[C(NH ₂ ) ₃ ]SbCl ₄ : An NLO Material with Enhanced Second Harmonic Response and Birefringence

Guanidine units have garnered significant attention in the study of ultraviolet (UV) nonlinear optical (NLO) materials due to their excellent microscopic optical properties, making them ideal systems for developing novel optical devices. A novel guanidinium-templated UV NLO crystal, [C(NH₂)₃]SbCl₄ (GSC), was successfully synthesized via an aqueous solution method. GSC crystallizes in the orthorhombic chiral space group P2₁2₁2₁, featuring a seesaw shaped [SbCl₄]⁻ anion. Remarkably, GSC exhibits a moderate second harmonic generation (SHG) response of 1.1 × KDP at 1064 nm and a large refractive index difference of 0.2 at 546 nm. This NLO effect originates from the synergistic interaction between the [C(NH₂)₃]⁺ unit and the stereochemically active [SbCl₄]⁻ unit. This work provides a candidate for the guanidine family of NLO materials.