SnPQ3 (Q = S, Se, S/Se): A Series of Lone-Pair Cationic Chalcogenophosphates Exhibiting Balanced NLO Activity Originating from SnQ8 Units.

Two chalcogenophosphates, SnPS2.86Se0.14 (1) and SnPSe3 (2), are isostructural and crystallize in the monoclinic noncentrosymmetric space group Pn. Their three-dimensional (3D) structures are constructed by [Sn(1)Q8] hendecahedra and [Sn(2)Q8] dodecahedra by sharing Q vertices and edges, leaving cavities for isolated [P2Q6] (Q = S/Se, Se) dimers. A second-harmonic-generation (SHG) measurement indicates that 1 is phase-matchable with a response of approximately 1.2 × AgGaS2 (AGS), which is verified by the theoretical calculation result. The powder sample of 1 exhibits a high laser-induced damage threshold of 3.9 × AGS. For comparison, the known SnPS3 (3) was also synthesized and evaluated using the same method. The chemical composition-NLO performance relationship of 1-3 is also discussed. Dipole moment calculation results suggest that [SnQ8] polyhedra make the main contribution to their excellent nonlinear optical (NLO) performance.