Tailored synthesis of nonlinear optical quaternary chalcohalides: Ba 4 Ge 3 S 9 Cl 2 , Ba 4 Si 3 Se 9 Cl 2 and Ba 4 Ge 3 Se 9 Cl 2 .

Three novel zero-dimensional quaternary chalcohalides, Ba4 Ge3 S9 Cl2 , Ba4 Si3 Se9 Cl2 and Ba4 Ge3 Se9 Cl2 , which crystallize in the polar noncentrosymmetric space group P63 (no. 173), have been rationally synthesized by a tailored approach on the basis of unique [M3 Q9 ]6- (M = Ge, Q = S; M = Ge/Si, Q = Se) units with Ba2+ cations and Cl- anions occupying the interspaces. The [M3 Q9 ]6- units which consist of three Q-corner-sharing [MQ4 ]4- tetrahedra, are arranged along the 63 screw axis. Remarkably, Ba4 Ge3 S9 Cl2 exhibits a strong powder second harmonic generation (SHG) response that is 2.4 times that of benchmark AgGaS2 at a laser radiation of 2.05 μm in the same particle size range of 46-74 μm. Furthermore, theoretical studies based on the density functional theory helped to gain insight into the origin of the SHG.