Atomic‐Shell Amplification of Intrinsic Coordination Asymmetry Enables Giant Optical Nonlinearity

Developing bulk nonlinear optical (NLO) crystals that simultaneously combine outstanding optical performance with excellent crystal growth capability remains a major challenge. Here, we introduce an intrinsic coordination asymmetry strategy that exploits highly polarizable tetrahedral Hg–S coordination and conformationally flexible ligands to amplify local structural distortions within discrete homoanionic modules. Guided by this concept, Cc ‐(NH 4 ) 2 Hg(SCN) 4 ( Cc ‐AHT) and its C 2/ c polymorph were synthesized. Structural and theoretical investigations reveal that the flexible (SCN) − ligands propagate distortions originating in the [HgS 4 ] core throughout the [Hg(SCN) 4 ] 2− module, establishing an atomic‐shell distortion amplification model that quantitatively traces the evolution of local coordination asymmetry. Cc ‐AHT exhibits a giant second‐harmonic generation response (14 × KH 2 PO 4 and 1.2 × KTiOPO 4 at 1064 nm), a band gap of 3.25 eV, and large birefringence (Δ n = 0.341 at 1064 nm), while centimeter‐sized single crystals can be readily grown under mild conditions. This work establishes intrinsic coordination asymmetry and its atomic‐shell amplification as a chemically guided strategy for designing high‐performance homoanionic NLO materials and provides a general framework for understanding how local structural distortions propagate through coordination modules containing conformationally flexible ligands.