KHgGeSe: A Quaternary Hg-Based Selenide with Nonlinear Optical Properties.

A new quaternary Hg-based selenide KHgGeSe was successfully synthesized and characterized as an engaging infrared (IR) nonlinear optical (NLO) material, which crystallizes in the 2 space group of the noncentrosymmetric monoclinic crystal system. The crystal structure of KHgGeSe is characterized by the stacking of countless layers with K cations filling the interlayers. The UV-vis-NIR diffuse reflectance spectrum shows that KHgGeSe possesses an indirect band gap of 2.13 eV. The second harmonic generation (SHG) response of KHgGeSe is approximately 1.1 times that of AgGaS (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. KHgGeSe also has a large powder laser-induced damage threshold (∼ 2.3 × AGS). As revealed by theoretical calculations, the optical properties of KHgGeSe are predominantly determined by [HgSe] and [GeSe] tetrahedra, and KHgGeSe possesses a calculated optical band gap of 1.44 eV together with a maximum SHG coefficient of -7.70 pm V.