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The novel I 4 -II-IV 2 -VI 7 quaternary diamond-like semiconductors (DLSs), Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 , were prepared from the constituent elemental powders at elevated temperatures. Cu 4 ZnGe 2 S 7 crystallizes in the polar, chiral space group C 2 and adopts the Cu 4 NiSi 2 S 7 structure-type, which can be considered a derivative of the cubic diamond structure. The cadmium analog takes on the Cu 5 Si 2 S 7 structure-type, which is related to the hexagonal diamond structure, and displays polar, Cc space group symmetry. Both compounds contain [Ge 2 S 7 ]6- anions created by corner-sharing GeS 4 tetrahedra. The crystal structures were carefully evaluated using: 1) extended connectivity tables, 2) minimum bounding ellipsoid analysis, and 3) bond valence sum (BVS) and global instability index (G) calculations. Both compounds are thermally stable up to temperatures exceeding 950 °C and melt incongruently. As assessed using diffuse reflectance spectroscopy, Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 possess optical bandgaps of 1.65 eV and 1.90 eV, respectively, which are narrower than the analogous I 2 -II-IV-VI 4 DLSs. Electronic structure calculations show that both compounds are direct-bandgap semiconductors with significant contributions of Cu- d and S- p orbitals to the states at the top of the valence band. While Cu 4 CdGe 2 S 7 displays a weak second harmonic generation (SHG) response, the second-order nonlinear optical susceptibility, χ(2), of Cu 4 ZnGe 2 S 7 is 14.8 ± 1.5 pm V-1, assessed using the Kurtz-Perry powder method and an optical-grade AgGaSe 2 (AGSe) reference. Both compounds exhibit laser-induced damage threshold values similar to AGSe, ∼0.1 GW cm-2, for λ = 1.064 µm using picosecond laser pulses. Most importantly, Cu 4 ZnGe 2 S 7 exhibits a phase-matching behavior at λ = 3.1 µm. These results suggest that Cu 4 ZnGe 2 S 7 holds potential for use in low-powered laser applications involving efficient wave mixing in the mid-IR region. [Display omitted] • Cu 4 ZnGe 2 S 7 is a novel diamond-like material with the Cu 4 NiSi 2 S 7 structure type. • Cu 4 CdGe 2 S 7 is a novel diamond-like material with the Cu 5 Si 2 S 7 structure type. • Optical bandgaps of Cu 4 ZnGe 2 S 7 and Cu 4 CdGe 2 S 7 are 1.65 and 1.90 eV, respectively. • Cu 4 ZnGe 2 S 7 has χ (2) value of 14.8 ± 1.5 pm V− 1 and is phase matching for λ = 3.1 µm. • Cu 4 ZnGe 2 S 7 is a candidate for low-powered laser applications in the mid-IR region. [ABSTRACT FROM AUTHOR]

• Cu 4 ZnGe 2 Se 7 is a new diamond-like semiconductor with the Cu 4 NiSi 2 S 7 structure type. • CZGSe and Cu 4 ZnGe 2 Se 7 possess optical bandgaps of 1.38 and 0.91 eV, respectively. • CZGSe and Cu 4 ZnGe 2 Se 7 are air stable and have wide windows of optical transparency. • CZGSe exhibits a strong second harmonic generation response at λ = 2900 nm. • CZGSe is an excellent candidate for low-powered infrared nonlinear optical devices. [Display omitted] In this study, two quaternary diamond-like semiconductors (DLSs) in the Cu-Zn-Ge-Se system, the known Cu 2 ZnGeSe 4 (also referred to as CZGSe) and the new Cu 4 ZnGe 2 Se 7 , are compared in terms of their crystal structures, electronic structures and physicochemical properties. Both compounds were prepared by high-temperature, solid-state synthesis at 800 °C. Single crystal X-ray diffraction was used to determine the structure of Cu 4 ZnGe 2 Se 7. The structures of both Cu 2 ZnGeSe 4 and Cu 4 ZnGe 2 Se 7 can be considered as derivatives of cubic diamond. Cu 4 ZnGe 2 Se 7 , with a reduced symmetry due to the cation ordering pattern, adopts the Cu 4 NiSi 2 S 7 structure type, with space group C 2. The corner-sharing tetrahedra in Cu 4 ZnGe 2 Se 7 are slightly distorted because the charge for some of the S2- anions is not compensated by the first-nearest-neighbor cations. Rietveld refinements using synchrotron X-ray powder diffraction data were used to assess the phase purity of the samples and confirm the bulk structural behavior. Diffuse reflectance UV/Vis/NIR spectroscopy shows that Cu 2 ZnGeSe 4 and Cu 4 ZnGe 2 Se 7 have direct optical bandgaps of 1.38 and 0.91 eV, respectively. Electronic structure calculations implementing density functional theory confirm the direct bandgap for Cu 4 ZnGe 2 Se 7 , with a calculated value of 0.62 eV. Both compounds are air stable, thermally stable up to relatively high temperatures, undergo phase transitions and have wide windows of optical clarity. The Kurtz-Perry powder technique was used to determine the second harmonic generation (SHG) responses using a commercial AgGaSe 2 standard. While Cu 4 ZnGe 2 Se 7 displays a weak SHG response, Cu 2 ZnGeSe 4 exhibits a response that is greater than several benchmark materials, with a large SHG coefficient, χ (2) , of 43 ± 6 pm/V at λ = 2900 nm. [ABSTRACT FROM AUTHOR]