Intriguing Dimensional Transition Inducing Variable Birefringence in KNaSnS and RbNaSnSe.

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, KNaSnS and RbNaSnSe, have been synthesized by the solid-state method. We have used the single crystal X-ray diffraction to determine their structures. KNaSnS crystallizes in the monoclinic space group 2/ and exhibits a three-dimensional framework constructed by the corner-sharing SnS and SnS units, whereas RbNaSnSe crystallizes in the tetragonal space group 4/ and features a zero-dimensional [SnSe] trimer built by the three edge-sharing SnSe tetrahedra. The physical property measurements indicate that RbNaSnSe 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 RbNaSnSe, the birefringence of KNaSnS is relatively small, ∼0.070. The study of their structure-property relationship indicates that the different connection modes of SnQ (Q = S, Se; = 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.