Li2Mg2Si2S6 and Li2Mg2Ge2S6 have been prepared as colorless, translucent, hexagonally shaped crystals via traditional high-temperature, solid-state synthesis. Single crystal X-ray diffraction and X-ray powder diffraction coupled with Rietveld refinement were used to solve and refine the crystal structures and assess phase purity, respectively. The title compounds are of a new structure type, crystallizing in the noncentrosymmetric space group P31m (No. 157). These compounds possess [M2S6]6- staggered ethane-like units nestled within the holes of the magnesium-sulfide layers created by edge-sharing MgS6 units, whereas the holes in the lithium-sulfide layers, created by LiS6 edge-sharing octahedra, are vacant. The magnesium-sulfide and lithium-sulfide layers are connected through the face sharing of octahedra creating a three-dimensional framework. Li2Mg2Si2S6 has a second harmonic generation response approximately 25 times that of alpha-quartz, using a Nd:YAG laser at 1064 nm with a repetition rate of 20 HZ and a pulse width of 7 ns. Electronic structure calculations show that this compound is a semiconductor with a direct bandgap. Additional physicochemical characterization including diffuse reflectance UV-Vis-NIR spectroscopy, attenuated total reflectance FT-IR spectroscopy and phase matchability will be discussed. The discovery of these compounds opens new routes for synthesizing noncentrosymmetric lithium sulfides as potential candidates for IR NLO applications.

Li2Mg2Si2S6 and Li2Mg2Ge2S6: two new quaternary lithium sulfides with potential in infrared nonlinear optics

255th National Meeting (2018)

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