Na4MgM2Se6 (M = Si, Ge): The First Noncentrosymmetric Compounds with Special Ethane-like [M2Se6]6– Units Exhibiting Large Laser-Damage Thresholds

Two new noncentrosymmetric compounds, Na4MgM2Se6 (I, M = Si; II, M = Ge), that contain special ethane-like [M2Se6](6-) units were reported for the first time. Remarkably, they exhibit high laser-damage thresholds [9 (I) and 7 (II) × benchmark AgGaS2] and moderate second-harmonic-generation responses with type I phase matching.