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

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.