A(2)SrM(IV)S(4) (A = Li, Na; M(IV) = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A(2)SrM(IV)S(4) (A = Li, Na; M(IV) = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS(2) as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS(2)) with phase-matching behavior, which satisfy the balance conditions (E(g) ≥ 3.0 eV and d(ij) ≥ 0.5 × benchmark AgGaS(2)) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na(2)SrM(IV)S(4) have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na(2)M(II)SnS(4) (M(II): Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.