Functional Chalcogenide Na₂HgSn₂Se₆ and K₂MnGe₂Se₆ Exhibiting Flexible Chain Structure and Intriguing Birefringence Tunability

The two functional chalcogenides K₂MnGe₂Se₆ and Na₂HgSn₂Se₆, featuring a straight-chain structure, have been successfully prepared and fully characterized. K₂MnGe₂Se₆ shows paramagnetic behavior. The birefringence of Na₂HgSn₂Se₆ is as large as 0.3107 and derives from the superposition of the polarizabilities of its fundamental building blocks, on the basis of first-principles calculations. Moreover, the flexible framework of the A₂M^IIM^IV₂Se₆ family enables a variety of heterogeneous substitutions and thus offers possible birefringence tunability, which may inspire the design and exploratory synthesis of IR birefringent materials.