Metals containing d-orbitals are typically characterized by strong deformation and polarization, yet they tend to induce narrow bandgaps that render them little-appreciated by high-power nonlinear optical (NLO) crystals. Incorporating highly electropositive polycations into d-orbital-containing chalcogenides to modify them into salt-inclusion chalcogenides (SICs) that are competitive in NLO materials, is a viable solution to this predicament. In the present work, two isostructural SICs [K4Cl][MGa9S16] (M = Mn, 1; Hg, 2) are successfully synthesized by the high-temperature molten-salt growth method. Both compounds demonstrate commendable second-harmonic-generation (SHG) responses (0.6–1.0 × AgGaS2 @1910 nm), which can be attributed to their well-designed [MGa9S16]3− anionic frameworks; and compound 2 exhibits the widest optical bandgap (3.41 eV) among the Hg-based NLO chalcogenides. Also, an interesting dual-band photoluminescence emission centered at ∼650 and ∼718 nm is detected in 1 at 77 K, with long lifetimes of 0.94 and 1.35 ms, respectively.
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