  Over decades, guided by the anionic group theory, the majority work has been
focused on the anionic-moiety of a nonlinear optical material, however, the
property guided structure modification and design on the cationic-moiety has
long been neglected. Herein, we report for the first time a coordination
strategy on the cationic-moiety, as demonstrated by the first exmple,
[Ag(NH3)2]2SO4 vs Ag2SO4, the coordination of the plus one Ag+ cation by the
neutral ligand forming the [Ag(NH3)2]+ cationic-moiety drives the formation of
the noncentrosymmetric tetragonal P-421c structure which exhibits a remarkable
property improvement, including a strong SHG intersity (1.4*KDP vs 0 @1064 nm),
and a large birefringence (delta(n)cal.: 0.102 vs 0.012 @1064 nm). Furthermore,
we discover that owing to the strong hydrogen bonds and spatial confinement of
[SO4]2- anions, the cation [Ag(NH3)2]+ bends parallel to the crystallographic c
axis with a bond angle of N-Ag-N = 174.35deg, generating a permenant dipole
moment z = -0.12 D that is responsible for the large birefringence that
enlarges the range of the phase matching SHG laser output. We believe what we
discover is only a glimpse of the long been neglected crystal engineering on
the cationic-moiety. Encourage by this work, exciting works on other ionic
compound systems shall flood in.
