Chiral and Polar Duality Design of Heteroanionic Compounds: Sr 18 Ge 9 O 5 S 31 Based on [Sr 3 OGeS 3 ] 2+ and [Sr 3 SGeS 3 ] 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 [Sr 3 OGeS 3 ] 2+ and [Sr 3 SGeS 3 ] 2+ groups to design and synthesize a new oxychalcogenide Sr 18 Ge 9 O 5 S 31 with chiral-polar duality is proposed. The presence of two enantiomers of Sr 18 Ge 9 O 5 S 31 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 Sr 18 Ge 9 O 5 S 31 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.