Atomic-Scale\nPolar Helix in Inorganic Crystals

Chiral structures, noncentrosymmetric objects with a\ngiven handedness,\nemerge on all scales in nature. The most well-known chiral form, <i>the helix</i>, has numerous materializations not only on the\nmicroscopic scale (DNA, cholesteric liquid crystals, and spin helices)\nbut also on macroscopic and even cosmological scales. The ongoing\nquest for new types of chiral structures is fueled by a wide range\nof fascinating phenomena observed in chiral materials, such as nonreciprocal\ntransport and optical processes, electro-optical effects, and chiral\namplification and induction. Here, we report a novel route to chirality\nin antipolar GaTa<sub>4</sub>Se<sub>8</sub> crystals, where the rotation\nof the electric polarization vector through the unit cell traces out\na helix. The determination of atomic positions using X-ray diffraction\ncombined with ab initio calculations reveals that quasi-molecular\nTa<sub>4</sub>Se<sub>4</sub> clusters distort upon a phase transition\nand evoke significant local electric polarization within structural\nlayers of the unit cell. This polarization is found to rotate in 90°\nsteps between neighboring layers, either clockwise or anticlockwise.\nA similar analysis performed on two archetypal chiral compounds, α-quartz\nand tellurium, implies that antipolar crystals with screw-axis symmetry\nmay generally host atomic-scale polarization helices with emergent\nfunctionalities.