Rare-Earth Selenophosphates Cs 2 YP 2 Se 7 and KYP 2 Se 6 Synthesized by the RFMOBQ Routine: Structural Evolution and Nonlinear Optical Activity

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials and have become promising candidates for achieving good integrated performance; however, the subgroup RE chalcophosphates have been rarely studied, especially Y-based ones. Here, a new quaternary RE selenophosphate, Cs 2 YP 2 Se 7 ( 1 ), and the first NLO-active Y-based selenophosphate, KYP 2 Se 6 ( 2 ), were synthesized via the reactive flux AI (A = K, Cs) assisted RE x O y –B–Se solid state method developed by us. They crystallize in the monoclinic P 2 1 / n and orthorhombic chiral P 2 1 2 1 2 1 structure, respectively. The structure of 1, featuring {[Y 2 P 4 Se 14 ] 4– } ∞ layers constructed by YSe 6, PSe 4 and P 2 Se 6 units, transforms to the structure of 2, constructed from {[YP 2 Se 6 ] − } ∞ layers built by YSe 8 and P 2 Se 6 units under the size effect of A + cations. The symmetry breaking enables second-harmonic generation (SHG) response in 2, and theoretical calculations reveal that the synergistic effect between YSe 8 and P 2 Se 6 units contributes to the SHG activity.