From AgHgI(SO₄) to AgHg₃I_3.2Cl_1.8(SO₄): Symmetry Breaking of the Belousovite‐Type Topology by Mixed Halides to Achieve Strong Nonlinear Optical Effect

Abstract Investigations and structural designs of new sulfates are essential and challenging in the nonlinear optical (NLO) field. Herein, a new mixed metal sulfate halide, AgHgI(SO 4 ) ( AHIS ) is synthesized. AHIS features the centrosymmetric (CS) Belousovite‐type structure and displays moderate birefringence (0.19 at 546 nm). To break this structure, the study proposes a strategy of simultaneously introducing the discrepant Lewis bases I − and Cl − , which results in the CS to non‐centrosymmetric (NCS) transformation and a novel NCS compound, AgHg 3 I 3.2 Cl 1.8 (SO 4 ) ( AHIClS ). It exhibits a very strong second‐harmonic generation response (11.8 × KH 2 PO 4 ) under 1064 nm laser radiation, large birefringence (0.39 at 546 nm), and a high laser‐induced damage threshold of 52.9 MW cm −2 , hence it is a promising NLO crystal. Its SHG effect far exceeds those of most sulfates. The relationship between different Lewis bases and overall structures has been analyzed, which can offer valuable insights into the research of NLO materials.