Giant Optical Anisotropy from Tiny Atomic Displacements Resolved Using Theory and Microscopy

Hexagonal perovskite sulfides of the form <i>A</i>_1+<i>x</i><i>B</i>S₃ (<i>A</i>,<i> B</i> = metals) have a quasi-one-dimensional crystal structure with face-shared (<i>B</i>S₆) octahedral chains. Giant optical anisotropy has been reported in one such compound, BaTiS₃. Replacing, Ba²⁺ with isovalent Sr²⁺ to form SrTiS₃ results in further enhancement of the optical anisotropy. Furthermore, Sr_1+<i>x</i>TiS₃ is observed to display periodic structural modulations, where the periodicity <i>x </i>can be modulated by changing the synthesis conditions.&lt;!--[endif]----&gt; However, the origins of giant optical anisotropy in these compounds and their connection with structural modulations remain unresolved. Here, we combine density-functional theory calculations with atomic-scale characterization using STEM to reveal that subtle periodic modulations in Sr_1+<i>x</i>TiS₃ structures and sub-Angström displacements in BaTiS3 enables their record-breaking optical anisotropy.<br/>Acknowledgements: This work was supported by ARO through the MURI program with award number W911NF-21-1-0327, and NSF through DMR-2122070 and DMR-2122071.<br/><br/><br/>&lt;!--![endif]----&gt;