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

Abstract 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 (≈2.5 × AGS), large bandgap (3.61 eV), and wide mid‐infrared transparent region (≈15.3 µm). 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.