Two Nonlinear Optical Thiophosphates CuHgPS and AgHgPS Activated by Their Tetrahedra-Stacking Architecture.

Infrared (IR) lasers are very critical in military and civil affairs, but it is also challenging and difficult to develop new infrared nonlinear optical (NLO) crystals. Herein, two new mixed-metal thiophosphates, CuHgPS and AgHgPS were discovered with the noncentrosymmetric (NCS) space group 2 (No. 31) and (No. 9). CuHgPS displays a three-dimensional (3D) defective diamond-like structure stacked by [CuHgPS] layers. AgHgPS is characterized by a 3D framework consisting of distorted tetrahedrons. Moreover, the optical spectra show the band gaps of CuHgPS and AgHgPS are 2.12 and 1.85 eV, respectively, with a broad transparent range of 0.7-16.0 μm. In these two compounds, the dipole moments of nonlinear active units are strengthened due to the high-valence element P and the Hg-S bonds with large susceptibility. Therefore, AgHgPS exhibits a moderate second harmonic generation (SHG) response that is half that of AgGaS (AGS) at 30-45 μm, while CuHgPS performs a phase-matching (PM) behavior with a good SHG signal of 0.8 × AGS at 150-200 μm. The origin of NLO performance and electronic structures were revealed by the calculated dipole moments of distorted tetrahedra and theoretical calculations on the basis of density functional theory.