Lone pair effects on ternary infrared nonlinear optical materials.

First-principles studies of the crystal structures, electronic structures and optical properties of noncentrosymmetrical (NCS) K3AsS4, Li3AsS3, Pb9As4S15 and Ag3AsS3 have been performed by means of density functional theory. Via a theoretical method to compute the intensity of the lone pair stereochemical activity of an As-S group, the correlated mechanism among the crystal structures, the stereo-chemical activity of lone pairs on As and the second harmonic generation (SHG) response has been clarified. The results prove that the SHG response is not only attributed to the lone pair stereochemical activity of the As-S group but also related to the direction of the forming layers in the crystal structure arrangement. Besides, the quantitative method for the stereo-chemical activity of lone pairs is universal, which is valid for other lone pair systems like those containing Pb2+, Bi3+, Sn2+, etc. The findings facilitate the exploration of materials that may exhibit a relatively large second order NLO reaction and can be used in infrared applications.