d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K2CdGe3S8 and K2CdGe3Se8, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K2CdGe3S8 and K2CdGe3Se8 were accurately determined by single-crystal X-ray diffraction and crystallize in the K2FeGe3S8 structure type. K2CdGe3S8 is isostructural to K2FeGe3S8 with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K2CdGe3S8 is 18× K2FeGe3S8 for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K2CdGe3S8 over K2FeGe3S8 are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS4] tetrahedra within K2CdGe3S8 exhibit a higher degree of distortion and larger volume compared to the [FeS4] tetrahedra in K2FeGe3S8. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d10-elements are better for SHG susceptibility than d6-elements in this study. K2CdGe3S8 is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS2, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS2), and good stability.