Strategic Integration Between Trigonal Architecture Construction and Molecular Polarization Enhancement for Record SHG Effect in Selenates

Although possessing similar structural and electronic configurations with the tetrahedral nonlinear optical (NLO) active units, such as [SO 4 ], [PO 4 ] and so on, the development of selenates with [SeO 4 ] units have been long term hindered due to the lack of effective approaches to enhance polarizability and optical anisotropy. Here, a strategic integration between trigonal architecture construction and molecular polarization enhancement was proposed, which resulted in the discovery of an exceedingly performant selenate optical material, namely Hg 6 O 2 H(SeO 4 ) 3 I 3 ( HSOI ). This compound possess unique pseudo‐planar [Hg 3 OI 3 ] secondary building units (SBUs), which formed the cloverleaf‐shaped layer and further construct the sandwich structure with the polar [SeO 4 H] FBUs inserted between the layers. Notably, HSOI exhibits an exceptionally large second harmonic generation (SHG) response (14.6×KDP), achieving a new record that is 170% higher than the previous value of nonlinear metal selenates. Meanwhile, a record high birefringence of selenates (∼ 0.2) was also achieved in HSOI . Theoretical calculations were carried out to reveal its optical origins and provide new insights for further structure‐driven functional materials design.