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

KZnGeS belonging to the noncentrosymmetric space group P2 of the monoclinic system was discovered via a solid-state method. It possesses two-dimensional [ZnGeS] 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 KZnGeS is 3.36(2) eV. According to powder second-harmonic-generation (SHG) measurements, the SHG response of KZnGeS is about 0.9 times that of AgGaS at the particle size range of 20-41 μm. Experimental results demonstrate that KZnGeS 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, KZnGeS 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 KZnGeS are dominantly determined by the [GeS] tetrahedra as well as a small contribution from the [ZnS] tetrahedra.