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, Cs₂YP₂Se₇ (<b>1</b>), and the first NLO-active Y-based selenophosphate, KYP₂Se₆ (<b>2</b>), were synthesized via the reactive flux AI (A = K, Cs) assisted RE_<i>x</i>O_<i>y</i>-B-Se solid state method developed by us. They crystallize in the monoclinic <i>P</i>2₁/<i>n</i> and orthorhombic chiral <i>P</i>2₁2₁2₁ structure, respectively. The structure of <b>1</b>, featuring {[Y₂P₄Se₁₄]^4-}_∞ layers constructed by YSe₆, PSe₄ and P₂Se₆ units, transforms to the structure of <b>2</b>, 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 <b>2</b>, and theoretical calculations reveal that the synergistic effect between YSe₈ and P₂Se₆ units contributes to the SHG activity.