Nonlinear optical effects in two mercury cyanamide/guanidinium chlorides Hg3(NCN)2Cl2 and Hg2(C(NH2)3)Cl5

Abstract

Two solid-state mixed-anion mercury salts, mercury cyanamide chloride, Hg₃(NCN)₂Cl₂, and mercury guanidinium chloride, Hg₂(C(NH₂)₃)Cl₅, were synthesized in phase-pure form, and their optical data were studied by means of UV-Vis measurements and electronic structure theory, revealing that both compounds exhibit direct band gap-transitions. Moreover, their second-harmonic generation effects were measured using powder samples, and the underlying mechanisms were elucidated by first-principles analyses. We demonstrate that the parallel arrangement of the NCN²⁻ groups in the crystal structure of Hg₃(NCN)₂Cl₂ enhances the superposition of dipole moments and produces a large nonlinear optical response. In comparison, the C(NH₂)₃⁺ groups in Hg₂(C(NH₂)₃)Cl₅ arrange in opposite polar orientations, which results only in a small SHG signal.