The modulation of structural polarity facilitating the gradient enhancement of second harmonic generation effects in hybrid metal halides

Nonlinear optical (NLO) crystals have important application value in optical communication, laser technology, and optical display. By enhancing the polarization, the SHG effect of the material can be effectively improved. In this study, three novel nonlinear optical crystals (C6H5N2)2MI4 (M = Zn, Cd, Hg) were synthesized by combining strong polarization ability halogen I, d10 metal cations and the highly polarized organic π-conjugated ring (C6H5N2)+. By adjusting the metal ions, the SHG effect can be enhanced successively from (C6H5N2)2ZnI4 to (C6H5N2)2HgI4. Their second harmonic (SHG) effects are 2.80 × KDP, 3.02 × KDP, and 4.25 × KDP, respectively. Among them, (C6H5N2)2HgI4 shows the strongest SHG effect in the mercury-pyridine hybrid metal halide system. Theoretical calculations indicate that the excellent SHG properties of these compounds are attributed to the synergistic effect between the organic π-conjugated (C6H5N2)+ cation and the [MI4]2− tetrahedral anion. The results show that the SHG effect of the compound can be effectively regulated by adjusting its polarity. This study also provides a new strategy for designing high-performance NLO organic-inorganic halide crystals and offers insights for a deep understanding of the key role of hybrid tetrahedra in enhancing the SHG response.