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

The first examples of rare‐earth polyiodates, namely, REI 5 O 14 (RE=Y and Gd), have been prepared by hydrothermal reactions of RE 2 O 3 and H 5 IO 6 in H 3 PO 4 (≥85 wt % in H 2 O), with extremely high yields (>95 %). They crystalize in the polar space group Cm and feature a brand‐new semicircle‐shaped [I 5 O 14 ] 3− pentameric polyiodate anion composed of two IO 3 and three IO 4 polyhedra. Remarkably, both compounds exhibit very large second‐harmonic generation (SHG) signals (14× and 15×KH 2 PO 4 (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.