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 A(2)M(II)M(2)(IV)Se(6) 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.