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

K2ZnGe3S8 belonging to the noncentrosymmetric space group P21 of the monoclinic system was discovered via a solid-state method. It possesses two-dimensional [ZnGe3S8]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 K2ZnGe3S8 is 3.36(2) eV. According to powder second-harmonic-generation (SHG) measurements, the SHG response of K2ZnGe3S8 is about 0.9 times that of AgGaS2 at the particle size range of 20-41 μm. Experimental results demonstrate that K2ZnGe3S8 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, K2ZnGe3S8 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 K2ZnGe3S8 are dominantly determined by the [GeS4] tetrahedra as well as a small contribution from the [ZnS4] tetrahedra.