K2 Sb(P2 O7 )F: Cairo Pentagonal Layer with Bifunctional Genes Reveal Optical Performance.

Discovering new functional genes, designing perfect crystal structures, and developing high-performance materials are the goals being pursued by scientists. Herein, the first antimony pyrophosphate, K2 Sb(P2 O7 )F, possessing an optimal layered structure, is reported, where the perfect structural arrangement induces excellent optical properties. K2 Sb(P2 O7 )F not only displays a sharply enhanced birefringence (0.157@546 nm) compared to the existing phosphate optical materials, but also exhibits a strong second-harmonic generation response (4.0×KDP). Remarkably, a new bifunctional gene, the square-pyramidal SbO4 F group, was discovered, and a unique two-dimensional arrangement of Cairo pentagonal tiling units was observed in inorganic compounds for the first time.