Perfectly encoding π-conjugated anions in the RE${}_5$(C${}_3$N${}_3$O${}_3$)(OH)${}_{12}$ (RE=Y, Yb, Lu) family with strong second harmonic generation response and balanced birefringence

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE${}_5$(C${}_3$N${}_3$O${}_3$)(OH)${}_{12}$ (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH${}_2$PO${}_4$), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C${}_3$N${}_3$O${}_3$)${}^{3-}$ anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area. (© 2022 Wiley‐VCH GmbH)