Mixed-Anion Inorganic Compounds: A Favorable Candidate for Infrared Nonlinear Optical Materials

Infrared nonlinear optical (IR-NLO) materials are the key components in solid-state lasers for military and civilian applications. The development of IR-NLO materials remains urgent because the existing commercial materials AgGaS2 (AGS), AgGaSe2, and ZnGeP2 are limited for their own intrinsic shortcomings, for example, the lower laser damage thresholds (LDTs), the non-phase-matchable feature, and the strong two-photon absorption, respectively, exhibited in AgGaSe2 and ZnGeP2 at a conventional laser of 1 μm. Chalcogenides, halides, and iodates are known to be common sources of potential IR-NLO materials with good properties, of which the most in-depth studied is usually constructed by single anions. Recently, mixed-anion inorganic compounds are thought to be promising IR-NLO materials which can optimize comprehensive performances, especially balance the incompatibility between a large NLO coefficient and a high LDT in one material. This review focuses on the syntheses and crystal structures of mixed-anion inorganic IR-NLO materials, as well as the relationships of structure–property. Examples include four kinds of such compounds, i.e., mixed-anion chalcogenides, mixed-anion halides, mixed-anion chalcohalides, and other miscellaneous mixed-anion compounds including adducts, oxychalcogenides, oxyhalides, iodate fluorides, selenite fluorides, and halide borates. This work gives some exploring directions for novel IR-NLO materials, including how to optimize the NLO performances of the existing materials, from the inferred structure–property relationships of the potential mixed-anion inorganic NLO compounds.