Correlated disorder has been shown to enhance and modulate magnetic,
electrical, dipolar, electrochemical and mechanical properties of materials.
However, the possibility of obtaining novel optical and opto-electronic
properties from such correlated disorder remains an open question. Here, we
show unambiguous evidence of correlated disorder in the form of anisotropic,
sub-angstrom-scale atomic displacements modulating the refractive index tensor
and resulting in the giant optical anisotropy observed in BaTiS3, a
quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction
studies reveal the presence of antipolar displacements of Ti atoms within
adjacent TiS6 chains along the c-axis, and three-fold degenerate Ti
displacements in the a-b plane. 47/49Ti solid-state NMR provides additional
evidence for those Ti displacements in the form of a three-horned NMR lineshape
resulting from low symmetry local environment around Ti atoms. We used scanning
transmission electron microscopy to directly observe the globally disordered Ti
a-b plane displacements and find them to be ordered locally over a few unit
cells. First-principles calculations show that the Ti a-b plane displacements
selectively reduce the refractive index along the ab-plane, while having
minimal impact on the refractive index along the chain direction, thus
resulting in a giant enhancement in the optical anisotropy. By showing a strong
connection between correlated disorder and the optical response in BaTiS3, this
study opens a pathway for designing optical materials with high refractive
index and functionalities such as a large optical anisotropy and nonlinearity.Comment: 24 pages, 3 figure