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Abstract Polarimetric infrared (IR) detection bolsters IR thermography by leveraging the polarization of light. Optical anisotropy, i.e., birefringence and dichroism, can be leveraged to achieve polarimetric detection. Recently, giant optical anisotropy is discovered in quasi‐1D narrow‐bandgap hexagonal perovskite sulfides, A1+xTiS3, specifically BaTiS3and Sr9/8TiS3. In these materials, the critical role of atomic‐scale structure modulations in the unconventional electrical, optical, and thermal properties raises the broader question of the nature of other materials that belong to this family. To address this issue, for the first time, high‐quality single crystals of a largely unexplored member of the A1+xTiX3(X = S, Se) family, BaTiSe3are synthesized. Single‐crystal X‐ray diffraction determined the room‐temperature structure with theP31cspace group, which is a superstructure of the earlier reportedP63/mmcstructure. The crystal structure of BaTiSe3features antiparallelc‐axis displacements similar to but of lower symmetry than BaTiS3, verified by the polarization dependent Raman spectroscopy. Fourier transform infrared (FTIR) spectroscopy is used to characterize the optical anisotropy of BaTiSe3, whose refractive index along the ordinary (E⊥c) and extraordinary (E‖c) optical axes is quantitatively determined by combining ellipsometry studies with FTIR. With a giant birefringence Δn∼ 0.9, BaTiSe3emerges as a new candidate for miniaturized birefringent optics for mid‐wave infrared to long‐wave infrared imaging.
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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 (Δn = 2.1) was reported in a quasi-one-dimensional chalcogenide, Sr9/8TiS3. Compared to SrTiS3, the excess Sr in Sr9/8TiS3 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 c-axis, hence resulting in the colossal optical anisotropy. Here, further enhancement of the colossal optical anisotropy to Δn = 2.5 in Sr8/7TiS3 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 SrxTiS3 compounds with x = [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 < x < 1.5. As x increases, an indirect-to-direct band gap transition is predicted for x ≥ 8/7 along with an increased occupancy of Ti-dz2 states. Together, these two factors result in a theoretically predicted maximum birefringence of Δn = 2.5 for Sr8/7TiS3. Single crystals of Sr8/7TiS3 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 Sr8/7TiS3, which is further corroborated by atomic-scale observations using scanning transmission electron microscopy. Polarization-resolved Fourier-transform infrared spectroscopy of Sr8/7TiS3 crystals shows Δn ≈ 2.5, in excellent agreement with the theoretical predictions. Overall, these findings demonstrate the compositional tunability of optical properties in SrxTiS3 compounds by control over atomic scale modulations and suggest that similar strategies could be extended to other compounds having modulated structures.
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- PAR ID:
- 10644492
- Publisher / Repository:
- ACS
- Date Published:
- Journal Name:
- ACS Nano
- ISSN:
- 1936-0851
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/acsnano.5c07992
