Tailored synthesis of nonlinear optical quaternary chalcohalides: Ba4Ge3S9Cl2, Ba4Si3Se9Cl2 and Ba4Ge3Se9Cl2.

Three novel zero-dimensional quaternary chalcohalides, Ba4Ge3S9Cl2, Ba4Si3Se9Cl2 and Ba4Ge3Se9Cl2, which crystallize in the polar noncentrosymmetric space group P63 (no. 173), have been rationally synthesized by a tailored approach on the basis of unique [M3Q9]6- (M = Ge, Q = S; M = Ge/Si, Q = Se) units with Ba2+ cations and Cl- anions occupying the interspaces. The [M3Q9]6- units which consist of three Q-corner-sharing [MQ4]4- tetrahedra, are arranged along the 63 screw axis. Remarkably, Ba4Ge3S9Cl2 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.