REI5 O14 (RE=Y and Gd): Promising SHG Materials Featuring the Semicircle-Shaped I5 O14 3- Polyiodate Anion.

The first examples of rare-earth polyiodates, namely, REI5 O14 (RE=Y and Gd), have been prepared by hydrothermal reactions of RE2 O3 and H5 IO6 in H3 PO4 (≥85 wt % in H2 O), with extremely high yields (>95 %). They crystalize in the polar space group Cm and feature a brand-new semicircle-shaped [I5 O14 ]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×KH2 PO4 (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.