Partial Congener Substitution Induced Centrosymmetric to Noncentrosymmetric Transformation Witnessed by K3Ga3(Ge7–x M x )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 K3Ga3Ge7Se20 (P21/c) to the new isostructural NCS species K3Ga3(Ge6.17Sn0.83)­Se20 (1) and K3Ga3(Ge4.95Si2.05)­Se20 (2) (Pc). Their structures feature a {[Ga3(Ge7–x M x )­Se20]3–}∞ (M = Si, Sn) polyanionic framework built from the basic functional motif M′Se4 (M′ = Ga, Ge, M) tetrahedra, similar to but slightly distorted from that of K3Ga3Ge7Se20. Their NCS structures are verified by the NLO activities. The NLO response of 1 is ∼1/3 times that of the benchmark AgGaS2 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.