EuHgGeSe and EuHgSnS: Two Quaternary Eu-Based Infrared Nonlinear Optical Materials with Strong Second-Harmonic-Generation Responses.

Metal chalcogenides play a critical role in the infrared (IR) nonlinear optical (NLO) field. However, Eu-based chalcogenide-type IR NLO materials are still scarce up to now. In this paper, two new quaternary Eu-based chalcogenides, EuHgGeSe and EuHgSnS, containing the "NLO active groups" [HgQ] (Q = S, Se) and [GeSe]/[SnS] were synthesized through traditional high-temperature solid-state reactions. They possess noncentrosymmetric structures, crystallizing in the 2 space group, and exhibit strong phase-matchable second-harmonic-generation (SHG) responses (3.1× and 1.77× that of AgGaS for EuHgGeSe and EuHgSnS, respectively). Meanwhile, the optical band gaps of EuHgGeSe (1.97 eV) and EuHgSnS (2.14 eV) were determined from UV-vis-NIR diffuse reflectance spectra. Differential scanning calorimetry (DSC) analyses reveal the congruent-melting behavior of EuHgGeSe. Furthermore, structural analysis and theoretical calculations verify the critical driving effects of [HgQ] tetrahedra on the strong SHG activity. The overall results demonstrate that EuHgGeSe and EuHgSnS are potential IR NLO materials.