Strongest Second Harmonic Generation in the Polar R₃MTQ₇ Family: Atomic Distribution Induced Nonlinear Optical Cooperation

The well-known polar R 3 MTQ 7 is a large family of noncentrosymmetric chalcogenides, and despite of adopting the same crystal structure type, its members show distinctively different nonlinear optical (NLO) properties, which is quite unusual. Yet, the intrinsic reason remains unknown. Herein, we report the discovery of six new members, La 3 Ga 0.5 (Ge 0.5 /Ga 0.5 )S 7 ( 1 ), La 3 In 0.5 (Ge 0.5 /In 0.5 )S 7 ( 2 ), Sm 3 Ga 0.5 (Ge 0.5 /Ga 0.5 )S 7 ( 3 ), La 3 In 0.33 GeS 7 ( 4 ), Sm 3 In 0.33 GeS 7 ( 5 ), and Gd 3 In 0.33 GeS 7 ( 6 ). Remarkably, polycrystalline 1 and 2 show the strongest second harmonic generation (SHG) of this family, 4.8 and 1.8 times that of the benchmark AgGaS 2 at 2.05 μm in the same particle size of 74–106 μm. For the first time we reveal that for the R 3 MTQ 7 family the atomic distribution mainly determines the NLO property, and members showing strong SHG must have a formula of R 3 M 0.5 TQ 7 . Furthermore, we illustrate whether the building unit MS 6 octahedron is half occupied ( 1 – 3 ) or one-third occupied ( 4 – 6 ) is total energy driven and charge balance controlled.