Halogen-Rich Design Strategy: Rational Synthesis of High-Performance Tetrahedron-Based Chalcohalides for Advanced...

Based on the latest anisotropic structure building units with diverse chemical bonds (ABUCB) concept, tetrahedron-based chalcohalides constructed from the heteroanionic [MCh_4-xX_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: A₄Ga₄Se₂X₁₂ (X = Cl: 1, Rb; 2, Cs; X = Br: 7, Rb) together with novel A₄M₄Se₃Cl₁₀ (M = Ga: 3, Rb; 4, Cs; M = Al: 6, Rb), Cs₄Ga₄Se₄Cl₈ (5) and Cs₃Al₆Se₁₀Cl (8). The noncentrosymmetric 4 distinguishes itself among all NLO chalcohalides, exhibiting strong second-harmonic generation response (4.27 ⨯ AgGaS₂ @ 1570 nm), wide band gap (4.05 eV), highest laser-induced damage threshold (50 ⨯ AgGaS₂), phase-matching compatible birefringence (0.066 @ 546 nm), and the broadest IR transparency window (0.26-25 µm).