Rational Design of Non‐Centrosymmetric Sn(II)‐Based Chalcogenides Exhibiting Superior Infrared Nonlinear Optical Performances

ABSTRACT Non‐centrosymmetric Sn(II)‐based chalcogenides hold great promise as infrared (IR) nonlinear optical (NLO) materials; however, their rational design and synthesis remain a substantial challenge due to the facile oxidation of Sn(II) to Sn(IV). Herein, by synergistically assembling Sn(II)/Pb(II) cations with GeS 4 tetrahedra and MX 6 (M═Pb, Sn, Na; X═Cl, Br) octahedral groups, two new Sn(II)‐chalcogenide IR NLO crystals, [Sn 0.5 Br 3 ][Sn 3 GeS 4 ] and [Na 0.5 Sn 0.25 Br 3 ][Sn 3 GeS 4 ], were successfully synthesized using a unique quenching‐reannealing method. Notably, they achieved 10‐ and 2–4‐fold enhancements in the SHG effect and birefringence, respectively, compared with their isostructural homolog, [Pb 0.5 Cl 3 ][Pb 3 GeS 4 ] (0.6 × AgGaS 2 , ∆n = 0.030). This unusual property disparity originates from the difference in stereochemical activity of lone pair electrons between Sn and Pb cations. Furthermore, owing to the property‐modulating effect of strongly electropositive Na + cations, [Na 0.5 Sn 0.25 Br 3 ][Sn 3 GeS 4 ] exhibits a well‐balanced set of optical properties, including strong SHG effect (6.4 × AgGaS 2 ), moderate birefringence (0.074), wide optical transparency range (0.47–12.5 µm), indicating it is a potential IR NLO materials. This work opens an avenue for the design of Sn (II)‐based IR NLO materials and the synthesis of metastable chalcogenides.