K2ZnGe3S8: A Congruent-Melting Infrared Nonlinear-Optical Material with a Large Band Gap

K 2 ZnGe 3 S 8 belonging to the noncentrosymmetric space group P 2 1 of the monoclinic system was discovered via a solid-state method. It possesses two-dimensional [ZnGe 3 S 8 ] 2– layers, with alkali-metal cations K + located between the layers. On the basis of UV–vis–near-IR diffuse-reflectance spectrometry, the band gap of K 2 ZnGe 3 S 8 is 3.36(2) eV. According to powder second-harmonic-generation (SHG) measurements, the SHG response of K 2 ZnGe 3 S 8 is about 0.9 times that of AgGaS 2 at the particle size range of 20–41 μm. Experimental results demonstrate that K 2 ZnGe 3 S 8 keeps a good balance between a large band gap (3.36 eV) and a moderate SHG response. Moreover, according to the differential scanning calorimetry measurements, K 2 ZnGe 3 S 8 melts congruently at around 1023 K and recrystallizes at about 963 K. Therefore, it is possible to obtain bulk single crystals via the Bridgman–Stockbarger method. The first-principles calculations indicate that the optical properties of K 2 ZnGe 3 S 8 are dominantly determined by the [GeS 4 ] tetrahedra as well as a small contribution from the [ZnS 4 ] tetrahedra.