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Metal chalcogenides, oxides, and halides are the three most typical materials classes for mid-infrared nonlinear optical (mid-IR NLO) crystals that can expand the spectral ranges of solid-state lasers to the mid-IR region. However, it is difficult for each of them to satisfy all the property requirements of mid-IR NLO crystals owing to their own defects. On the contrary, combining multiple anionic units as much as possible to construct heteroanionic compounds is an effective strategy for the design of new high-performance IR NLO crystals. Although in recent research, a series of heteroanionic materials containing two types of anions (i.e., chalcogenides-halides, oxychalcogenides, and oxyhalides) have been reported, the compounds that contain all three types of anions have still been unexplored. Herein, we report the first IR NLO crystal Ba2F2Ge2O3S2, which contains all three typical anions (O2–, S2–, and F–) and satisfies the key property requirements as a promising IR NLO candidate, including a strong SHG response (∼1.4× AgGaS2), a large band gap (Eg = 4.45 eV), and wide IR transmission (0.24–9.0 μm). This research indicates that coupling different types of anions in one compound is a more attractive strategy to explore new IR NLO materials with excellent properties.
Liu et al. (Mon,) studied this question.