K₂HgGe₃Se₈: A Quaternary Hg-Based Selenide with Nonlinear Optical Properties

A new quaternary Hg-based selenide K₂HgGe₃Se₈ was successfully synthesized and characterized as an engaging infrared (IR) nonlinear optical (NLO) material, which crystallizes in the <i>P</i>2₁ space group of the noncentrosymmetric monoclinic crystal system. The crystal structure of K₂HgGe₃Se₈ is characterized by the stacking of countless <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mmultiscripts><mml:msup><mml:mrow><mml:mo>[</mml:mo><mml:mrow><mml:mi>H</mml:mi><mml:mi>g</mml:mi></mml:mrow><mml:msub><mml:mrow><mml:mi>G</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mrow><mml:mi>S</mml:mi><mml:mi>e</mml:mi></mml:mrow><mml:mn>8</mml:mn></mml:msub><mml:mo>]</mml:mo></mml:mrow><mml:mrow><mml:mn>2</mml:mn><mml:mo>-</mml:mo></mml:mrow></mml:msup><mml:mprescripts></mml:mprescripts><mml:mi>∞</mml:mi><mml:mn>2</mml:mn></mml:mmultiscripts></mml:math> layers with K⁺ cations filling the interlayers. The UV-vis-NIR diffuse reflectance spectrum shows that K₂HgGe₃Se₈ possesses an indirect band gap of 2.13 eV. The second harmonic generation (SHG) response of K₂HgGe₃Se₈ is approximately 1.1 times that of AgGaS₂ (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. K₂HgGe₃Se₈ also has a large powder laser-induced damage threshold (∼ 2.3 × AGS). As revealed by theoretical calculations, the optical properties of K₂HgGe₃Se₈ are predominantly determined by [HgSe₄] and [GeSe₄] tetrahedra, and K₂HgGe₃Se₈ possesses a calculated optical band gap of 1.44 eV together with a maximum SHG coefficient <i>d</i>₂₂ of -7.70 pm V^-1.