Disorder and diffuse scattering in single-chirality (TaSe₄)₂I crystals

The quasi-one-dimensional chiral compound (TaSe4)2I has been extensively studied as a prime example of a topological Weyl semimetal. Upon crossing its phase transition temperature TCDW approximate to 263 K, (TaSe4)2I exhibits incommensurate charge density wave (CDW) modulations described by the well-defined propagation vector similar to(0.05, 0.05, 0.11), oblique to the TaSe4 chains. Although optical and transport properties greatly depend on chirality, there is no systematic report about chiral domain size for (TaSe4)2I. In this study, our single-crystal scattering refinements reveal a bulk iodine deficiency, and Flack parameter measurements on multiple crystals demonstrate that separate (TaSe4)2I crystals have uniform handedness, supported by direct imaging and helicitydependent terahertz emission spectroscopy. Our single-crystal x-ray scattering and calculated diffraction patterns identify multiple diffuse features and create a real-space picture of the temperature-dependent (TaSe4)2I crystal structure. The short-range diffuse features are present at room temperature and decrease in intensity as the CDW modulation develops. These transverse displacements, along with electron pinning from the iodine deficiency, help explain why (TaSe4)2I behaves as an electronic semiconductor at temperatures above and below TCDW, despite a metallic band structure calculated from density functional theory of the ideal structure.