Atomic-Scale Polar Helix in Inorganic Crystals

Abstract

Chiral structures, noncentrosymmetric objects with a given handedness, emerge on all scales in nature. The most well-known chiral form, the helix, has numerous materializations not only on the microscopic scale (DNA, cholesteric liquid crystals, and spin helices) but also on macroscopic and even cosmological scales. The ongoing quest for new types of chiral structures is fueled by a wide range of fascinating phenomena observed in chiral materials, such as nonreciprocal transport and optical processes, electro-optical effects, and chiral amplification and induction. Here, we report a novel route to chirality in antipolar GaTa₄Se₈ crystals, where the rotation of the electric polarization vector through the unit cell traces out a helix. The determination of atomic positions using X-ray diffraction combined with ab initio calculations reveals that quasi-molecular Ta₄Se₄ clusters distort upon a phase transition and evoke significant local electric polarization within structural layers of the unit cell. This polarization is found to rotate in 90° steps between neighboring layers, either clockwise or anticlockwise. A similar analysis performed on two archetypal chiral compounds, α-quartz and tellurium, implies that antipolar crystals with screw-axis symmetry may generally host atomic-scale polarization helices with emergent functionalities.