(C6H5N2)HgCl3: Discovery of a Polar Hg‐Based Hybrid Halide as Preeminent Nonlinear Optical and Birefringent Material Activated by π‐Conjugated Organic Cation Substitution

Abstract Hybrid organic–inorganic halides have attracted widespread attention due to their multiple optical performances. The combination of distorted metal cation polyhedra and π‐conjugated organic ions is an effective strategy to derive hybrid nonlinear optical (NLO) and birefringent crystals. Here, a new acentric 4‐cyanopyridine mercury halide, (C 6 H 5 N 2 )HgCl 3 , have been obtained via replacing an inorganic cation with an organic cation based on parent compound CsHgCl 3 . (C 6 H 5 N 2 )HgCl 3 shows the strongest second harmonic generation (SHG) effect among mercury‐pyridine hybrid metal halides, with an intensity of 3.04 times of KH 2 PO 4 (KDP) at 1.064 µm laser radiation. The crystal structure of (C 6 H 5 N 2 )HgCl 3 is composed of specially [HgCl 3 ] ∞ chains and [(C 6 H 5 N 2 ) + ] cations. Additionally, (C 6 H 5 N 2 )HgCl 3 shows an outstanding experimental birefringence (Δn = 0.360@0.546 µm), which is the largest value among mercury‐based hybrid halides, and its birefringence is greater than those of all inorganic metal halide birefringent crystals. The discovery of (C 6 H 5 N 2 )HgCl 3 indicates the potential advantage of mercury‐based hybrid halides in optical materials.