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

The well-known polar R3MTQ7 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, La3Ga0.5(Ge0.5/Ga0.5)­S7 (1), La3In0.5(Ge0.5/In0.5)­S7 (2), Sm3Ga0.5(Ge0.5/Ga0.5)­S7 (3), La3In0.33GeS7 (4), Sm3In0.33GeS7 (5), and Gd3In0.33GeS7 (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 AgGaS2 at 2.05 μm in the same particle size of 74–106 μm. For the first time we reveal that for the R3MTQ7 family the atomic distribution mainly determines the NLO property, and members showing strong SHG must have a formula of R3M0.5TQ7. Furthermore, we illustrate whether the building unit MS6 octahedron is half occupied (1–3) or one-third occupied (4–6) is total energy driven and charge balance controlled.