Partial Congener Substitution Induced Centrosymmetric to Noncentrosymmetric Transformation Witnessed by KGa(GeM)Se (M = Si, Sn) and Their Nonlinear Optical Properties.

Crystallizing in a noncentrosymmetric (NCS) structure is the essential requirement for a crystal to be second-order nonlinear optical (NLO) active. Here, a simple strategy of partial congener substitution is introduced to induce transformation of the known centrosymmetric KGaGeSe (2/) to the new isostructural NCS species KGa(GeSn)Se () and KGa(GeSi)Se () (). Their structures feature a {[Ga(GeM)Se]} (M = Si, Sn) polyanionic framework built from the basic functional motif M'Se (M' = Ga, Ge, M) tetrahedra, similar to but slightly distorted from that of KGaGeSe. Their NCS structures are verified by the NLO activities. The NLO response of is ∼1/3 times that of the benchmark AgGaS under a 2.1 μm laser radiation, while exhibits a weak effect, and both are NLO phase-matchable. Their optical band gaps are measured to be 2.02 and 2.12 eV, and they are transparent in the ranges of 0.61-25 and 0.58-25 μm, respectively.