Noncentrosymmetric Na6Pb3P4S16 and Centrosymmetric K2MIIP2S6 (MII = Mg and Zn) Displaying Multiple Membered-Ring Configurations and Strong Optical Anisotropy

Three thiophosphates including noncentrosymmetric Na₆Pb₃P₄S₁₆ and centrosymmetric K₂M^IIP₂S₆ (M^II = Mg and Zn) were successfully synthesized in vacuum-sealed silica tubes. Note that interesting multiple six membered-rings (6-MRs) including 6-NaS₆-MRs and 6-KS_n-MRs (n = 6 and 7) formed by A⁺-centered polyhedra were discovered in the structures of title thiophosphates and these MR-composed three-dimensional (3D) tunnels show great possibility to facilitate the filling of various structural blocks (such as zero-dimensional (0D) Pb₃S₁₀ trimers or one-dimensional (1D) (M^IIS_n)_n chains). Na₆Pb₃P₄S₁₆ exhibits the strongest nonlinear optical (NLO) response (5.4 × AgGaS₂) with phase-matching (PM) behavior among the known Pb-based PM NLO sulfides, which is much larger than that of Pb₃P₂S₈ (3.5 × AgGaS₂); it was verified that such large second harmonic generation (SHG) response in Na₆Pb₃P₄S₁₆ can be attributed to the huge contribution of stereochemically active PbS₄ units based on the SHG-density and dipole-moment calculations. Moreover, title thiophosphates show large birefringences (Δn = 0.102-0.21), which indicates that incorporation of [P₂S₆] dimers or polarized PbS₄ units into structures provides positive benefits for the onset of strong optical anisotropy.