[Ag(NH 3 ) 2 ] 2 SO 4 : A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials **

Abstract Over recent decades, guided by the anionic group theory of nonlinear optical (NLO) materials, rational design strategies have been primarily focused on anionic moieties; consequently, structural modification and design of cationic moieties have long been neglected. Herein, we report a strategy for the coordination of cationic moieties that substantially enhances the optical properties of NLO materials. For an example with well‐known crystal structure, [Ag(NH 3 ) 2 ] 2 SO 4 , we demonstrate that the coordination of the Ag + cation by the neutral ligand drives the formation of a noncentrosymmetric tetragonal P 4 ‾ 2 1 c structure as a positive uniaxial crystal. The bending of the [Ag(NH 3 ) 2 ] + cationic moiety parallel to the z ‐axis generates an anisotropic arrangement of the dipoles, i.e., a dipole of 0.12 D along the z ‐direction, yet zero dipole in the xy ‐plane, which interacts anisotropically with the incident light oscillating electromagnetic wave, leading to optical anisotropy with a large birefringence. The incident beam of 589.3 nm normal to the (110) crystal plane measures Δ n obv. =0.08, and [Ag(NH 3 ) 2 ] 2 SO 4 also exhibits a phase‐matching NLO response 1.4 times that of KH 2 PO 4 (KDP) ( obv . 1.4×KDP @1064 nm; cal. d 36 =1.50 pm V −1 ).