Hierarchical Ordering Decouples the SHG–Birefringence–LIDT Trade‐Off in a Single‐Phase Mid‐Infrared Nonlinear Optical Crystal

ABSTRACT The pursuit of high‐performance infrared nonlinear optical crystals is perpetually constrained by the inherent trade‐offs among second‐harmonic generation (SHG) efficiency, laser‐induced damage threshold (LIDT), and birefringence for phase‐matching (PM). Traditional compositional engineering, which seeks new formulas, is fundamentally limited by a compound's intrinsic electronic structure. Here, we introduce a paradigm shift of performance optimization through hierarchical order editing within a fixed chemical composition. In the model compound [Hg 3 Se 2 ][ZnCl 4 ], we demonstrate tuning of its hierarchical order encompassing both rotational twinning and superstructure by adjusting synthetic conditions, which yields three distinct variants ( 1 – 3 ) from a single formula. Their optical properties evolve progressively from negligible SHG ( 1 ), to weak non‐PM SHG ( 2 ), and finally to strong PM SHG alongside the highest LIDT ( 3 ). We reveal that this tunability stems from the decoupled roles of different ordering parameters, twinning primarily governs the SHG coefficient and birefringence, while superstructural modulation dominantly influences the LIDT. This work establishes hierarchical order editing​ as a powerful and general strategy for property optimization, transcending the limitations of compositional design.