[Ag(NH<sub>3</sub>)<sub>2</sub>]<sub>2</sub>SO<sub>4</sub>: A Strategy for the Coordination of Cationic Moieties to Design Nonlinear Optical Materials**
Yi‐Chang Yang, Xin Liu, Jing Lu, Li‐Ming Wu, Ling Chen
Abstract
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 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 ).