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Our exploration in vanadate selenites by using boric acid flux method has successfully lead into a new phase, namely, γ-V 2 Se 2 O 9. Single crystal X-ray diffraction indicates its a new structure which crystallizes in monoclinic space group P 2 1 /c with a lattice parameter of a ​= ​7.4857 (12) Å, b ​= ​7.4388 (8) Å, c ​= ​13.042 (2) Å, β ​= ​106.619 (9)°. Its crystal structure features novel 2D electroneutral [V 2 Se 2 O 9 ] layers composed of 1D [V 2 SeO 9 ]4- chains that are further bridged by Se (1)O 3 triangular pyramids. [V 2 SeO 9 ]4- chain consist corner-sharing of VO 6 and tridentate Se (2)O 3. Unlike previously reported α - and β -V 2 Se 2 O 9 with isolated [V 4 O 18 ]16− tetramers, corner-sharing of VO 6 octahedra lead into a 1D [V 2 O 9 ]8- chain in γ-V 2 Se 2 O 9. X-ray powder diffraction and thermal behavior analysis were performed to study its phase purity and thermal stability. Optical diffuse reflectance spectra associated with Tauc's fitting indicates a direct bandgap semiconductor with an energy value of 2.34 ​eV, which consistent with the theoretical prediction. As a compare, the electronic bandgaps of three polymorphs were studied and shows a sequence of α -V 2 Se 2 O 9 < β -V 2 Se 2 O 9 <γ-V 2 Se 2 O 9. Density of states were also calculated to reveal the origin of the bandgaps and bond interactions. [Display omitted] γ-V 2 Se 2 O 9 was obtained by boric acid flux method in vanadate selenite system. Its crystal structure features novel 2D electroneutral [V 2 Se 2 O 9 ] layers composed of 1D [V 2 SeO 9 ]4- chains that are further bridged by Se (1)O 3 triangular pyramids. [V 2 SeO 9 ]4- chain consist corner-sharing of VO 6 and tridentate Se (2)O 3. UV–Vis diffuse reflectance spectroscopy coupled with Tauc's analysis indicates a direct semiconductor with a bandgap value of 2.34 ​eV, and its energy transition type was further supported by first-principles electronic structure calculation. • γ-V 2 Se 2 O 9 was obtained by boric acid flux method for the first time. • Unlike α - and β -V 2 Se 2 O 9 with isolated [V 4 O 18 ]16− tetramers, corner-sharing of VO 6 forms 1D [V 2 O 9 ]8- chains in γ-V 2 Se 2 O 9. • UV–Vis diffuse reflectance spectroscopy coupled with Tauc's fitting indicates γ-V 2 Se 2 O 9 as a direct bandgap of 2.34 ​eV. [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]