Hybrid Metal‐Halide Infrared Nonlinear Optical Crystals of (TMEDA)MI₅ (M = Sb, Bi) with High Stability

Abstract Organic–inorganic metal halides (OIMHs) with unique structural flexibility possess excellent photoelectric properties. They are regarded as next‐generation photovoltaic materials, phosphors, semiconductors, and ferroelectrics. The metal‐halide units in OIMHs are good microscopic building blocks of nonlinear optical crystals for laser wavelength conversion. However, most OIMHs are absent from nonlinear optics owing to their macroscopic nonlinear optical (NLO)‐inactive centrosymmetric crystal structure. In this study, two new lead‐free OIMHs, (TMEDA)SbI₅ and (TMEDA)BiI₅ (where TMEDA²⁺ is N,N,N′‐trimethylethylenediammonium), having 1D structure, crystallized in the orthorhombic system with a non‐centrosymmetric P2₁2₁2₁ space group, are synthesized. Remarkably, upon 2090 nm laser irradiation, both compounds possess a strong infrared (IR) nonlinear optical response of the same magnitude as AgGaS₂, which is a benchmark semiconductor‐type nonlinear optical crystal. In addition, under the excitation of ultraviolet and visible lights, both compounds produce self‐trapped exciton‐induced red‐light emission. First‐principles electronic structure calculations reveal that the optical properties originate from the electronic transitions within the inorganic metal‐halide group. The obtained results indicate that both compounds are potential photoelectric materials for laser frequency conversion and fluorescence, and the observation of NLO effect in these two compounds verifies that OIMHs are also good candidates for NLO crystals.