Noncentrosymmetric Na<sub>6</sub>Pb<sub>3</sub>P<sub>4</sub>S<sub>16</sub> and Centrosymmetric K<sub>2</sub>M<sup>II</sup>P<sub>2</sub>S<sub>6</sub> (M<sup>II</sup> = Mg and Zn) Displaying Multiple Membered-Ring Configurations and Strong Optical Anisotropy
Chenyao Zhao, Dazhi Lu, Xinyu Tian, Jingjing Xu, Bingbing Zhang, Kui Wu, Haohai Yu, Huaijin Zhang
Abstract
Three thiophosphates including noncentrosymmetric Na 6 Pb 3 P 4 S 16 and centrosymmetric K 2 M II P 2 S 6 (M II = Mg and Zn) were successfully synthesized in vacuum-sealed silica tubes. Note that interesting multiple six membered-rings (6-MRs) including 6-NaS 6 -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) Pb 3 S 10 trimers or one-dimensional (1D) (M II S n ) n chains). Na 6 Pb 3 P 4 S 16 exhibits the strongest nonlinear optical (NLO) response (5.4 × AgGaS 2 ) with phase-matching (PM) behavior among the known Pb-based PM NLO sulfides, which is much larger than that of Pb 3 P 2 S 8 (3.5 × AgGaS 2 ); it was verified that such large second harmonic generation (SHG) response in Na 6 Pb 3 P 4 S 16 can be attributed to the huge contribution of stereochemically active PbS 4 units based on the SHG-density and dipole-moment calculations. Moreover, title thiophosphates show large birefringences (Δ n = 0.102–0.21), which indicates that incorporation of [P 2 S 6 ] dimers or polarized PbS 4 units into structures provides positive benefits for the onset of strong optical anisotropy.