Chiral Lone‐Pair Helices with Handedness Coupling to Electric‐Strain Fields

Abstract Ferrochiral materials with an achiral‐to‐chiral phase transition and switchable chirality have unique application opportunities, enabling control of the angular momentum of circularly polarized lattice vibrations (chiral phonons) and chirality‐related electronic phenomena. Materials that fall into this class are, however, extremely rare and often accompanied by other types of ferroic order that interfere with the ferrochiral responses. In this work, two tetragonal tungsten bronzes (TTBs), K ₄ Bi ₂ Nb ₁₀ O ₃₀ and Rb ₄ Bi ₂ Nb ₁₀ O ₃₀ , are proposed as ideal ferrochiral candidates. Using high‐resolution X‐ray powder diffraction (XRD) combined with transmission electron microscopy, the incommensurately modulated and chiral structures are solved. Temperature‐dependent XRD reveals that both materials undergo an achiral‐to‐chiral phase transition from P 4/ mbm to P 42 ₁ 2(00γ) q 00. The chirality originates from a cooperative helical displacement of Bi ³⁺ atoms perpendicular to the c direction and represents the primary order parameter. As a secondary effect of the ferrochiral order, a spatially varying piezoelectric response is observed, consistent with the polycrystalline nature of the investigated materials. Through invariant analysis, an external electric‐strain‐field coupling with the piezoelectricity is proposed as a conjugate field for switching chirality, establishing TTBs as a versatile playground for the emergent field of ferrochirality.