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 SnQn (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.