Partial Congener Substitution Induced Centrosymmetric to Noncentrosymmetric Transformation Witnessed by K3Ga3(Ge7–xMx)Se20 (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 K 3 Ga 3 Ge 7 Se 20 ( P 2 1 / c ) to the new isostructural NCS species K 3 Ga 3 (Ge 6.17 Sn 0.83 )Se 20 ( 1 ) and K 3 Ga 3 (Ge 4.95 Si 2.05 )Se 20 ( 2 ) ( Pc ). Their structures feature a {[Ga 3 (Ge 7– x M x )Se 20 ] 3– } ∞ (M = Si, Sn) polyanionic framework built from the basic functional motif M′Se 4 (M′ = Ga, Ge, M) tetrahedra, similar to but slightly distorted from that of K 3 Ga 3 Ge 7 Se 20 . Their NCS structures are verified by the NLO activities. The NLO response of 1 is ∼1/3 times that of the benchmark AgGaS 2 under a 2.1 μm laser radiation, while 2 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.