Layered Compounds AFBiS₂: Superior Birefringent Crystals

First-principles calculations predict that SrFBiS₂ has a superior birefringence of 1.28, which is larger than the giant birefringence of LaOBiS₂ Wang H, Chin. Phys. Lett. 31 2013 047802. An AF layer (i.e., SrF) is shown to be important by the further investigations on AFBiS₂ (A=Mg, Ca, Ba). Here MgFBiS₂ exhibits the largest inherent birefringence (～1.66) among single-crystal compounds. The origin of SBF is also discussed based on the electronic structure and refractive index.