Building-block approach to the discovery of Na8Mn2(Ge2Se6)2: A polar chalcogenide exhibiting promising harmonic generation signals with a high laser-induced damage threshold

•The building block approach has been utilized to prepare Na8Mn2(Ge2Se6)2 at 750 °C.•The polar structure of Na8Mn2(Ge2Se6)2 features the ethane-like [Ge2Se6]6- unit.•Na8Mn2(Ge2Se6)2 is a paramagnetic semiconductor with an optical bandgap of 1.95 eV.•The compound displays modest second-harmonic and strong third harmonic generation.•Na8Mn2(Ge2Se6)2 boasts a larger laser-induced damage threshold, ~9 × AgGaSe2. [Display omitted] A new polar quaternary chalcogenide, Na8Mn2(Ge2Se6)2, has been synthesized using the building-block approach by reacting preformed Na6Ge2Se6 and MnCl2 at 750 °C. The structure consists of layers of [Na(1)Mn(Ge2Se6)]3– stacked perpendicular to the c-axis and sodium ions occupying the interlayer space. An indirect bandgap of 1.52 eV has been calculated using density functional theory, which is expectedly underestimated compared to the observed optical bandgap of 1.95 eV derived from diffuse reflectance spectroscopic measurements in the UV/Vis/NIR region. Magnetic measurements confirm the paramagnetic nature of Na8Mn2(Ge2Se6)2 with an experimental magnetic moment of 5.8 μB in good agreement with the theoretical spin only moment of 5.92 μB for high spin Mn2+. Na8Mn2(Ge2Se6)2 exhibits a potentially wide region of transparency in the measured range of 2.5–25 µm. Na8Mn2(Ge2Se6)2 shows a modest second-harmonic generation (SHG) response but with a high laser-induced damage threshold (LIDT) of ~9x AgGaSe2. Third harmonic generation (THG) measurements indicate that Na8Mn2(Ge2Se6)2 displays a high THG coefficient (1.9x AgGaSe2) at λ = 1800 nm.