K 2 HgGe 3 Se 8 : A Quaternary Hg-Based Selenide with Nonlinear Optical Properties.

A new quaternary Hg-based selenide K 2 HgGe 3 Se 8 was successfully synthesized and characterized as an engaging infrared (IR) nonlinear optical (NLO) material, which crystallizes in the P 2 1 space group of the noncentrosymmetric monoclinic crystal system. The crystal structure of K 2 HgGe 3 Se 8 is characterized by the stacking of countless [ H g G e 3 S e 8 ] 2 - ∞ 2 layers with K + cations filling the interlayers. The UV-vis-NIR diffuse reflectance spectrum shows that K 2 HgGe 3 Se 8 possesses an indirect band gap of 2.13 eV. The second harmonic generation (SHG) response of K 2 HgGe 3 Se 8 is approximately 1.1 times that of AgGaS 2 (AGS) within the particle size of 20-50 μm along with a nonphase-matching (NPM) behavior at 2090 nm, as indicated by the SHG test. K 2 HgGe 3 Se 8 also has a large powder laser-induced damage threshold (∼ 2.3 × AGS). As revealed by theoretical calculations, the optical properties of K 2 HgGe 3 Se 8 are predominantly determined by [HgSe 4 ] and [GeSe 4 ] tetrahedra, and K 2 HgGe 3 Se 8 possesses a calculated optical band gap of 1.44 eV together with a maximum SHG coefficient d 22 of -7.70 pm V -1 .