Toward Atomic-Scale Control over Structural Modulations in Quasi-1D Chalcogenides for Colossal Optical Anisotropy

Optically anisotropic materials are sought after for tailoring the polarization of light. Recently, colossal optical anisotropy (Δ<i>n</i> = 2.1) was reported in a quasi-one-dimensional chalcogenide, Sr_9/8TiS₃. Compared to SrTiS₃, the excess Sr in Sr_9/8TiS₃ leads to periodic structural modulations and introduces additional electrons, which undergo charge ordering on select Ti atoms to form a highly polarizable cloud oriented along the <i>c</i>-axis, hence resulting in the colossal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to Δ<i>n</i> = 2.5 in Sr_8/7TiS₃ is reported through control over the periodicity of the atomic-scale modulations. The role of structural modulations in tuning the optical properties in a series of Sr_<i>x</i>TiS₃ compounds with <i>x</i> = [1, 9/8, 8/7, 6/5, 5/4, 4/3, 3/2] is investigated using density-functional-theory (DFT) calculations. The structural modulations arise from various stacking sequences of face-sharing TiS₆ octahedra and twist-distorted trigonal prisms and are found to be thermodynamically stable for 1 < <i>x</i> < 1.5. As <i>x</i> increases, an indirect-to-direct band gap transition is predicted for <i>x</i> ≥ 8/7 along with an increased occupancy of Ti-<i>d</i>_<i>z</i>² states. Together, these two factors result in a theoretically predicted maximum birefringence of Δ<i>n</i> = 2.5 for Sr_8/7TiS₃. Single crystals of Sr_8/7TiS₃ were grown using a molten-salt flux method. Single-crystal X-ray diffraction measurements confirm the presence of long-range order with a periodicity corresponding to Sr_8/7TiS₃, which is further corroborated by atomic-scale observations using scanning transmission electron microscopy. Polarization-resolved Fourier-transform infrared spectroscopy of Sr_8/7TiS₃ crystals shows Δ<i>n</i> ≈ 2.5, in excellent agreement with the theoretical predictions. Overall, these findings demonstrate the compositional tunability of optical properties in Sr_<i>x</i>TiS₃ compounds by control over atomic scale modulations and suggest that similar strategies could be extended to other compounds having modulated structures.