Achieving High‐Birefringence Thiophosphates From Low Polarizability Anisotropy Units by an Axial Linear Matching Strategy

Birefringent crystals with transparency spanning the mid‐ to far‐infrared regions are critical for infrared optical technologies. However, their development remains constrained by the inherent challenge of simultaneously achieving high birefringence and a broad IR transmission window. Taking the thiophosphate Nb 4 P 2 S 21 , characterized by birefringence‐active S–S units, as a template, we developed a linear‐matching strategy to enhance its optical anisotropy. This approach involves retaining the strongly anisotropic [Nb 2 PS 10 ] backbone while replacing the highly flexible C 2 v ‐symmetric [S 3 ] 2− bridging units with D ∞ h ‐symmetric [LiS 2 ] 3− and [S 2 ] 2− groups, which possess lower polarizability anisotropy. Although such a high‐to‐low substitution typically reduces birefringence, the rigid linear groups couple effectively with the chain, resulting in enhanced birefringence of 0.45 and 0.53 at 546 nm for CsLiNb 4 P 2 S 20 and Nb 4 P 2 S 20 , respectively. Moreover, both crystals exhibit broad infrared transparency (2.5–16.7 µm), and are air stable, highlighting their potential applications as mid‐ to far‐infrared birefringent optical materials. Our design strategy provides a new perspective for the design of crystals with large birefringence.