KREP 2 Se 6 (RE = Sm, Gd, Tb): The First Rare-Earth Selenophosphates with Remarkable Nonlinear Optical Activities Realized by Synergistic Effect of RE- and P-Based Motifs.

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials because of their nice integrated performances; however, very few RE chalcophosphates are involved for this topic. Here, three quaternary RE selenophosphates, KSmP 2 Se 6 (1), KGdP 2 Se 6 (2), and KTbP 2 Se 6 (3), are profoundly studied for their NLO potentials. Their noncentrosymmetric P21 structures feature RESe8-bicapped trigonal prisms and ethane-like [P 2 Se 6 ] 4 - dimers built {[REP 2 Se 6 ]-}∞ layers. As the first studied NLO-active RE selenophosphates, 1-3 exhibit second harmonic generation (SHG)responses ≈0.34-1.08 × AgGaS 2 at 2.10 µm and laser-induced damage thresholds (LIDTs) ≈1.43-4.33 × AgGaS 2 , and they all show phase-matchable behaviors, indicating their wonderful balanced NLO properties. Theoretical calculations demonstrate that the synergistic effect between RESe 8 and P 2 Se 6 units makes the major contribution to the SHG responses.