Chiral and Polar Duality Design of Heteroanionic Compounds: Sr₁₈Ge₉O₅S₃₁ Based on [Sr₃OGeS₃]²⁺ and [Sr₃SGeS₃]²⁺ Groups

Chirality and polarity are the two most important and representative symmetry-dependent properties. For polar structures, all the twofold axes perpendicular to the principal axis of symmetry should be removed. For chiral structures, all the mirror-related symmetries and inversion axes should be removed. Especially for duality (polarity and chirality), all of the above symmetries should be broken and that also represents the highest-level challenge. Herein, a new symmetry-breaking strategy that employs heteroanionic groups to construct hourglass-like [Sr3OGeS3](2+) and [Sr3SGeS3](2+) groups to design and synthesize a new oxychalcogenide Sr18Ge9O5S31 with chiral-polar duality is proposed. The presence of two enantiomers of Sr18Ge9O5S31 is confirmed by the single-crystal X-ray diffraction. Its optical activity and ferroelectricity are also studied by solid-state circular dichroism spectroscopy and piezoresponse force microscopy, respectively. Further property measurements show that Sr18Ge9O5S31 possesses excellent nonlinear optical properties, including the strong second harmonic generation efficiency (approximate to 2.5 x AGS), large bandgap (3.61 eV), and wide mid-infrared transparent region (approximate to 15.3 mu m). These indicate that the unique microstructure groups of heteroanionic materials are conducive to realizing symmetry-breaking and are able to provide some inspiration for exploring the chiral-polar duality materials.