Giant Birefringence in Layered Compound LaOBiS₂

Birefringent crystals (BFCs) have been extensively used in imaging spectrometers, laser devices, and optical components. Seeking new BFCs is important for both fundamental research and industrial applications. Employing first-principles density functional theory, we find that LaOBiS₂ (space group: P4/nmm) appearing in the parent phase of the 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 also discussed.