Synthesis, Structure, and Characterization of Two Mixed-Cation Quaternary Chalcogenides K₂BaSnQ₄ (Q = S, Se)

Two new isostructural metal chalcogenides, K₂BaSnS₄ and K₂BaSnSe₄, have been isolated for the first time. They feature the intriguing ∞ 1[BaSnQ₄]^2- (Q = S, Se) tunnel structures which are separated by K⁺ cations. Experiments combined with theoretical calculations demonstrate that K₂BaSnS₄ possesses a large energy gap (3.09 eV), which is conducive for large laser-induced damage thresholds (LDTs), and maintains a relatively moderate SHG response (0.5 × AgGaS₂) in the meanwhile. Moreover, K₂BaSnS₄ possesses good phase-matchability, which stems from its suitable birefringence (0.04@2.09 μm).