [Bi 2 O 9 F 2 ] chains, which were further connected with neighbouring chains through bridged selenite groups to construct the 3D framework. Because of the relatively ordered alignment of [BiO x F y ] and SeO 3 groups, they have large second-harmonic generation (SHG) responses of about 15.0( 1 ) and 14.4( 2 ) times that of KH 2 PO 4 (KDP) under 1064 nm laser radiation and 0.40( 1 ) and 0.36( 2 ) times that of AgGaS 2 (AGS) under 2.05 μm laser radiation, respectively. In addition, the SHG response of ( 1 ) was the largest among those of all reported metal selenites. In addition, all of them had high thermal stability (∼400 °C), wide energy band gaps (over 3.72 eV), and an excellent IR transparent region, suggesting that they had great potential as high power mid-infrared nonlinear optical crystals.