Optimized arrangement of non-π-conjugated PO₃NH₃ units leads to enhanced ultraviolet optical nonlinearity in NaPO₃NH₃

The key to achieving nonlinear optical (NLO) crystals with superior optical properties lies in the development of new high-performance functional units and their precise arrangement. In this study, we identify the polar covalent tetrahedron (PO₃NH₃)⁻, with its remarkable optical properties, as a novel deep-ultraviolet (DUV) NLO-active unit for the first time. To ensure the (PO₃NH₃)⁻ units are accurately aligned, we selected NaNH₄PO₃F·H₂O as a template. Subsequently, we successfully synthesized NaPO₃NH₃, wherein the (PO₃NH₃)⁻ units were arranged in an orderly manner. This excellent ordered arrangement of (PO₃NH₃)⁻ units enables NaPO₃NH₃ to exhibit not only a significant SHG effect (1.2 × KDP), but also the largest birefringence (0.062@546.1 nm) among DUV non-π-conjugated phosphate systems.