Na2Ba[Na2Sn2S7]: Structural Tolerance Factor‐Guided NLO Performance Improvement

The strong mutual coupling of and even the opposite change in the key parameters, such as the band gap (Eg) and second‐order harmonic generation (SHG), leads to the extreme scarcity in high‐performance IR nonlinear optical (NLO) chalcogenides. Herein, we report 8 new sulfides, Na2Ba[(AgxNa1−x)2Sn2S7] (1, x=0; 1 series, x=0.1–0.6; Na2Ba[(Li0.58Na0.42)2Sn2S7], 1‐0.6Li); Na2Sr[Cu2Sn2S7] (2); and Na2Ba[Cu2Sn2S7] (3). We use the structural tolerance factor ( tIexp ${{t}_{I}^{exp}}$ ) to connect the chemical composition, crystal structure, and NLO properties. Guided by these correlations, a better balance between Eg and SHG is realized in 1, which exhibits a large Eg of 3.42 eV and excellent NLO properties (SHG: 1.5×AGS; laser‐induced damage threshold: 12×AGS), representing the best performance among the known Hg‐ or As‐free sulfides to date. Guided by the relationships between the structural tolerance factor and the dopant concentration and second‐order susceptibility, a better balance between Eg and SHG is realized in Na2Ba [Na2Sn2S7] 1, which exhibits a large Eg of 3.42 eV and excellent NLO properties (SHG: 1.5×AGS; laser‐induced damage threshold: 12×AGS), representing the best performance among the known Hg‐ or As‐free sulfides to date.