[La3Br][GeS4]2: Cationic-Moiety-Guided Strategy Enables Record Birefringence and Broad Optical Window

Birefringent crystals are essential for polarization control in optical systems, yet achieving both large birefringence (Δn) and a broad transmission window from ultraviolet to infrared remains a significant challenge. Here, we synthesize a novel birefringent crystal [La3Br][GeS4]2 (LGSB) with ultrahigh birefringence and a broadband optical transparency window, where a trigonal-planar [La3Br] cationic moiety serves as a structural “puppeteer” to modulate the ordered assembly of [GeS4] tetrahedra. More notably, theoretical calculations confirm that the non-bonding S 3p in [GeS4] and the non-bonding Br 4p in [La3Br] are both aligned and spread along a consistent direction. These unique traits endow LGSB with exceptional birefringence (Δnobv. = 0.344@546 nm), surpassing currently reported sulfur-containing chalcogenide halides, even outperforming benchmark crystals YVO4 (0.230@546 nm) and TiO2 (0.300@546 nm). Meanwhile, it delivers an ultrabroad transmission range of 0.36−23.2 μm. Thus, LGSB sets a new standard for wide-bandwidth, high-anisotropy materials and shows strong prospects for multi-band optical parametric oscillators (OPOs) and advanced photonic devices.