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The Kurtz-Perry powder technique (KPPT) is widely utilized for the rapid characterization of newly grown nonlinear optical (NLO) materials available in pristine powder form. It assesses the effective nonlinear susceptibility of randomly oriented powder in comparison with a reference material. This technique becomes especially versatile when equipped with a broad wavelength range and intensity tunability because it provides key NLO properties of the materials under investigation, such as the frequency dispersion of the NLO susceptibility, phase-matching (PM) range, and laser-induced damage threshold (LIDT) as a guideline. In this study, wavelength-dependent NLO experiments (λ = 1100 – 3700 nm) were conducted on polycrystalline LiInSe 2 , prepared by high-temperature, solid-state synthesis. Our LiInSe 2 was found to be phase-matchable for λ > 2500 nm and the effective NLO susceptibility was estimated as 20.0 ± 1.9 pm/V, assessed at the static limit using commercial-grade single crystals of AgGaS 2 and AgGaSe 2 that were ground to powders. Alternatively, the effective NLO susceptibility was determined to be twice as large, ~40 pm/V, when utilizing a homemade, microcrystalline AgGaSe 2 reference at the static limit. This severe overestimation of the NLO susceptibility that can occur while working with a homemade standard underscores the importance of using a commercial-grade reference with the KPPT. Furthermore, this study also discusses the LIDT of LiInSe 2 based on intensity-dependent SHG at 1064 nm with picosecond pulses, and compares with those of the commercial-grade AgGaS 2 and AgGaSe 2 , determined under identical conditions. As defect levels can cause widely varying apparent color, thus optical bandgap, in LiInSe 2 , this study stresses the importance of using a high-quality standard when exploiting the KPPT for NLO materials characterization. The role of defects is discussed in terms of χ 2 , phase matching onset and LIDT, parameters that predominantly affect the evaluation of new NLO materials. [Display omitted] • A high-quality NLO reference is necessary for the Kurtz-Perry powder technique. • Broadband measurements best characterize the behavior of candidate NLO materials. • The laser-induced damage threshold of LiInSe 2 is 1.0 GW/cm2 (λ = 1064 nm, τ = 30 ps). • LiInSe 2 has a χ(2) value of ~20 pm/V and is phase matchable for λ > 2500 nm. [ABSTRACT FROM AUTHOR]

Highlights: [•] The title compounds were prepared by a high-temperature, direct-combination route. [•] The room temperature structure of Li4SnS4 is reported in space group Pnma. [•] The Li(2)S4 tetrahedra invert upon going from the RT to low temperature structure. [•] Electronic structure calculations indicate the Li–S bonding to be primarily ionic. [•] Li4GeS4 and Li4SnS4 have shown to be stable up to ∼850 and 950°C respectively. [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]