Alkali Metal Substitution-Induced Structural Transformation for RbREP₂Se₆ (RE = Gd, Tb, Dy) with Second-Harmonic Generation Responses

Rare-earth (RE) chalcogenides have been extensively studied as infrared nonlinear optical (NLO) materials, and they demonstrate high potential as candidates with integrated performances; however, the subgroup of RE chalcophosphates has been rarely studied. Herein, three new quaternary RE selenophosphates, RbGdP₂Se₆ (<b>1</b>), RbTbP₂Se₆ (<b>2</b>), and RbDyP₂Se₆ (<b>3</b>), were synthesized using the reactive flux RbI-assisted RE_<i>x</i>O_<i>y</i>-B-Se solid-state method. They crystallize in the orthorhombic chiral space group <i>P</i>2₁2₁2₁ and are the first RE chalcophosphate NLO materials crystallizing in this space group. Their structures feature {[REP₂Se₆]^-}_∞ layers constructed by RESe₈ bicapped trigonal prisms and ethane-like P₂Se₆ dimers. <b>1</b>-<b>3</b> exhibit phase-matchable SHG responses of ∼0.2-0.3 × AgGaS₂ at 2.10 μm and optical bandgaps of ∼2.40-2.51 eV. Theoretical calculations reveal that the synergistic effect between the RESe₈ and P₂Se₆ motifs contributes to the SHG responses.