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, A2Bi2(SeO3)3F2 [A = K(1) and Rb(2)], has been synthesized through a mild hydrothermal method. These isostructural compounds all had one-dimensional [Bi2O9F2] chains, which were further connected with neighbouring chains through bridged selenite groups to construct the 3D framework. Because of the relatively ordered alignment of [BiOxFy] and SeO3 groups, they have large second-harmonic generation (SHG) responses of about 15.0(1) and 14.4(2) times that of KH2PO4 (KDP) under 1064 nm laser radiation and 0.40(1) and 0.36(2) times that of AgGaS2 (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.