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-(NH₄)₂Hg(SCN)₄ (Cc-AHT) and its C2/c polymorph were synthesized. Structural and theoretical investigations reveal that the flexible (SCN)^- ligands propagate distortions originating in the [HgS₄] 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 × KH₂PO₄ and 1.2 × KTiOPO₄ at 1064 nm), a band gap of 3.25 eV, and large birefringence (Δn = 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.