KREP₂Se₆ (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.