  Birefringent crystals (BFC) have been extensively used in imaging
spectrometer, laser devices, and optical components. Seeking new BFC is of
important for both fundamental research and industrial application. Employed
first-principles density functional theory (DFT), we found that LaOBiS2 (space
group P4/nmm), the parent phase of recently discovered BiS2-superconductor,
exhibits superior birefringence with a maximum value of 2.94, which is 13.4
times larger than the extensively used YVO4 (0.22). Furthermore, LaOBiS2 is an
indirect semiconductor with a band-gap of 0.9 eV, which is suitable for optical
applications. The origin of giant inherent birefringence is discussed.
