[C₂N₄H₇O][NH₂SO₃]: High-Performance Ultraviolet Nonlinear Optical Crystal with Ditrigon Coupled Guanylurea Group

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 ([C₂N₄H₇O]⁺, abbreviated as GU), was investigated using a unit coupling strategy. Furthermore, by modifying the KBe₂BO₃F₂ template structure at the molecular level, the first metal-free guanylurea sulfamate [C₂N₄H₇O][NH₂SO₃] 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.