Quadruple‐Bidentate Nitrate‐Ligated A2Hg(NO3)4 (A=K, Rb): Strong Second‐Harmonic Generation and Sufficient Birefringence

Abstract The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO‐active modules in an optimal additive fashion. We report herein the first NLO‐active mercuric nitrates A 2 Hg(NO 3 ) 4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d 10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo‐diamond‐like structures with parallel [Hg(NO 3 ) 4 ] modules, and leading to strong powder second‐harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH 2 PO 4 . In combination with the simple solution preparation of centimeter‐scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single‐crystal structure analysis reveal that the newly‐developed highly condensed and distorted [Hg(NO 3 ) 4 ] module, with an Hg 2+ cation that is quadruply bidentate nitrate‐ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high‐performing next‐generation NLO materials.