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The new, quaternary diamond-like semiconductor (DLS) Cu4MnGe2S7 was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu4MnGe2S7 features [Ge2S7]6− units and adopts the Cu5Si2S7 structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu2MnGeS4 with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu2MnGeS4 and Cu4MnGe2S7 compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz–Perry powder method, the second-order nonlinear optical susceptibility, χ(2), values for Cu2MnGeS4 and Cu4MnGe2S7 were estimated to be 16.9 ± 2.0 pm V−1 and 2.33 ± 0.86 pm V−1, respectively, by comparing with an optical-quality standard reference material, AgGaSe2 (AGSe). Cu2MnGeS4 was found to be phase matchable at λ = 3100 nm, whereas Cu4MnGe2S7 was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu4MnGe2S7 precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu2MnGeS4 was estimated to be ∼0.1 GW cm−2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu4MnGe2S7 could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations. [ABSTRACT FROM AUTHOR]

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]