Li2Mg2Si2S6 and Li2Mg2Ge2S6: Two nonlinear optical sulfides featuring a unique, polar trigonal structure incorporating ethane‐like anions

The new compounds Li2Mg2Si2S6 and Li2Mg2Ge2S6 have been prepared via traditional high‐temperature, solid‐state synthesis. The title compounds crystallize in the polar, noncentrosymmetric, trigonal space group P31m (No. 157) and adopt a new structure type featuring staggered, ethane‐like (T2S6)6− units, where T=Si or Ge. These (T2S6)6− units are nestled within the holes of magnesium‐sulfide “layers” that are created through the edge‐sharing of MgS6 octahedra. The holes found in the lithium‐sulfide “layers”, created by LiS6 edge‐sharing octahedra, remain vacant, containing no (T2S6)6− anionic group. Through the face sharing of the respective MgS6 and LiS6 octahedra, the magnesium‐sulfide and lithium‐sulfide “layers” are stitched together resulting in an overall three‐dimensional structure. The optical bandgaps of Li2Mg2Si2S6 and Li2Mg2Ge2S6 are 3.24 and 3.18 eV, respectively, as estimated from optical diffuse reflectance UV‐Vis‐NIR spectroscopy. The compounds exhibit second harmonic generation responses of ∼0.24×KDP and ∼2.92×α‐quartz for Li2Mg2Si2S6 and ∼0.17×KDP and ∼2.08×α‐quartz for Li2Mg2Ge2S6, using a Nd:YAG laser at 1.064 μm. Electronic structure calculations were performed using density functional theory and the linearized augmented plane‐wave approach within the WIEN2k software package. Examination of the electronic band structures shows that these compounds are indirect bandgap semiconductors.