[C ₂ N ₄ H ₇ O][NH ₂ SO ₃ ]: High‐Performance Ultraviolet Nonlinear Optical Crystal with Ditrigon Coupled Guanylurea Group

Abstract The Development of high‐performance short‐wave ultraviolet (UV) nonlinear optical (NLO) crystals is critical in the field of optoelectronic functional crystals. High‐performance NLO functional units are crucial for achieving desired target crystals. In this study, a promising group, guanylurea ([C 2 N 4 H 7 O] + , abbreviated as GU), was investigated using a unit coupling strategy. Furthermore, by modifying the KBe 2 BO 3 F 2 template structure at the molecular level, the first metal‐free guanylurea sulfamate [C 2 N 4 H 7 O][NH 2 SO 3 ] was successfully synthesized. This crystal achieved an effective balance between the short UV cutoff edge (227 nm), strong second‐harmonic generation (SHG) response (6.2×KDP), and large birefringence (0.225@1064 nm). In addition, the remarkable phase‐matching capability provides the potential for a fourth‐harmonic generation (266 nm) in the Nd: YAG laser. Theoretical calculations indicated that GU and its favorable arrangement were primarily responsible for the large SHG effect and birefringence. Our study demonstrates that the GU group is promising for the development of UV NLO crystals.