REI5O14 (RE=Y and Gd): Promising SHG Materials Featuring the Semicircle‐Shaped I5O143− Polyiodate Anion

The first examples of rare‐earth polyiodates, namely, REI₅O₁₄ (RE=Y and Gd), have been prepared by hydrothermal reactions of RE₂O₃ and H₅IO₆ in H₃PO₄ (≥85 wt % in H₂O), with extremely high yields (>95 %). They crystalize in the polar space group Cm and feature a brand‐new semicircle‐shaped I₅O₁₄]^3? pentameric polyiodate anion composed of two IO₃ and three IO₄ polyhedra. Remarkably, both compounds exhibit very large second‐harmonic generation (SHG) signals (14× and 15×KH₂PO₄ (KDP) upon 1064 nm laser radiation for Y and Gd compounds, respectively). Our work shows that the hydrothermal reaction in a phosphoric acid medium facilitates the formation of rare‐earth polyiodates.