Halogen‐Rich Design Strategy: Rational Synthesis of High‐Performance Tetrahedron‐Based Chalcohalides for Advanced Infrared Optical Materials

ABSTRACT Based on the latest anisotropic structure building units with diverse chemical bonds (ABUCB) concept, tetrahedron‐based chalcohalides constructed from the heteroanionic [MCh 4‐x X x ] tetrahedra are expected to display superior nonlinear optical (NLO) performance relative to conventional single‐anion chalcogenides. However, reported chalcohalides indicate a pronounced bias toward chalcogen‐rich (Ch‐rich) compositions. Over 95% of known chalcohalides fall into the Ch‐rich regime, leaving the intrinsic advantages of halogen‐rich (X‐rich) structures largely untapped. We reveal that this gap is not due to structural instability, but rather a dual “synthetic trap” of thermodynamic factor and stoichiometric constraint. Guided with the X‐rich design strategy, we overcame these long‐standing key constraints and realizing the first X‐rich chalcohalides: A 4 Ga 4 Se 2 X 12 (X = Cl: 1, Rb; 2, Cs; X = Br: 7, Rb) together with novel A 4 M 4 Se 3 Cl 10 (M = Ga: 3, Rb ; 4, Cs; M = Al: 6, Rb), Cs 4 Ga 4 Se 4 Cl 8 ( 5 ) and Cs 3 Al 6 Se 10 Cl ( 8 ). The noncentrosymmetric 4 distinguishes itself among all NLO chalcohalides, exhibiting strong second‐harmonic generation response (4.27 ⨯ AgGaS 2 @ 1570 nm), wide band gap (4.05 eV), highest laser‐induced damage threshold (50 ⨯ AgGaS 2 ), phase‐matching compatible birefringence (0.066 @ 546 nm), and the broadest IR transparency window (0.26–25 µm).