The Birefringence Modulation in Short‐Wave Ultraviolet Sulfates with Functional π‐Conjugated Cations and Polymerized Heteroleptic Tetrahedral Anions

Abstract Sulfates with a large bandgap are always promising optical candidates. However, due to insufficient birefringence, their applications are restricted. Here, in terms of this deficiency, two strategies are proposed to rationally modulate the optical anisotropy of sulfates by cations and anions, respectively. For the cation modulation, the [C(NH₂)₃]Al(SO₄)₂·6H₂O crystal is investigated, which exhibits an unprecedented birefringence (0.098 @ 546 nm) compared with other deep‐ultraviolet transparent sulfates, resulting from the synergy of π‐conjugated [C(NH₂)₃]⁺ and non‐π‐conjugated [SO₄]²⁻ groups. For the anion modulation, a novel functional group, [S₃O₆]²⁻, with a large polarizability anisotropy, is designed. Thus, a new crystal, NaKS₃O₆, is obtained, which exhibits a large birefringence of 0.130 @ 546 nm with a short ultraviolet absorption cutoff edge of 215 nm. These results confirm the validity of the strategies. Therefore, the strategies play an important role in improving sulfate's birefringence and then promoting their potential applications in advanced optical systems.