Layered compounds AFBiS2: superior birefringent crystals

Stimulated by the discovery of the giant birefringence (GBF) in LaOBiS2 (H. Wang, arxiv: 1304.5032), we check SrFBiS2 and find a superior birefringence up to 1.28. Naturally, AF layer (i.e., SrF) is considered to be of important and is demonstrated by the further study on AFBiS2 (A=Mg, Ca, Ba). Interestingly, MgFBiS2 shows the largest birefringence of 1.65. Furthermore, the origin of GBF is discussed in detail.