KREP 2 S 6 (RE = Sm, Gd, Tb, Dy): A Series of Polar Rare-Earth Thiophosphates with High Laser-Induced Damage Thresholds and Moderate Nonlinear-Optical Responses Synergistically Contributed by Isolated P 2 S 6 Units and {[RE 2 S 15 ] 24- } ∞ Layers.

As one of the potential candidates of nonlinear-optical (NLO) materials, rare-earth chalcophosphates have demonstrated promising properties. Here, KREP 2 S 6 (RE = Sm, Gd, Tb, Dy) were synthesized using the facile RE 2 O 3 -B-S solid-state method. They crystallize with a monoclinic chiral P 2 1 structure, and their layer structures are built by isolated ethane-like P 2 S 6 dimers and RES 8 bicapped trigonal prisms built {[RE 2 S 15 ] 24- } ∞ layers. By comparing the structures with related ones, the change of the alkali metal or RE 3+ ions can cause structural transformation. Their band gaps are tunable between 2.58 and 3.79 eV, and their powder samples exhibit good NLO properties. Theoretical calculations suggest that the NLO properties are mainly contributed by P 2 S 6 units and {[RE 2 S 15 ] 24- } ∞ layers synergistically, in which {[RE 2 S 15 ] 24- } ∞ layers and P 2 S 6 units dominate the contribution to the band gap and second-harmonic-generation response, respectively. This work enriches the application of rare-earth chalcophosphates as NLO materials.