Rare-Earth Selenophosphates Cs₂YP₂Se₇ and KYP₂Se₆ Synthesized by the RFMOBQ Routine: Structural Evolution and...

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, Cs₂YP₂Se₇ (1), and the first NLO-active Y-based selenophosphate, KYP₂Se₆ (2), were synthesized via the reactive flux AI (A = K, Cs) assisted RE_xO_y-B-Se solid state method developed by us. They crystallize in the monoclinic P2₁/n and orthorhombic chiral P2₁2₁2₁ structure, respectively. The structure of 1, featuring {[Y₂P₄Se₁₄]^4-}_∞ layers constructed by YSe₆, PSe₄ and P₂Se₆ units, transforms to the structure of 2, constructed from {[YP₂Se₆]^-}_∞ layers built by YSe₈ and P₂Se₆ 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 YSe₈ and P₂Se₆ units contributes to the SHG activity.