Correlated Disorder of the Sub-Angstrom Atomic Displacements in BaTiS₃ Causes Giant Optical Anisotropy

Correlated disorder arising from electronic instabilities leads to emergent electronic and magnetic orderings. The connection between correlated disorder and function has been studied in magnetic, electronic, and dipolar systems, but the emergence of novel optical phenomena from correlated disorder remains poorly understood. We show unambiguous evidence of the role of correlated sub-Angstrom atomic displacements in explaining the origin of giant optical anisotropy in BaTiS₃ (BTS). We carried out structure analysis by combining synchrotron single crystal X-ray diffraction (SC-XRD), scanning transmission electron microscopy (STEM) and nuclear magnetic resonance (NMR) studies supported by first principles calculations to resolve this open question. SC-XRD reveals the antipolar shifts between adjacent TiS₃ chains following a P6₃cm space group, commensurately modulated the previously reported structure. More importantly, the refined structure of BTS shows an emergent three-fold correlated disorder of Ti a-b plane displacements towards adjacent S atoms. First principles calculations reveal preference for Γ5 distortion mode Ti displacements along the BTS a-b plane in agreement with the experiments. Finally, the magnitude of Ti a-b plane displacements strongly correlates with the increase in optical anisotropy of BTS. This study shows the broader connection between correlated disorder and function, especially in realizing emergent physical properties such as giant optical anisotropy.