A Rigid‐Flexible Cooperative Bond‐Engineering Strategy for the Rational Design of Advanced Infrared Nonlinear Optical Crystals

ABSTRACT Infrared (IR) nonlinear optical (NLO) materials have long been sought, but their development has been hindered by conflicting microstructural requirements, particularly the challenge of simultaneously achieving a strong second‐harmonic generation (SHG) response, sufficient birefringence, and a wide bandgap. We herein propose a “rigid‐flexible cooperative bond‐engineering” strategy that precisely modulates the band structure and module configuration by rationally assembling d 10 cations and alkali metal cations to construct a unique rigid–flexible hybrid bonding framework. Guided by this concept, a series of novel IR NLO materials A 2 Li 4‐x Ag x (PS 4 ) 2 and A 2 Li 4‐x Ag x (AsS 4 ) 2 (A = Na, K) were successfully designed and synthesized. Experimental and computational studies reveal that the flexible components of the hybrid bonds drive the formation of distorted motifs and endow them with high field responsiveness, while the rigid components raise the conduction band minimum to a higher energy. As a result, the title compounds have achieved a rare balance between strong SHG response, large birefringence, and relatively wide bandgap. This work not only provides a series of promising IR NLO materials but also opens a new avenue for the customized design of novel optoelectronic materials.