Synthesis and SHG properties of the melamine-based material (C3N6H7)ZnX3(C3N6H6) (X = Cl, Br)

Abstract

The zinc halide–melamine system was investigated by conducting exploratory solid-state reactions and hydrothermal syntheses, yielding Zn₃(OH)₂Cl₄(C₃N₆H₆)₃ and (C₃N₆H₇)ZnX₃(C₃N₆H₆) with X = Cl and Br. All compounds were characterized by single-crystal X-ray diffraction, revealing that there are two modifications of the melaminium–zinc-chloride–melamine compound, crystallizing with non-centrosymmetric space groups (Pna2₁ and P2₁). The presence of delocalized π-electron systems in compounds containing cyclic (B₃O₆)³⁻, (C₃N₃O₃)³⁻, or melamine derivatives is known to give rise to significant second harmonic generation (SHG) properties, provided that the given structure has no inversion center. All crystal structures feature Zn²⁺ in a tetrahedral coordination environment, forming layered structures. Spectroscopic measurements on phase-pure orthorhombic (C₃N₆H₇)ZnBr₃(C₃N₆H₆) resulted in direct and indirect band gaps of 4.74 eV and 4.46 eV respectively. Measurements of the nonlinear optical response performed on single crystals of three different compounds revealed significant intensities following the sequence monoclinic (C₃N₆H₇)ZnCl₃(C₃N₆H₆) > orthorhombic (C₃N₆H₇)ZnBr₃(C₃N₆H₆) > orthorhombic (C₃N₆H₇)ZnCl₃(C₃N₆H₆). Their SHG properties are compared with those of KTP, one of the most widely used SHG materials.