Intriguing Dimensional Transition Inducing Variable Birefringence in K₂Na₂Sn₃S₈ and Rb₃NaSn₃Se₈

The birefringent crystals capable of modulating the polarization of lights are of the current research interests. Although many oxide crystals have been discovered and widely used in UV and visible regions, the birefringent crystals in the infrared (IR) region are still rare. Herein, two new chalcogenides, K₂Na₂Sn₃S₈ and Rb₃NaSn₃Se₈, have been synthesized by the solid-state method. We have used the single crystal X-ray diffraction to determine their structures. K₂Na₂Sn₃S₈ crystallizes in the monoclinic space group C2/c and exhibits a three-dimensional framework constructed by the corner-sharing SnS₄ and SnS₅ units, whereas Rb₃NaSn₃Se₈ crystallizes in the tetragonal space group P4/nbm and features a zero-dimensional [Sn₃Se₈]^4- trimer built by the three edge-sharing SnSe₄ tetrahedra. The physical property measurements indicate that Rb₃NaSn₃Se₈ has a wide IR transparent window up to 20 μm and large birefringence, ∼0.196, suggesting its potential application as a birefringent crystal in the IR region. However, compared with Rb₃NaSn₃Se₈, the birefringence of K₂Na₂Sn₃S₈ is relatively small, ∼0.070. The study of their structure-property relationship indicates that the different connection modes of SnQ (Q = S, Se; n = 4, 5) polyhedra are the main reason for the large difference of birefringence between the two compounds. These studies will provide a new insight for the origin of birefringence and will facilitate the exploration of new IR birefringent crystals.