BiS_2-Layer Gives Giant Birefringence: First-Principles Calculations

Birefringence attracts increasing attention due to its extensive applications in the imaging spectrometer, laser devices and optical components. Stimulated by the discovery of the giant birefringence (GBF) in LaOBiS_2 and AFBiS_2, we explore its origin by checking three compounds with and without the BiS_2 layer: Bi_2OS_2 (i.e., BiOBiS_2), Bi_2O_2S and LaOAlS_2. It is demonstrated that GBF appears only in the compounds with the BiS_2 layer and is furthermore confirmed in MOBiS_2 (M=Al, Ga, In and Sc) compounds. The result is useful for discovering/developing new-type birefringent crystal or even multifunctional materials.