NaNO ₃ [(NH ₂ ) ₂ SO ₂ ] ₂ : A Halogen‐Free KBBF‐Type Nitrate Crystal with Record Birefringence for Ultraviolet Applications

ABSTRACT Birefringent crystals are crucial optical materials that play a pivotal role in modulating and detecting the polarization state of light. Their birefringence depends not only on the intrinsic anisotropy of microscopic functional units but also closely relates to the spatial alignment of these units within the crystal lattice. In this work, using KBe 2 BO 3 F 2 (KBBF) as the blueprint for structural design, a new ultraviolet birefringent material, NaNO 3 [(NH 2 ) 2 SO 2 ] 2 , was designed and obtained by strategically substituting the planar [BO 3 ] units in the KBBF with [NO 3 ] units that possess superior polarizability anisotropy. NaNO 3 [(NH 2 ) 2 SO 2 ] 2 constitutes the first report of a halogen‐free KBBF‐type optical material incorporating [NO 3 ] units. In the structure of NaNO 3 [(NH 2 ) 2 SO 2 ] 2 , the [NO 3 ] units exhibit a highly uniform alignment, endowing the crystal with a large birefringence of 0.130 @ 546 nm—the highest value reported to date among inorganic sulfamates and sulfonamides. NaNO 3 [(NH 2 ) 2 SO 2 ] 2 also shows exceptionally high birefringence when compared to mixed‐anion nitrate compounds and KBBF‐like compounds containing [MO 3 ] units. Theoretical calculations have confirmed that the aligned [NO 3 ] units are the primary source of the birefringence. This work points to new directions in the exploration and design of high‐performance KBBF‐type optical materials through unit substitution and alignment control.