Synergistic Enhancement of Optical Anisotropy in Nitrate Sulfamate for UV Applications: Role of Functional Group Alignment and Hydrogen Bond Network

To address the increasing demand for UV optical devices, development of novel materials combining high birefringence with a broad UV transmission window is crucial. In this work, a targeted structural design strategy centered on introducing cation-ordered sites and optimizing hydrogen bond networks was proposed and executed, which successfully aligns the polarized [NO₃] and [SO₃NH₂] groups in parallel and achieves synergistic enhancement of optical anisotropy. Guided by this strategy, four new UV nitrate sulfamates were synthesized. Among them, the calculated birefringence of K₂(NO₃)(SO₃NH₂) (0.122 at 546 nm), Rb₂(NO₃)(SO₃NH₂) (0.116 at 546 nm) and K_0.5(NH₄)_1.5(NO₃)(SO₃NH₂) (0.126 at 546 nm) display about 2.7-, 2.5- and 2.8-fold higher than that of Rb_1.7(NH₄)_0.3(NO₃)(SO₃NH₂) (0.045 at 546 nm), with these values exceeding all the birefringence reported to date for metal sulfamates, nitrate sulfamates and co-crystal. More importantly, this study elucidates how the regulation of cation-occupied cooperative hydrogen bond networks drives structural transformations and clarifies the underlying origins of birefringence and its enhancement. These findings provide valuable insights and guidance for the rational design of short-wavelength UV birefringent materials.