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

Abstract

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.