Birefringent　crystals　play　an　irreplaceable　role　in　optical　systems　by　adjusting　the　polarization　state　of　light　in　optical　devices.　This　work　successfully　synthesized　a　new　thiophosphate　phase　of　β-Pb3P2S8　through　the　high-temperature　solid-state　spontaneous　crystallization　method.　Different　from　the　cubic　α-Pb3P2S8,　the　β-Pb3P2S8　crystallizes　in　the　orthorhombic　Pbcn　space　group.　Notably,　β-Pb3P2S8　shows　a　large　band　gap　of　2.37　eV　in　lead-based　chalcogenides,　wide　infrared　transparent　window　(2.5−15　μm),　and　excellent　thermal　stability.　Importantly,　the　experimental　birefringence　shows　the　largest　value　of　0.26@550　nm　in　chalcogenides,　even　larger　than　the　commercialized　oxide　materials.　The　Barder　charge　analysis　result　indicates　that　the　exceptional　birefringence　effect　is　mainly　from　the　Pb2+　and　S2−　in　the　[PbSn]　polyhedrons.　Meanwhile,　the　parallelly　arranged　polyhedral　layers　could　improve　the　structural　anisotropic.　Therefore,　this　work　supports　a　new　method　for　designing　chalcogenides　with　exceptional　birefringence　effect　in　the　infrared　region.　©　2024