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.